[PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Barry Song]

From: Barry Song <v-songbaohua@oppo.com>

userfaultfd_move() checks whether the PTE entry is present or a
swap entry.

- If the PTE entry is present, move_present_pte() handles folio
  migration by setting:

  src_folio->index = linear_page_index(dst_vma, dst_addr);

- If the PTE entry is a swap entry, move_swap_pte() simply copies
  the PTE to the new dst_addr.

This approach is incorrect because, even if the PTE is a swap entry,
it can still reference a folio that remains in the swap cache.

This creates a race window between steps 2 and 4.
 1. add_to_swap: The folio is added to the swapcache.
 2. try_to_unmap: PTEs are converted to swap entries.
 3. pageout: The folio is written back.
 4. Swapcache is cleared.
If userfaultfd_move() occurs in the window between steps 2 and 4,
after the swap PTE has been moved to the destination, accessing the
destination triggers do_swap_page(), which may locate the folio in
the swapcache. However, since the folio's index has not been updated
to match the destination VMA, do_swap_page() will detect a mismatch.

This can result in two critical issues depending on the system
configuration.

If KSM is disabled, both small and large folios can trigger a BUG
during the add_rmap operation due to:

 page_pgoff(folio, page) != linear_page_index(vma, address)

[   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
[   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
[   13.337716] memcg:ffff00000405f000
[   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
[   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
[   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
[   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
[   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
[   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
[   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
[   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
[   13.340190] ------------[ cut here ]------------
[   13.340316] kernel BUG at mm/rmap.c:1380!
[   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
[   13.340969] Modules linked in:
[   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
[   13.341470] Hardware name: linux,dummy-virt (DT)
[   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
[   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
[   13.342018] sp : ffff80008752bb20
[   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
[   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
[   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
[   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
[   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
[   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
[   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
[   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
[   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
[   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
[   13.343876] Call trace:
[   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P)
[   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320
[   13.344333]  do_swap_page+0x1060/0x1400
[   13.344417]  __handle_mm_fault+0x61c/0xbc8
[   13.344504]  handle_mm_fault+0xd8/0x2e8
[   13.344586]  do_page_fault+0x20c/0x770
[   13.344673]  do_translation_fault+0xb4/0xf0
[   13.344759]  do_mem_abort+0x48/0xa0
[   13.344842]  el0_da+0x58/0x130
[   13.344914]  el0t_64_sync_handler+0xc4/0x138
[   13.345002]  el0t_64_sync+0x1ac/0x1b0
[   13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
[   13.345504] ---[ end trace 0000000000000000 ]---
[   13.345715] note: a.out[107] exited with irqs disabled
[   13.345954] note: a.out[107] exited with preempt_count 2

If KSM is enabled, Peter Xu also discovered that do_swap_page() may
trigger an unexpected CoW operation for small folios because
ksm_might_need_to_copy() allocates a new folio when the folio index
does not match linear_page_index(vma, addr).

This patch also checks the swapcache when handling swap entries. If a
match is found in the swapcache, it processes it similarly to a present
PTE.
However, there are some differences. For example, the folio is no longer
exclusive because folio_try_share_anon_rmap_pte() is performed during
unmapping.
Furthermore, in the case of swapcache, the folio has already been
unmapped, eliminating the risk of concurrent rmap walks and removing the
need to acquire src_folio's anon_vma or lock.

Note that for large folios, in the swapcache handling path, we directly
return -EBUSY since split_folio() will return -EBUSY regardless if
the folio is under writeback or unmapped. This is not an urgent issue,
so a follow-up patch may address it separately.

Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI")
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Cc: Tangquan Zheng <zhengtangquan@oppo.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/userfaultfd.c | 76 ++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 67 insertions(+), 9 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 867898c4e30b..2df5d100e76d 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -18,6 +18,7 @@
 #include <asm/tlbflush.h>
 #include <asm/tlb.h>
 #include "internal.h"
+#include "swap.h"
 
 static __always_inline
 bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
@@ -1072,16 +1073,14 @@ static int move_present_pte(struct mm_struct *mm,
 	return err;
 }
 
-static int move_swap_pte(struct mm_struct *mm,
+static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
 			 unsigned long dst_addr, unsigned long src_addr,
 			 pte_t *dst_pte, pte_t *src_pte,
 			 pte_t orig_dst_pte, pte_t orig_src_pte,
 			 pmd_t *dst_pmd, pmd_t dst_pmdval,
-			 spinlock_t *dst_ptl, spinlock_t *src_ptl)
+			 spinlock_t *dst_ptl, spinlock_t *src_ptl,
+			 struct folio *src_folio)
 {
-	if (!pte_swp_exclusive(orig_src_pte))
-		return -EBUSY;
-
 	double_pt_lock(dst_ptl, src_ptl);
 
 	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
@@ -1090,10 +1089,20 @@ static int move_swap_pte(struct mm_struct *mm,
 		return -EAGAIN;
 	}
 
+	/*
+	 * The src_folio resides in the swapcache, requiring an update to its
+	 * index and mapping to align with the dst_vma, where a swap-in may
+	 * occur and hit the swapcache after moving the PTE.
+	 */
+	if (src_folio) {
+		folio_move_anon_rmap(src_folio, dst_vma);
+		src_folio->index = linear_page_index(dst_vma, dst_addr);
+	}
+
 	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
 	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
-	double_pt_unlock(dst_ptl, src_ptl);
 
+	double_pt_unlock(dst_ptl, src_ptl);
 	return 0;
 }
 
@@ -1137,6 +1146,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 			  __u64 mode)
 {
 	swp_entry_t entry;
+	struct swap_info_struct *si = NULL;
 	pte_t orig_src_pte, orig_dst_pte;
 	pte_t src_folio_pte;
 	spinlock_t *src_ptl, *dst_ptl;
@@ -1318,6 +1328,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 				       orig_dst_pte, orig_src_pte, dst_pmd,
 				       dst_pmdval, dst_ptl, src_ptl, src_folio);
 	} else {
+		struct folio *folio = NULL;
+
 		entry = pte_to_swp_entry(orig_src_pte);
 		if (non_swap_entry(entry)) {
 			if (is_migration_entry(entry)) {
@@ -1331,9 +1343,53 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 			goto out;
 		}
 
-		err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte,
-				    orig_dst_pte, orig_src_pte, dst_pmd,
-				    dst_pmdval, dst_ptl, src_ptl);
+		if (!pte_swp_exclusive(orig_src_pte)) {
+			err = -EBUSY;
+			goto out;
+		}
+
+		si = get_swap_device(entry);
+		if (unlikely(!si)) {
+			err = -EAGAIN;
+			goto out;
+		}
+		/*
+		 * Verify the existence of the swapcache. If present, the folio's
+		 * index and mapping must be updated even when the PTE is a swap
+		 * entry. The anon_vma lock is not taken during this process since
+		 * the folio has already been unmapped, and the swap entry is
+		 * exclusive, preventing rmap walks.
+		 *
+		 * For large folios, return -EBUSY immediately, as split_folio()
+		 * also returns -EBUSY when attempting to split unmapped large
+		 * folios in the swapcache. This issue needs to be resolved
+		 * separately to allow proper handling.
+		 */
+		if (!src_folio)
+			folio = filemap_get_folio(swap_address_space(entry),
+					swap_cache_index(entry));
+		if (!IS_ERR_OR_NULL(folio)) {
+			if (folio && folio_test_large(folio)) {
+				err = -EBUSY;
+				folio_put(folio);
+				goto out;
+			}
+			src_folio = folio;
+			src_folio_pte = orig_src_pte;
+			if (!folio_trylock(src_folio)) {
+				pte_unmap(&orig_src_pte);
+				pte_unmap(&orig_dst_pte);
+				src_pte = dst_pte = NULL;
+				/* now we can block and wait */
+				folio_lock(src_folio);
+				put_swap_device(si);
+				si = NULL;
+				goto retry;
+			}
+		}
+		err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
+				orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
+				dst_ptl, src_ptl, src_folio);
 	}
 
 out:
@@ -1350,6 +1406,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 	if (src_pte)
 		pte_unmap(src_pte);
 	mmu_notifier_invalidate_range_end(&range);
+	if (si)
+		put_swap_device(si);
 
 	return err;
 }
-- 
2.39.3 (Apple Git-146)

Re: [PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Peter Xu]

On Wed, Feb 26, 2025 at 01:14:00PM +1300, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> userfaultfd_move() checks whether the PTE entry is present or a
> swap entry.
> 
> - If the PTE entry is present, move_present_pte() handles folio
>   migration by setting:
> 
>   src_folio->index = linear_page_index(dst_vma, dst_addr);
> 
> - If the PTE entry is a swap entry, move_swap_pte() simply copies
>   the PTE to the new dst_addr.
> 
> This approach is incorrect because, even if the PTE is a swap entry,
> it can still reference a folio that remains in the swap cache.
> 
> This creates a race window between steps 2 and 4.
>  1. add_to_swap: The folio is added to the swapcache.
>  2. try_to_unmap: PTEs are converted to swap entries.
>  3. pageout: The folio is written back.
>  4. Swapcache is cleared.
> If userfaultfd_move() occurs in the window between steps 2 and 4,
> after the swap PTE has been moved to the destination, accessing the
> destination triggers do_swap_page(), which may locate the folio in
> the swapcache. However, since the folio's index has not been updated
> to match the destination VMA, do_swap_page() will detect a mismatch.
> 
> This can result in two critical issues depending on the system
> configuration.
> 
> If KSM is disabled, both small and large folios can trigger a BUG
> during the add_rmap operation due to:
> 
>  page_pgoff(folio, page) != linear_page_index(vma, address)
> 
> [   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
> [   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
> [   13.337716] memcg:ffff00000405f000
> [   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
> [   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
> [   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
> [   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
> [   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
> [   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
> [   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
> [   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
> [   13.340190] ------------[ cut here ]------------
> [   13.340316] kernel BUG at mm/rmap.c:1380!
> [   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
> [   13.340969] Modules linked in:
> [   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
> [   13.341470] Hardware name: linux,dummy-virt (DT)
> [   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
> [   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
> [   13.342018] sp : ffff80008752bb20
> [   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
> [   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
> [   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
> [   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
> [   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
> [   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
> [   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
> [   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
> [   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
> [   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
> [   13.343876] Call trace:
> [   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P)
> [   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320
> [   13.344333]  do_swap_page+0x1060/0x1400
> [   13.344417]  __handle_mm_fault+0x61c/0xbc8
> [   13.344504]  handle_mm_fault+0xd8/0x2e8
> [   13.344586]  do_page_fault+0x20c/0x770
> [   13.344673]  do_translation_fault+0xb4/0xf0
> [   13.344759]  do_mem_abort+0x48/0xa0
> [   13.344842]  el0_da+0x58/0x130
> [   13.344914]  el0t_64_sync_handler+0xc4/0x138
> [   13.345002]  el0t_64_sync+0x1ac/0x1b0
> [   13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
> [   13.345504] ---[ end trace 0000000000000000 ]---
> [   13.345715] note: a.out[107] exited with irqs disabled
> [   13.345954] note: a.out[107] exited with preempt_count 2
> 
> If KSM is enabled, Peter Xu also discovered that do_swap_page() may
> trigger an unexpected CoW operation for small folios because
> ksm_might_need_to_copy() allocates a new folio when the folio index
> does not match linear_page_index(vma, addr).
> 
> This patch also checks the swapcache when handling swap entries. If a
> match is found in the swapcache, it processes it similarly to a present
> PTE.
> However, there are some differences. For example, the folio is no longer
> exclusive because folio_try_share_anon_rmap_pte() is performed during
> unmapping.
> Furthermore, in the case of swapcache, the folio has already been
> unmapped, eliminating the risk of concurrent rmap walks and removing the
> need to acquire src_folio's anon_vma or lock.
> 
> Note that for large folios, in the swapcache handling path, we directly
> return -EBUSY since split_folio() will return -EBUSY regardless if
> the folio is under writeback or unmapped. This is not an urgent issue,
> so a follow-up patch may address it separately.
> 
> Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI")
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Brian Geffon <bgeffon@google.com>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: Kalesh Singh <kaleshsingh@google.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Cc: Lokesh Gidra <lokeshgidra@google.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> Cc: Nicolas Geoffray <ngeoffray@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: ZhangPeng <zhangpeng362@huawei.com>
> Cc: Tangquan Zheng <zhengtangquan@oppo.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>

Acked-by: Peter Xu <peterx@redhat.com>

Some nitpicks below, maybe no worth for a repost..

> ---
>  mm/userfaultfd.c | 76 ++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 67 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 867898c4e30b..2df5d100e76d 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -18,6 +18,7 @@
>  #include <asm/tlbflush.h>
>  #include <asm/tlb.h>
>  #include "internal.h"
> +#include "swap.h"
>  
>  static __always_inline
>  bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
> @@ -1072,16 +1073,14 @@ static int move_present_pte(struct mm_struct *mm,
>  	return err;
>  }
>  
> -static int move_swap_pte(struct mm_struct *mm,
> +static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
>  			 unsigned long dst_addr, unsigned long src_addr,
>  			 pte_t *dst_pte, pte_t *src_pte,
>  			 pte_t orig_dst_pte, pte_t orig_src_pte,
>  			 pmd_t *dst_pmd, pmd_t dst_pmdval,
> -			 spinlock_t *dst_ptl, spinlock_t *src_ptl)
> +			 spinlock_t *dst_ptl, spinlock_t *src_ptl,
> +			 struct folio *src_folio)
>  {
> -	if (!pte_swp_exclusive(orig_src_pte))
> -		return -EBUSY;
> -
>  	double_pt_lock(dst_ptl, src_ptl);
>  
>  	if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> @@ -1090,10 +1089,20 @@ static int move_swap_pte(struct mm_struct *mm,
>  		return -EAGAIN;
>  	}
>  
> +	/*
> +	 * The src_folio resides in the swapcache, requiring an update to its
> +	 * index and mapping to align with the dst_vma, where a swap-in may
> +	 * occur and hit the swapcache after moving the PTE.
> +	 */
> +	if (src_folio) {
> +		folio_move_anon_rmap(src_folio, dst_vma);
> +		src_folio->index = linear_page_index(dst_vma, dst_addr);
> +	}
> +
>  	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
>  	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
> -	double_pt_unlock(dst_ptl, src_ptl);
>  
> +	double_pt_unlock(dst_ptl, src_ptl);

Unnecessary line move.

>  	return 0;
>  }
>  
> @@ -1137,6 +1146,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>  			  __u64 mode)
>  {
>  	swp_entry_t entry;
> +	struct swap_info_struct *si = NULL;
>  	pte_t orig_src_pte, orig_dst_pte;
>  	pte_t src_folio_pte;
>  	spinlock_t *src_ptl, *dst_ptl;
> @@ -1318,6 +1328,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>  				       orig_dst_pte, orig_src_pte, dst_pmd,
>  				       dst_pmdval, dst_ptl, src_ptl, src_folio);
>  	} else {
> +		struct folio *folio = NULL;
> +
>  		entry = pte_to_swp_entry(orig_src_pte);
>  		if (non_swap_entry(entry)) {
>  			if (is_migration_entry(entry)) {
> @@ -1331,9 +1343,53 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>  			goto out;
>  		}
>  
> -		err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte,
> -				    orig_dst_pte, orig_src_pte, dst_pmd,
> -				    dst_pmdval, dst_ptl, src_ptl);
> +		if (!pte_swp_exclusive(orig_src_pte)) {
> +			err = -EBUSY;
> +			goto out;
> +		}
> +
> +		si = get_swap_device(entry);
> +		if (unlikely(!si)) {
> +			err = -EAGAIN;
> +			goto out;
> +		}
> +		/*
> +		 * Verify the existence of the swapcache. If present, the folio's
> +		 * index and mapping must be updated even when the PTE is a swap
> +		 * entry. The anon_vma lock is not taken during this process since
> +		 * the folio has already been unmapped, and the swap entry is
> +		 * exclusive, preventing rmap walks.
> +		 *
> +		 * For large folios, return -EBUSY immediately, as split_folio()
> +		 * also returns -EBUSY when attempting to split unmapped large
> +		 * folios in the swapcache. This issue needs to be resolved
> +		 * separately to allow proper handling.
> +		 */
> +		if (!src_folio)
> +			folio = filemap_get_folio(swap_address_space(entry),
> +					swap_cache_index(entry));
> +		if (!IS_ERR_OR_NULL(folio)) {
> +			if (folio && folio_test_large(folio)) {

Can drop this folio check as it just did check "!IS_ERR_OR_NULL(folio)"..

> +				err = -EBUSY;
> +				folio_put(folio);
> +				goto out;
> +			}
> +			src_folio = folio;
> +			src_folio_pte = orig_src_pte;
> +			if (!folio_trylock(src_folio)) {
> +				pte_unmap(&orig_src_pte);
> +				pte_unmap(&orig_dst_pte);
> +				src_pte = dst_pte = NULL;
> +				/* now we can block and wait */
> +				folio_lock(src_folio);
> +				put_swap_device(si);
> +				si = NULL;

Not sure if it can do any harm, but maybe still nicer to put swap before
locking folio.

Thanks,

> +				goto retry;
> +			}
> +		}
> +		err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
> +				orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
> +				dst_ptl, src_ptl, src_folio);
>  	}
>  
>  out:
> @@ -1350,6 +1406,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>  	if (src_pte)
>  		pte_unmap(src_pte);
>  	mmu_notifier_invalidate_range_end(&range);
> +	if (si)
> +		put_swap_device(si);
>  
>  	return err;
>  }
> -- 
> 2.39.3 (Apple Git-146)
> 

-- 
Peter Xu

Re: [PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Barry Song]

On Wed, Feb 26, 2025 at 2:00 PM Peter Xu <peterx@redhat.com> wrote:
>
> Acked-by: Peter Xu <peterx@redhat.com>
>

Thanks!

> Some nitpicks below, maybe no worth for a repost..

Hi Andrew,

Could you please help squash the below change?

From 42273506ba00723151e3a08b4ffd3f2c303e7ccc Mon Sep 17 00:00:00 2001
From: Barry Song <v-songbaohua@oppo.com>
Date: Wed, 26 Feb 2025 15:22:17 +1300
Subject: [PATCH] minor cleanup according to Peter Xu

According to Peter Xu:
1. Unnecessary line move.
2. Can drop this folio check as it just did check
   "!IS_ERR_OR_NULL(folio)"
3. Not sure if it can do any harm, but maybe still nicer
   to put swap before locking folio.

Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/userfaultfd.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 2df5d100e76d..2955e20f86bf 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1101,8 +1101,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
 
 	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
 	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
-
 	double_pt_unlock(dst_ptl, src_ptl);
+
 	return 0;
 }
 
@@ -1369,7 +1369,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 			folio = filemap_get_folio(swap_address_space(entry),
 					swap_cache_index(entry));
 		if (!IS_ERR_OR_NULL(folio)) {
-			if (folio && folio_test_large(folio)) {
+			if (folio_test_large(folio)) {
 				err = -EBUSY;
 				folio_put(folio);
 				goto out;
@@ -1380,10 +1380,10 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 				pte_unmap(&orig_src_pte);
 				pte_unmap(&orig_dst_pte);
 				src_pte = dst_pte = NULL;
-				/* now we can block and wait */
-				folio_lock(src_folio);
 				put_swap_device(si);
 				si = NULL;
+				/* now we can block and wait */
+				folio_lock(src_folio);
 				goto retry;
 			}
 		}
-- 
2.39.3 (Apple Git-146)

> Peter Xu
>

Thanks
Barry

Re: [PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Suren Baghdasaryan]

On Tue, Feb 25, 2025 at 5:00 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Feb 26, 2025 at 01:14:00PM +1300, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > userfaultfd_move() checks whether the PTE entry is present or a
> > swap entry.
> >
> > - If the PTE entry is present, move_present_pte() handles folio
> >   migration by setting:
> >
> >   src_folio->index = linear_page_index(dst_vma, dst_addr);
> >
> > - If the PTE entry is a swap entry, move_swap_pte() simply copies
> >   the PTE to the new dst_addr.
> >
> > This approach is incorrect because, even if the PTE is a swap entry,
> > it can still reference a folio that remains in the swap cache.
> >
> > This creates a race window between steps 2 and 4.
> >  1. add_to_swap: The folio is added to the swapcache.
> >  2. try_to_unmap: PTEs are converted to swap entries.
> >  3. pageout: The folio is written back.
> >  4. Swapcache is cleared.
> > If userfaultfd_move() occurs in the window between steps 2 and 4,
> > after the swap PTE has been moved to the destination, accessing the
> > destination triggers do_swap_page(), which may locate the folio in
> > the swapcache. However, since the folio's index has not been updated
> > to match the destination VMA, do_swap_page() will detect a mismatch.
> >
> > This can result in two critical issues depending on the system
> > configuration.
> >
> > If KSM is disabled, both small and large folios can trigger a BUG
> > during the add_rmap operation due to:
> >
> >  page_pgoff(folio, page) != linear_page_index(vma, address)
> >
> > [   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
> > [   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
> > [   13.337716] memcg:ffff00000405f000
> > [   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
> > [   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
> > [   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
> > [   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
> > [   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
> > [   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
> > [   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
> > [   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
> > [   13.340190] ------------[ cut here ]------------
> > [   13.340316] kernel BUG at mm/rmap.c:1380!
> > [   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
> > [   13.340969] Modules linked in:
> > [   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
> > [   13.341470] Hardware name: linux,dummy-virt (DT)
> > [   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
> > [   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
> > [   13.342018] sp : ffff80008752bb20
> > [   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
> > [   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
> > [   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
> > [   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
> > [   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
> > [   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
> > [   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
> > [   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
> > [   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
> > [   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
> > [   13.343876] Call trace:
> > [   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P)
> > [   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320
> > [   13.344333]  do_swap_page+0x1060/0x1400
> > [   13.344417]  __handle_mm_fault+0x61c/0xbc8
> > [   13.344504]  handle_mm_fault+0xd8/0x2e8
> > [   13.344586]  do_page_fault+0x20c/0x770
> > [   13.344673]  do_translation_fault+0xb4/0xf0
> > [   13.344759]  do_mem_abort+0x48/0xa0
> > [   13.344842]  el0_da+0x58/0x130
> > [   13.344914]  el0t_64_sync_handler+0xc4/0x138
> > [   13.345002]  el0t_64_sync+0x1ac/0x1b0
> > [   13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
> > [   13.345504] ---[ end trace 0000000000000000 ]---
> > [   13.345715] note: a.out[107] exited with irqs disabled
> > [   13.345954] note: a.out[107] exited with preempt_count 2
> >
> > If KSM is enabled, Peter Xu also discovered that do_swap_page() may
> > trigger an unexpected CoW operation for small folios because
> > ksm_might_need_to_copy() allocates a new folio when the folio index
> > does not match linear_page_index(vma, addr).
> >
> > This patch also checks the swapcache when handling swap entries. If a
> > match is found in the swapcache, it processes it similarly to a present
> > PTE.
> > However, there are some differences. For example, the folio is no longer
> > exclusive because folio_try_share_anon_rmap_pte() is performed during
> > unmapping.
> > Furthermore, in the case of swapcache, the folio has already been
> > unmapped, eliminating the risk of concurrent rmap walks and removing the
> > need to acquire src_folio's anon_vma or lock.
> >
> > Note that for large folios, in the swapcache handling path, we directly
> > return -EBUSY since split_folio() will return -EBUSY regardless if
> > the folio is under writeback or unmapped. This is not an urgent issue,
> > so a follow-up patch may address it separately.
> >
> > Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI")
> > Cc: Andrea Arcangeli <aarcange@redhat.com>
> > Cc: Suren Baghdasaryan <surenb@google.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Axel Rasmussen <axelrasmussen@google.com>
> > Cc: Brian Geffon <bgeffon@google.com>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Kalesh Singh <kaleshsingh@google.com>
> > Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> > Cc: Lokesh Gidra <lokeshgidra@google.com>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> > Cc: Nicolas Geoffray <ngeoffray@google.com>
> > Cc: Peter Xu <peterx@redhat.com>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: Shuah Khan <shuah@kernel.org>
> > Cc: ZhangPeng <zhangpeng362@huawei.com>
> > Cc: Tangquan Zheng <zhengtangquan@oppo.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>
> Acked-by: Peter Xu <peterx@redhat.com>
>
> Some nitpicks below, maybe no worth for a repost..

With Peter's nits addressed,

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks!

>
> > ---
> >  mm/userfaultfd.c | 76 ++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 67 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 867898c4e30b..2df5d100e76d 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -18,6 +18,7 @@
> >  #include <asm/tlbflush.h>
> >  #include <asm/tlb.h>
> >  #include "internal.h"
> > +#include "swap.h"
> >
> >  static __always_inline
> >  bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
> > @@ -1072,16 +1073,14 @@ static int move_present_pte(struct mm_struct *mm,
> >       return err;
> >  }
> >
> > -static int move_swap_pte(struct mm_struct *mm,
> > +static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
> >                        unsigned long dst_addr, unsigned long src_addr,
> >                        pte_t *dst_pte, pte_t *src_pte,
> >                        pte_t orig_dst_pte, pte_t orig_src_pte,
> >                        pmd_t *dst_pmd, pmd_t dst_pmdval,
> > -                      spinlock_t *dst_ptl, spinlock_t *src_ptl)
> > +                      spinlock_t *dst_ptl, spinlock_t *src_ptl,
> > +                      struct folio *src_folio)
> >  {
> > -     if (!pte_swp_exclusive(orig_src_pte))
> > -             return -EBUSY;
> > -
> >       double_pt_lock(dst_ptl, src_ptl);
> >
> >       if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > @@ -1090,10 +1089,20 @@ static int move_swap_pte(struct mm_struct *mm,
> >               return -EAGAIN;
> >       }
> >
> > +     /*
> > +      * The src_folio resides in the swapcache, requiring an update to its
> > +      * index and mapping to align with the dst_vma, where a swap-in may
> > +      * occur and hit the swapcache after moving the PTE.
> > +      */
> > +     if (src_folio) {
> > +             folio_move_anon_rmap(src_folio, dst_vma);
> > +             src_folio->index = linear_page_index(dst_vma, dst_addr);
> > +     }
> > +
> >       orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
> >       set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
> > -     double_pt_unlock(dst_ptl, src_ptl);
> >
> > +     double_pt_unlock(dst_ptl, src_ptl);
>
> Unnecessary line move.
>
> >       return 0;
> >  }
> >
> > @@ -1137,6 +1146,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >                         __u64 mode)
> >  {
> >       swp_entry_t entry;
> > +     struct swap_info_struct *si = NULL;
> >       pte_t orig_src_pte, orig_dst_pte;
> >       pte_t src_folio_pte;
> >       spinlock_t *src_ptl, *dst_ptl;
> > @@ -1318,6 +1328,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >                                      orig_dst_pte, orig_src_pte, dst_pmd,
> >                                      dst_pmdval, dst_ptl, src_ptl, src_folio);
> >       } else {
> > +             struct folio *folio = NULL;
> > +
> >               entry = pte_to_swp_entry(orig_src_pte);
> >               if (non_swap_entry(entry)) {
> >                       if (is_migration_entry(entry)) {
> > @@ -1331,9 +1343,53 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >                       goto out;
> >               }
> >
> > -             err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte,
> > -                                 orig_dst_pte, orig_src_pte, dst_pmd,
> > -                                 dst_pmdval, dst_ptl, src_ptl);
> > +             if (!pte_swp_exclusive(orig_src_pte)) {
> > +                     err = -EBUSY;
> > +                     goto out;
> > +             }
> > +
> > +             si = get_swap_device(entry);
> > +             if (unlikely(!si)) {
> > +                     err = -EAGAIN;
> > +                     goto out;
> > +             }
> > +             /*
> > +              * Verify the existence of the swapcache. If present, the folio's
> > +              * index and mapping must be updated even when the PTE is a swap
> > +              * entry. The anon_vma lock is not taken during this process since
> > +              * the folio has already been unmapped, and the swap entry is
> > +              * exclusive, preventing rmap walks.
> > +              *
> > +              * For large folios, return -EBUSY immediately, as split_folio()
> > +              * also returns -EBUSY when attempting to split unmapped large
> > +              * folios in the swapcache. This issue needs to be resolved
> > +              * separately to allow proper handling.
> > +              */
> > +             if (!src_folio)
> > +                     folio = filemap_get_folio(swap_address_space(entry),
> > +                                     swap_cache_index(entry));
> > +             if (!IS_ERR_OR_NULL(folio)) {
> > +                     if (folio && folio_test_large(folio)) {
>
> Can drop this folio check as it just did check "!IS_ERR_OR_NULL(folio)"..
>
> > +                             err = -EBUSY;
> > +                             folio_put(folio);
> > +                             goto out;
> > +                     }
> > +                     src_folio = folio;
> > +                     src_folio_pte = orig_src_pte;
> > +                     if (!folio_trylock(src_folio)) {
> > +                             pte_unmap(&orig_src_pte);
> > +                             pte_unmap(&orig_dst_pte);
> > +                             src_pte = dst_pte = NULL;
> > +                             /* now we can block and wait */
> > +                             folio_lock(src_folio);
> > +                             put_swap_device(si);
> > +                             si = NULL;
>
> Not sure if it can do any harm, but maybe still nicer to put swap before
> locking folio.
>
> Thanks,
>
> > +                             goto retry;
> > +                     }
> > +             }
> > +             err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
> > +                             orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
> > +                             dst_ptl, src_ptl, src_folio);
> >       }
> >
> >  out:
> > @@ -1350,6 +1406,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >       if (src_pte)
> >               pte_unmap(src_pte);
> >       mmu_notifier_invalidate_range_end(&range);
> > +     if (si)
> > +             put_swap_device(si);
> >
> >       return err;
> >  }
> > --
> > 2.39.3 (Apple Git-146)
> >
>
> --
> Peter Xu
>