* [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
@ 2025-09-30 6:05 Lance Yang
2025-09-30 6:31 ` David Hildenbrand
` (2 more replies)
0 siblings, 3 replies; 9+ messages in thread
From: Lance Yang @ 2025-09-30 6:05 UTC (permalink / raw)
To: akpm, david, lorenzo.stoakes
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, dev.jain, npache,
riel, Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable,
Lance Yang
From: Lance Yang <lance.yang@linux.dev>
When splitting an mTHP and replacing a zero-filled subpage with the shared
zeropage, try_to_map_unused_to_zeropage() currently drops several important
PTE bits.
For userspace tools like CRIU, which rely on the soft-dirty mechanism for
incremental snapshots, losing the soft-dirty bit means modified pages are
missed, leading to inconsistent memory state after restore.
As pointed out by David, the more critical uffd-wp bit is also dropped.
This breaks the userfaultfd write-protection mechanism, causing writes
to be silently missed by monitoring applications, which can lead to data
corruption.
Preserve both the soft-dirty and uffd-wp bits from the old PTE when
creating the new zeropage mapping to ensure they are correctly tracked.
Cc: <stable@vger.kernel.org>
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
v2 -> v3:
- ptep_get() gets called only once per iteration (per Dev)
- https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
v1 -> v2:
- Avoid calling ptep_get() multiple times (per Dev)
- Double-check the uffd-wp bit (per David)
- Collect Acked-by from David - thanks!
- https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
mm/migrate.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index ce83c2c3c287..bafd8cb3bebe 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
struct folio *folio,
+ pte_t old_pte,
unsigned long idx)
{
struct page *page = folio_page(folio, idx);
@@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
return false;
VM_BUG_ON_PAGE(!PageAnon(page), page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
- VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
+ VM_BUG_ON_PAGE(pte_present(old_pte), page);
if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
mm_forbids_zeropage(pvmw->vma->vm_mm))
@@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
pvmw->vma->vm_page_prot));
+
+ if (pte_swp_soft_dirty(old_pte))
+ newpte = pte_mksoft_dirty(newpte);
+ if (pte_swp_uffd_wp(old_pte))
+ newpte = pte_mkuffd_wp(newpte);
+
set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
@@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio,
while (page_vma_mapped_walk(&pvmw)) {
rmap_t rmap_flags = RMAP_NONE;
- pte_t old_pte;
+ pte_t old_pte = ptep_get(pvmw.pte);
pte_t pte;
swp_entry_t entry;
struct page *new;
@@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio *folio,
}
#endif
if (rmap_walk_arg->map_unused_to_zeropage &&
- try_to_map_unused_to_zeropage(&pvmw, folio, idx))
+ try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
continue;
folio_get(folio);
pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
- old_pte = ptep_get(pvmw.pte);
entry = pte_to_swp_entry(old_pte);
if (!is_migration_entry_young(entry))
--
2.49.0
^ permalink raw reply [flat|nested] 9+ messages in thread* Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 6:05 [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
@ 2025-09-30 6:31 ` David Hildenbrand
2025-09-30 6:51 ` Lance Yang
2025-09-30 6:33 ` Dev Jain
2025-09-30 10:56 ` [syzbot ci] " syzbot ci
2 siblings, 1 reply; 9+ messages in thread
From: David Hildenbrand @ 2025-09-30 6:31 UTC (permalink / raw)
To: Lance Yang, akpm, lorenzo.stoakes
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, dev.jain, npache,
riel, Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable
On 30.09.25 08:05, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
>
> When splitting an mTHP and replacing a zero-filled subpage with the shared
> zeropage, try_to_map_unused_to_zeropage() currently drops several important
> PTE bits.
>
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> incremental snapshots, losing the soft-dirty bit means modified pages are
> missed, leading to inconsistent memory state after restore.
>
> As pointed out by David, the more critical uffd-wp bit is also dropped.
> This breaks the userfaultfd write-protection mechanism, causing writes
> to be silently missed by monitoring applications, which can lead to data
> corruption.
>
> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
> creating the new zeropage mapping to ensure they are correctly tracked.
>
> Cc: <stable@vger.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Dev Jain <dev.jain@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> v2 -> v3:
> - ptep_get() gets called only once per iteration (per Dev)
> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
>
> v1 -> v2:
> - Avoid calling ptep_get() multiple times (per Dev)
> - Double-check the uffd-wp bit (per David)
> - Collect Acked-by from David - thanks!
> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>
> mm/migrate.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..bafd8cb3bebe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
>
> static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> struct folio *folio,
> + pte_t old_pte,
> unsigned long idx)
Nit:
static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
struct folio *folio, pte_t old_pte, unsigned long idx)
LGTM, Thanks!
--
Cheers
David / dhildenb
^ permalink raw reply [flat|nested] 9+ messages in thread* Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 6:31 ` David Hildenbrand
@ 2025-09-30 6:51 ` Lance Yang
0 siblings, 0 replies; 9+ messages in thread
From: Lance Yang @ 2025-09-30 6:51 UTC (permalink / raw)
To: David Hildenbrand
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, dev.jain, npache,
riel, Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable,
akpm, lorenzo.stoakes
On 2025/9/30 14:31, David Hildenbrand wrote:
> On 30.09.25 08:05, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When splitting an mTHP and replacing a zero-filled subpage with the
>> shared
>> zeropage, try_to_map_unused_to_zeropage() currently drops several
>> important
>> PTE bits.
>>
>> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
>> incremental snapshots, losing the soft-dirty bit means modified pages are
>> missed, leading to inconsistent memory state after restore.
>>
>> As pointed out by David, the more critical uffd-wp bit is also dropped.
>> This breaks the userfaultfd write-protection mechanism, causing writes
>> to be silently missed by monitoring applications, which can lead to data
>> corruption.
>>
>> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
>> creating the new zeropage mapping to ensure they are correctly tracked.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage
>> when splitting isolated thp")
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Suggested-by: Dev Jain <dev.jain@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> v2 -> v3:
>> - ptep_get() gets called only once per iteration (per Dev)
>> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-
>> lance.yang@linux.dev/
>>
>> v1 -> v2:
>> - Avoid calling ptep_get() multiple times (per Dev)
>> - Double-check the uffd-wp bit (per David)
>> - Collect Acked-by from David - thanks!
>> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-
>> lance.yang@linux.dev/
>>
>> mm/migrate.c | 14 ++++++++++----
>> 1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ce83c2c3c287..bafd8cb3bebe 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio,
>> struct list_head *list)
>> static bool try_to_map_unused_to_zeropage(struct
>> page_vma_mapped_walk *pvmw,
>> struct folio *folio,
>> + pte_t old_pte,
>> unsigned long idx)
>
> Nit:
>
> static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk
> *pvmw,
> struct folio *folio, pte_t old_pte, unsigned long idx)
Well, let me clean that up ;p
>
> LGTM, Thanks!
Cheers!
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 6:05 [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 6:31 ` David Hildenbrand
@ 2025-09-30 6:33 ` Dev Jain
2025-09-30 6:56 ` Lance Yang
2025-09-30 10:56 ` [syzbot ci] " syzbot ci
2 siblings, 1 reply; 9+ messages in thread
From: Dev Jain @ 2025-09-30 6:33 UTC (permalink / raw)
To: Lance Yang, akpm, david, lorenzo.stoakes
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, npache, riel,
Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable
On 30/09/25 11:35 am, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
>
> When splitting an mTHP and replacing a zero-filled subpage with the shared
> zeropage, try_to_map_unused_to_zeropage() currently drops several important
> PTE bits.
>
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> incremental snapshots, losing the soft-dirty bit means modified pages are
> missed, leading to inconsistent memory state after restore.
>
> As pointed out by David, the more critical uffd-wp bit is also dropped.
> This breaks the userfaultfd write-protection mechanism, causing writes
> to be silently missed by monitoring applications, which can lead to data
> corruption.
>
> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
> creating the new zeropage mapping to ensure they are correctly tracked.
>
> Cc: <stable@vger.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Dev Jain <dev.jain@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> v2 -> v3:
> - ptep_get() gets called only once per iteration (per Dev)
> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
>
> v1 -> v2:
> - Avoid calling ptep_get() multiple times (per Dev)
> - Double-check the uffd-wp bit (per David)
> - Collect Acked-by from David - thanks!
> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>
> mm/migrate.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..bafd8cb3bebe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
>
> static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> struct folio *folio,
> + pte_t old_pte,
> unsigned long idx)
Could have just added this in the same line as folio?
> {
> struct page *page = folio_page(folio, idx);
> @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> return false;
> VM_BUG_ON_PAGE(!PageAnon(page), page);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
> + VM_BUG_ON_PAGE(pte_present(old_pte), page);
>
> if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
> mm_forbids_zeropage(pvmw->vma->vm_mm))
> @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>
> newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
> pvmw->vma->vm_page_prot));
> +
> + if (pte_swp_soft_dirty(old_pte))
> + newpte = pte_mksoft_dirty(newpte);
> + if (pte_swp_uffd_wp(old_pte))
> + newpte = pte_mkuffd_wp(newpte);
> +
> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>
> dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
> @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio,
>
> while (page_vma_mapped_walk(&pvmw)) {
> rmap_t rmap_flags = RMAP_NONE;
> - pte_t old_pte;
> + pte_t old_pte = ptep_get(pvmw.pte);
> pte_t pte;
> swp_entry_t entry;
> struct page *new;
> @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio *folio,
> }
> #endif
> if (rmap_walk_arg->map_unused_to_zeropage &&
> - try_to_map_unused_to_zeropage(&pvmw, folio, idx))
> + try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
> continue;
>
> folio_get(folio);
> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
> - old_pte = ptep_get(pvmw.pte);
>
> entry = pte_to_swp_entry(old_pte);
> if (!is_migration_entry_young(entry))
Looks good, the special bit does not overlay on any arch with the soft-dirty bit.
It shouldn't overlay with uffd-wp as well since split_huge_zero_page_pmd does the
same bit preservation.
Reviewed-by: Dev Jain <dev.jain@arm.com>
^ permalink raw reply [flat|nested] 9+ messages in thread* Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 6:33 ` Dev Jain
@ 2025-09-30 6:56 ` Lance Yang
0 siblings, 0 replies; 9+ messages in thread
From: Lance Yang @ 2025-09-30 6:56 UTC (permalink / raw)
To: Dev Jain
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, npache, riel,
Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, akpm, linux-kernel, linux-mm, ioworker0,
stable, lorenzo.stoakes, david
On 2025/9/30 14:33, Dev Jain wrote:
>
> On 30/09/25 11:35 am, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When splitting an mTHP and replacing a zero-filled subpage with the
>> shared
>> zeropage, try_to_map_unused_to_zeropage() currently drops several
>> important
>> PTE bits.
>>
>> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
>> incremental snapshots, losing the soft-dirty bit means modified pages are
>> missed, leading to inconsistent memory state after restore.
>>
>> As pointed out by David, the more critical uffd-wp bit is also dropped.
>> This breaks the userfaultfd write-protection mechanism, causing writes
>> to be silently missed by monitoring applications, which can lead to data
>> corruption.
>>
>> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
>> creating the new zeropage mapping to ensure they are correctly tracked.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage
>> when splitting isolated thp")
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Suggested-by: Dev Jain <dev.jain@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> v2 -> v3:
>> - ptep_get() gets called only once per iteration (per Dev)
>> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-
>> lance.yang@linux.dev/
>>
>> v1 -> v2:
>> - Avoid calling ptep_get() multiple times (per Dev)
>> - Double-check the uffd-wp bit (per David)
>> - Collect Acked-by from David - thanks!
>> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-
>> lance.yang@linux.dev/
>>
>> mm/migrate.c | 14 ++++++++++----
>> 1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ce83c2c3c287..bafd8cb3bebe 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio,
>> struct list_head *list)
>> static bool try_to_map_unused_to_zeropage(struct
>> page_vma_mapped_walk *pvmw,
>> struct folio *folio,
>> + pte_t old_pte,
>> unsigned long idx)
>
> Could have just added this in the same line as folio?
Sure ;p
>
>> {
>> struct page *page = folio_page(folio, idx);
>> @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct
>> page_vma_mapped_walk *pvmw,
>> return false;
>> VM_BUG_ON_PAGE(!PageAnon(page), page);
>> VM_BUG_ON_PAGE(!PageLocked(page), page);
>> - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
>> + VM_BUG_ON_PAGE(pte_present(old_pte), page);
>> if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags &
>> VM_LOCKED) ||
>> mm_forbids_zeropage(pvmw->vma->vm_mm))
>> @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct
>> page_vma_mapped_walk *pvmw,
>> newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
>> pvmw->vma->vm_page_prot));
>> +
>> + if (pte_swp_soft_dirty(old_pte))
>> + newpte = pte_mksoft_dirty(newpte);
>> + if (pte_swp_uffd_wp(old_pte))
>> + newpte = pte_mkuffd_wp(newpte);
>> +
>> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>> dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
>> @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio,
>> while (page_vma_mapped_walk(&pvmw)) {
>> rmap_t rmap_flags = RMAP_NONE;
>> - pte_t old_pte;
>> + pte_t old_pte = ptep_get(pvmw.pte);
>> pte_t pte;
>> swp_entry_t entry;
>> struct page *new;
>> @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio
>> *folio,
>> }
>> #endif
>> if (rmap_walk_arg->map_unused_to_zeropage &&
>> - try_to_map_unused_to_zeropage(&pvmw, folio, idx))
>> + try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
>> continue;
>> folio_get(folio);
>> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
>> - old_pte = ptep_get(pvmw.pte);
>> entry = pte_to_swp_entry(old_pte);
>> if (!is_migration_entry_young(entry))
>
> Looks good, the special bit does not overlay on any arch with the soft-
> dirty bit.
> It shouldn't overlay with uffd-wp as well since split_huge_zero_page_pmd
> does the
> same bit preservation.
Yeah. Thanks for double-checking the bit overlaps!
Good to know we're on solid ground here ;)
>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
Cheers!
^ permalink raw reply [flat|nested] 9+ messages in thread
* [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 6:05 [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 6:31 ` David Hildenbrand
2025-09-30 6:33 ` Dev Jain
@ 2025-09-30 10:56 ` syzbot ci
2025-09-30 11:17 ` Lance Yang
2 siblings, 1 reply; 9+ messages in thread
From: syzbot ci @ 2025-09-30 10:56 UTC (permalink / raw)
To: akpm, apopple, baohua, baolin.wang, byungchul, david, dev.jain,
gourry, harry.yoo, ioworker0, jannh, joshua.hahnjy, lance.yang,
liam.howlett, linux-kernel, linux-mm, lorenzo.stoakes,
matthew.brost, npache, peterx, rakie.kim, riel, ryan.roberts,
stable, usamaarif642, vbabka, ying.huang, yuzhao, ziy
Cc: syzbot, syzkaller-bugs
syzbot ci has tested the following series
[v3] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
https://lore.kernel.org/all/20250930060557.85133-1-lance.yang@linux.dev
* [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
and found the following issue:
general protection fault in remove_migration_pte
Full report is available here:
https://ci.syzbot.org/series/a2021abd-c238-431c-a92e-cc29beb53cbf
***
general protection fault in remove_migration_pte
tree: torvalds
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
base: 1896ce8eb6c61824f6c1125d69d8fda1f44a22f8
arch: amd64
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
config: https://ci.syzbot.org/builds/84a2085e-d609-43ea-8b19-f9af8ea3d54a/config
C repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/c_repro
syz repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/syz_repro
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 5985 Comm: syz.0.27 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
Call Trace:
<TASK>
rmap_walk_anon+0x553/0x730 mm/rmap.c:2842
remove_migration_ptes mm/migrate.c:478 [inline]
migrate_folio_move mm/migrate.c:1394 [inline]
migrate_folios_move mm/migrate.c:1725 [inline]
migrate_pages_batch+0x200a/0x35c0 mm/migrate.c:1972
migrate_pages_sync mm/migrate.c:2002 [inline]
migrate_pages+0x1bcc/0x2930 mm/migrate.c:2111
migrate_to_node mm/mempolicy.c:1244 [inline]
do_migrate_pages+0x5ee/0x800 mm/mempolicy.c:1343
kernel_migrate_pages mm/mempolicy.c:1858 [inline]
__do_sys_migrate_pages mm/mempolicy.c:1876 [inline]
__se_sys_migrate_pages+0x544/0x650 mm/mempolicy.c:1872
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f922b98ec29
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffccaf966f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000100
RAX: ffffffffffffffda RBX: 00007f922bbd5fa0 RCX: 00007f922b98ec29
RDX: 0000200000000300 RSI: 0000000000000003 RDI: 0000000000000000
RBP: 00007f922ba11e41 R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f922bbd5fa0 R14: 00007f922bbd5fa0 R15: 0000000000000004
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
----------------
Code disassembly (best guess):
0: 43 20 48 89 rex.XB and %cl,-0x77(%r8)
4: 84 24 08 test %ah,(%rax,%rcx,1)
7: 01 00 add %eax,(%rax)
9: 00 49 8d add %cl,-0x73(%rcx)
c: 47 rex.RXB
d: 40 rex
e: 48 89 84 24 00 01 00 mov %rax,0x100(%rsp)
15: 00
16: 4c 89 64 24 50 mov %r12,0x50(%rsp)
1b: 4c 8b b4 24 70 01 00 mov 0x170(%rsp),%r14
22: 00
23: 4c 89 f0 mov %r14,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 4c 89 f7 mov %r14,%rdi
34: e8 22 3e ff ff call 0xffff3e5b
39: 49 8b 06 mov (%r14),%rax
3c: 48 rex.W
3d: 89 .byte 0x89
3e: 44 rex.R
3f: 24 .byte 0x24
***
If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syzbot@syzkaller.appspotmail.com
---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 10:56 ` [syzbot ci] " syzbot ci
@ 2025-09-30 11:17 ` Lance Yang
0 siblings, 0 replies; 9+ messages in thread
From: Lance Yang @ 2025-09-30 11:17 UTC (permalink / raw)
To: syzbot ci
Cc: syzbot, syzkaller-bugs, yuzhao, ziy, ying.huang, jannh, vbabka,
riel, usamaarif642, david, dev.jain, akpm, ryan.roberts, stable,
apopple, rakie.kim, gourry, matthew.brost, lorenzo.stoakes,
baohua, liam.howlett, ioworker0, harry.yoo, peterx, npache,
baolin.wang, linux-mm, joshua.hahnjy, linux-kernel, byungchul
Thanks for the report!
On 2025/9/30 18:56, syzbot ci wrote:
> syzbot ci has tested the following series
>
> [v3] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
> https://lore.kernel.org/all/20250930060557.85133-1-lance.yang@linux.dev
> * [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
>
> and found the following issue:
> general protection fault in remove_migration_pte
>
> Full report is available here:
> https://ci.syzbot.org/series/a2021abd-c238-431c-a92e-cc29beb53cbf
>
> ***
>
> general protection fault in remove_migration_pte
>
> tree: torvalds
> URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
> base: 1896ce8eb6c61824f6c1125d69d8fda1f44a22f8
> arch: amd64
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> config: https://ci.syzbot.org/builds/84a2085e-d609-43ea-8b19-f9af8ea3d54a/config
> C repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/c_repro
> syz repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/syz_repro
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
This is a known issue that I introduced in the v3 patch. I spotted
this exact NULL pointer dereference bug[1] myself and have already
sent out a v5 version[2] with the fix.
The root cause is that ptep_get() is called before the !pwmw.pte
check, which handles PMD-mapped THP migration entries.
[1]
https://lore.kernel.org/linux-mm/2d21c9bc-e299-4ca6-85ba-b01a1f346d9d@linux.dev
[2]
https://lore.kernel.org/linux-mm/20250930081040.80926-1-lance.yang@linux.dev
Thanks,
Lance
> CPU: 0 UID: 0 PID: 5985 Comm: syz.0.27 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
> Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
> RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
> RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
> FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
> Call Trace:
> <TASK>
> rmap_walk_anon+0x553/0x730 mm/rmap.c:2842
> remove_migration_ptes mm/migrate.c:478 [inline]
> migrate_folio_move mm/migrate.c:1394 [inline]
> migrate_folios_move mm/migrate.c:1725 [inline]
> migrate_pages_batch+0x200a/0x35c0 mm/migrate.c:1972
> migrate_pages_sync mm/migrate.c:2002 [inline]
> migrate_pages+0x1bcc/0x2930 mm/migrate.c:2111
> migrate_to_node mm/mempolicy.c:1244 [inline]
> do_migrate_pages+0x5ee/0x800 mm/mempolicy.c:1343
> kernel_migrate_pages mm/mempolicy.c:1858 [inline]
> __do_sys_migrate_pages mm/mempolicy.c:1876 [inline]
> __se_sys_migrate_pages+0x544/0x650 mm/mempolicy.c:1872
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f922b98ec29
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffccaf966f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000100
> RAX: ffffffffffffffda RBX: 00007f922bbd5fa0 RCX: 00007f922b98ec29
> RDX: 0000200000000300 RSI: 0000000000000003 RDI: 0000000000000000
> RBP: 00007f922ba11e41 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f922bbd5fa0 R14: 00007f922bbd5fa0 R15: 0000000000000004
> </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
> Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
> RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
> RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
> FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
> ----------------
> Code disassembly (best guess):
> 0: 43 20 48 89 rex.XB and %cl,-0x77(%r8)
> 4: 84 24 08 test %ah,(%rax,%rcx,1)
> 7: 01 00 add %eax,(%rax)
> 9: 00 49 8d add %cl,-0x73(%rcx)
> c: 47 rex.RXB
> d: 40 rex
> e: 48 89 84 24 00 01 00 mov %rax,0x100(%rsp)
> 15: 00
> 16: 4c 89 64 24 50 mov %r12,0x50(%rsp)
> 1b: 4c 8b b4 24 70 01 00 mov 0x170(%rsp),%r14
> 22: 00
> 23: 4c 89 f0 mov %r14,%rax
> 26: 48 c1 e8 03 shr $0x3,%rax
> * 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
> 2f: 74 08 je 0x39
> 31: 4c 89 f7 mov %r14,%rdi
> 34: e8 22 3e ff ff call 0xffff3e5b
> 39: 49 8b 06 mov (%r14),%rax
> 3c: 48 rex.W
> 3d: 89 .byte 0x89
> 3e: 44 rex.R
> 3f: 24 .byte 0x24
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
> Tested-by: syzbot@syzkaller.appspotmail.com
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at syzkaller@googlegroups.com.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
@ 2025-09-30 7:10 Lance Yang
2025-09-30 11:16 ` [syzbot ci] " syzbot ci
0 siblings, 1 reply; 9+ messages in thread
From: Lance Yang @ 2025-09-30 7:10 UTC (permalink / raw)
To: akpm, david, lorenzo.stoakes
Cc: peterx, ziy, baolin.wang, baohua, ryan.roberts, dev.jain, npache,
riel, Liam.Howlett, vbabka, harry.yoo, jannh, matthew.brost,
joshua.hahnjy, rakie.kim, byungchul, gourry, ying.huang, apopple,
usamaarif642, yuzhao, linux-kernel, linux-mm, ioworker0, stable,
Lance Yang
From: Lance Yang <lance.yang@linux.dev>
When splitting an mTHP and replacing a zero-filled subpage with the shared
zeropage, try_to_map_unused_to_zeropage() currently drops several important
PTE bits.
For userspace tools like CRIU, which rely on the soft-dirty mechanism for
incremental snapshots, losing the soft-dirty bit means modified pages are
missed, leading to inconsistent memory state after restore.
As pointed out by David, the more critical uffd-wp bit is also dropped.
This breaks the userfaultfd write-protection mechanism, causing writes
to be silently missed by monitoring applications, which can lead to data
corruption.
Preserve both the soft-dirty and uffd-wp bits from the old PTE when
creating the new zeropage mapping to ensure they are correctly tracked.
Cc: <stable@vger.kernel.org>
Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
v3 -> v4:
- Minor formatting tweak in try_to_map_unused_to_zeropage() function
signature (per David and Dev)
- Collect Reviewed-by from Dev - thanks!
- https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev/
v2 -> v3:
- ptep_get() gets called only once per iteration (per Dev)
- https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
v1 -> v2:
- Avoid calling ptep_get() multiple times (per Dev)
- Double-check the uffd-wp bit (per David)
- Collect Acked-by from David - thanks!
- https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
mm/migrate.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index ce83c2c3c287..21a2a1bf89f7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -296,8 +296,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
}
static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
- struct folio *folio,
- unsigned long idx)
+ struct folio *folio, pte_t old_pte, unsigned long idx)
{
struct page *page = folio_page(folio, idx);
pte_t newpte;
@@ -306,7 +305,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
return false;
VM_BUG_ON_PAGE(!PageAnon(page), page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
- VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
+ VM_BUG_ON_PAGE(pte_present(old_pte), page);
if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
mm_forbids_zeropage(pvmw->vma->vm_mm))
@@ -322,6 +321,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
pvmw->vma->vm_page_prot));
+
+ if (pte_swp_soft_dirty(old_pte))
+ newpte = pte_mksoft_dirty(newpte);
+ if (pte_swp_uffd_wp(old_pte))
+ newpte = pte_mkuffd_wp(newpte);
+
set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
@@ -344,7 +349,7 @@ static bool remove_migration_pte(struct folio *folio,
while (page_vma_mapped_walk(&pvmw)) {
rmap_t rmap_flags = RMAP_NONE;
- pte_t old_pte;
+ pte_t old_pte = ptep_get(pvmw.pte);
pte_t pte;
swp_entry_t entry;
struct page *new;
@@ -365,12 +370,11 @@ static bool remove_migration_pte(struct folio *folio,
}
#endif
if (rmap_walk_arg->map_unused_to_zeropage &&
- try_to_map_unused_to_zeropage(&pvmw, folio, idx))
+ try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
continue;
folio_get(folio);
pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
- old_pte = ptep_get(pvmw.pte);
entry = pte_to_swp_entry(old_pte);
if (!is_migration_entry_young(entry))
--
2.49.0
^ permalink raw reply [flat|nested] 9+ messages in thread* [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 7:10 [PATCH v4 1/1] " Lance Yang
@ 2025-09-30 11:16 ` syzbot ci
2025-09-30 11:20 ` Lance Yang
0 siblings, 1 reply; 9+ messages in thread
From: syzbot ci @ 2025-09-30 11:16 UTC (permalink / raw)
To: akpm, apopple, baohua, baolin.wang, byungchul, david, dev.jain,
gourry, harry.yoo, ioworker0, jannh, joshua.hahnjy, lance.yang,
liam.howlett, linux-kernel, linux-mm, lorenzo.stoakes,
matthew.brost, npache, peterx, rakie.kim, riel, ryan.roberts,
stable, usamaarif642, vbabka, ying.huang, yuzhao, ziy
Cc: syzbot, syzkaller-bugs
syzbot ci has tested the following series
[v4] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
https://lore.kernel.org/all/20250930071053.36158-1-lance.yang@linux.dev
* [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
and found the following issue:
general protection fault in remove_migration_pte
Full report is available here:
https://ci.syzbot.org/series/8cc7e52f-a859-4251-bd08-9787cdaf7928
***
general protection fault in remove_migration_pte
tree: linux-next
URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
base: 262858079afde6d367ce3db183c74d8a43a0e83f
arch: amd64
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
config: https://ci.syzbot.org/builds/97ee4826-5d29-472d-a85d-51543b0e45de/config
C repro: https://ci.syzbot.org/findings/f4819db2-21f2-4280-8bc4-942445398953/c_repro
syz repro: https://ci.syzbot.org/findings/f4819db2-21f2-4280-8bc4-942445398953/syz_repro
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 6025 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
RIP: 0010:remove_migration_pte+0x369/0x2320 mm/migrate.c:352
Code: 00 48 8d 43 20 48 89 44 24 68 49 8d 47 40 48 89 84 24 e8 00 00 00 4c 89 64 24 48 4c 8b b4 24 50 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 f8 3e ff ff 49 8b 06 48 89 44 24
RSP: 0018:ffffc90002fb73e0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88802957e300 RCX: 1ffffd40008c9006
RDX: 0000000000000000 RSI: 0000000000030dff RDI: 0000000000030c00
RBP: ffffc90002fb75d0 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff520005f6e34 R12: ffffea0004648008
R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004648000
FS: 00005555624de500(0000) GS:ffff8880b83fc000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000300 CR3: 000000010d8b8000 CR4: 00000000000006f0
Call Trace:
<TASK>
rmap_walk_anon+0x553/0x730 mm/rmap.c:2855
remove_migration_ptes mm/migrate.c:469 [inline]
migrate_folio_move mm/migrate.c:1381 [inline]
migrate_folios_move mm/migrate.c:1711 [inline]
migrate_pages_batch+0x202e/0x35e0 mm/migrate.c:1967
migrate_pages_sync mm/migrate.c:1997 [inline]
migrate_pages+0x1bcc/0x2930 mm/migrate.c:2106
migrate_to_node mm/mempolicy.c:1244 [inline]
do_migrate_pages+0x5ee/0x800 mm/mempolicy.c:1343
kernel_migrate_pages mm/mempolicy.c:1858 [inline]
__do_sys_migrate_pages mm/mempolicy.c:1876 [inline]
__se_sys_migrate_pages+0x544/0x650 mm/mempolicy.c:1872
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb18e18ec29
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffdca5c9838 EFLAGS: 00000246 ORIG_RAX: 0000000000000100
RAX: ffffffffffffffda RBX: 00007fb18e3d5fa0 RCX: 00007fb18e18ec29
RDX: 0000200000000300 RSI: 0000000000000003 RDI: 0000000000000000
RBP: 00007fb18e211e41 R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fb18e3d5fa0 R14: 00007fb18e3d5fa0 R15: 0000000000000004
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
RIP: 0010:remove_migration_pte+0x369/0x2320 mm/migrate.c:352
Code: 00 48 8d 43 20 48 89 44 24 68 49 8d 47 40 48 89 84 24 e8 00 00 00 4c 89 64 24 48 4c 8b b4 24 50 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 f8 3e ff ff 49 8b 06 48 89 44 24
RSP: 0018:ffffc90002fb73e0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88802957e300 RCX: 1ffffd40008c9006
RDX: 0000000000000000 RSI: 0000000000030dff RDI: 0000000000030c00
RBP: ffffc90002fb75d0 R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff520005f6e34 R12: ffffea0004648008
R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004648000
FS: 00005555624de500(0000) GS:ffff8880b83fc000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000300 CR3: 000000010d8b8000 CR4: 00000000000006f0
----------------
Code disassembly (best guess):
0: 00 48 8d add %cl,-0x73(%rax)
3: 43 20 48 89 rex.XB and %cl,-0x77(%r8)
7: 44 24 68 rex.R and $0x68,%al
a: 49 8d 47 40 lea 0x40(%r15),%rax
e: 48 89 84 24 e8 00 00 mov %rax,0xe8(%rsp)
15: 00
16: 4c 89 64 24 48 mov %r12,0x48(%rsp)
1b: 4c 8b b4 24 50 01 00 mov 0x150(%rsp),%r14
22: 00
23: 4c 89 f0 mov %r14,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 4c 89 f7 mov %r14,%rdi
34: e8 f8 3e ff ff call 0xffff3f31
39: 49 8b 06 mov (%r14),%rax
3c: 48 rex.W
3d: 89 .byte 0x89
3e: 44 rex.R
3f: 24 .byte 0x24
***
If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syzbot@syzkaller.appspotmail.com
---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
2025-09-30 11:16 ` [syzbot ci] " syzbot ci
@ 2025-09-30 11:20 ` Lance Yang
0 siblings, 0 replies; 9+ messages in thread
From: Lance Yang @ 2025-09-30 11:20 UTC (permalink / raw)
To: syzbot ci
Cc: syzbot, liam.howlett, usamaarif642, rakie.kim, david, dev.jain,
ioworker0, syzkaller-bugs, ziy, byungchul, baolin.wang, akpm,
apopple, vbabka, joshua.hahnjy, gourry, linux-mm, matthew.brost,
lorenzo.stoakes, ying.huang, linux-kernel, baohua, npache,
yuzhao, ryan.roberts, riel, stable, peterx, jannh, harry.yoo
On 2025/9/30 19:16, syzbot ci wrote:
> syzbot ci has tested the following series
>
> [v4] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
> https://lore.kernel.org/all/20250930071053.36158-1-lance.yang@linux.dev
> * [PATCH v4 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
>
> and found the following issue:
> general protection fault in remove_migration_pte
>
> Full report is available here:
> https://ci.syzbot.org/series/8cc7e52f-a859-4251-bd08-9787cdaf7928
>
> ***
>
> general protection fault in remove_migration_pte
>
> tree: linux-next
> URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> base: 262858079afde6d367ce3db183c74d8a43a0e83f
> arch: amd64
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> config: https://ci.syzbot.org/builds/97ee4826-5d29-472d-a85d-51543b0e45de/config
> C repro: https://ci.syzbot.org/findings/f4819db2-21f2-4280-8bc4-942445398953/c_repro
> syz repro: https://ci.syzbot.org/findings/f4819db2-21f2-4280-8bc4-942445398953/syz_repro
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
This is a known issue that I introduced in the v3 patch. I spotted
this exact NULL pointer dereference bug[1] myself and have already
sent out a v5 version[2] with the fix.
The root cause is that ptep_get() is called before the !pwmw.pte
check, which handles PMD-mapped THP migration entries.
[1]
https://lore.kernel.org/linux-mm/2d21c9bc-e299-4ca6-85ba-b01a1f346d9d@linux.dev
[2]
https://lore.kernel.org/linux-mm/20250930081040.80926-1-lance.yang@linux.dev
Thanks,
Lance
> CPU: 0 UID: 0 PID: 6025 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x369/0x2320 mm/migrate.c:352
> Code: 00 48 8d 43 20 48 89 44 24 68 49 8d 47 40 48 89 84 24 e8 00 00 00 4c 89 64 24 48 4c 8b b4 24 50 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 f8 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002fb73e0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff88802957e300 RCX: 1ffffd40008c9006
> RDX: 0000000000000000 RSI: 0000000000030dff RDI: 0000000000030c00
> RBP: ffffc90002fb75d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff520005f6e34 R12: ffffea0004648008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004648000
> FS: 00005555624de500(0000) GS:ffff8880b83fc000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 000000010d8b8000 CR4: 00000000000006f0
> Call Trace:
> <TASK>
> rmap_walk_anon+0x553/0x730 mm/rmap.c:2855
> remove_migration_ptes mm/migrate.c:469 [inline]
> migrate_folio_move mm/migrate.c:1381 [inline]
> migrate_folios_move mm/migrate.c:1711 [inline]
> migrate_pages_batch+0x202e/0x35e0 mm/migrate.c:1967
> migrate_pages_sync mm/migrate.c:1997 [inline]
> migrate_pages+0x1bcc/0x2930 mm/migrate.c:2106
> migrate_to_node mm/mempolicy.c:1244 [inline]
> do_migrate_pages+0x5ee/0x800 mm/mempolicy.c:1343
> kernel_migrate_pages mm/mempolicy.c:1858 [inline]
> __do_sys_migrate_pages mm/mempolicy.c:1876 [inline]
> __se_sys_migrate_pages+0x544/0x650 mm/mempolicy.c:1872
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fb18e18ec29
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffdca5c9838 EFLAGS: 00000246 ORIG_RAX: 0000000000000100
> RAX: ffffffffffffffda RBX: 00007fb18e3d5fa0 RCX: 00007fb18e18ec29
> RDX: 0000200000000300 RSI: 0000000000000003 RDI: 0000000000000000
> RBP: 00007fb18e211e41 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fb18e3d5fa0 R14: 00007fb18e3d5fa0 R15: 0000000000000004
> </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x369/0x2320 mm/migrate.c:352
> Code: 00 48 8d 43 20 48 89 44 24 68 49 8d 47 40 48 89 84 24 e8 00 00 00 4c 89 64 24 48 4c 8b b4 24 50 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 f8 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002fb73e0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff88802957e300 RCX: 1ffffd40008c9006
> RDX: 0000000000000000 RSI: 0000000000030dff RDI: 0000000000030c00
> RBP: ffffc90002fb75d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff520005f6e34 R12: ffffea0004648008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004648000
> FS: 00005555624de500(0000) GS:ffff8880b83fc000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 000000010d8b8000 CR4: 00000000000006f0
> ----------------
> Code disassembly (best guess):
> 0: 00 48 8d add %cl,-0x73(%rax)
> 3: 43 20 48 89 rex.XB and %cl,-0x77(%r8)
> 7: 44 24 68 rex.R and $0x68,%al
> a: 49 8d 47 40 lea 0x40(%r15),%rax
> e: 48 89 84 24 e8 00 00 mov %rax,0xe8(%rsp)
> 15: 00
> 16: 4c 89 64 24 48 mov %r12,0x48(%rsp)
> 1b: 4c 8b b4 24 50 01 00 mov 0x150(%rsp),%r14
> 22: 00
> 23: 4c 89 f0 mov %r14,%rax
> 26: 48 c1 e8 03 shr $0x3,%rax
> * 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
> 2f: 74 08 je 0x39
> 31: 4c 89 f7 mov %r14,%rdi
> 34: e8 f8 3e ff ff call 0xffff3f31
> 39: 49 8b 06 mov (%r14),%rax
> 3c: 48 rex.W
> 3d: 89 .byte 0x89
> 3e: 44 rex.R
> 3f: 24 .byte 0x24
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
> Tested-by: syzbot@syzkaller.appspotmail.com
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at syzkaller@googlegroups.com.
^ permalink raw reply [flat|nested] 9+ messages in thread
end of thread, other threads:[~2025-09-30 11:20 UTC | newest]
Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2025-09-30 6:05 [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage Lance Yang
2025-09-30 6:31 ` David Hildenbrand
2025-09-30 6:51 ` Lance Yang
2025-09-30 6:33 ` Dev Jain
2025-09-30 6:56 ` Lance Yang
2025-09-30 10:56 ` [syzbot ci] " syzbot ci
2025-09-30 11:17 ` Lance Yang
2025-09-30 7:10 [PATCH v4 1/1] " Lance Yang
2025-09-30 11:16 ` [syzbot ci] " syzbot ci
2025-09-30 11:20 ` Lance Yang
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