[PATCH v2] mm/memfd: fix information leak in hugetlb folios

[PATCH v2] mm/memfd: fix information leak in hugetlb folios

[Deepanshu Kartikey]

When allocating hugetlb folios for memfd, three initialization steps
are missing:

1. Folios are not zeroed, leading to kernel memory disclosure to userspace
2. Folios are not marked uptodate before adding to page cache
3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()

The memfd allocation path bypasses the normal page fault handler
(hugetlb_no_page) which would handle all of these initialization steps.
This is problematic especially for udmabuf use cases where folios are
pinned and directly accessed by userspace via DMA.

Fix by matching the initialization pattern used in hugetlb_no_page():
- Zero the folio using folio_zero_user() which is optimized for huge pages
- Mark it uptodate with folio_mark_uptodate()
- Take hugetlb_fault_mutex before adding to page cache to prevent races

The folio_zero_user() change also fixes a potential security issue where
uninitialized kernel memory could be disclosed to userspace through
read() or mmap() operations on the memfd.

Reported-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@gmail.com/ [v1]
Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b
Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Cc: stable@vger.kernel.org
Suggested-by: Oscar Salvador <osalvador@suse.de>
Suggested-by: David Hildenbrand <david@redhat.com>
Tested-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---

v1 -> v2:
- Use folio_zero_user() instead of folio_zero_range() (optimized for huge pages)
- Add folio_mark_uptodate() before adding to page cache
- Add hugetlb_fault_mutex locking around hugetlb_add_to_page_cache()
- Add Fixes: tag and Cc: stable for backporting
- Add Suggested-by: tags for Oscar and David
---
 mm/memfd.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mm/memfd.c b/mm/memfd.c
index 1d109c1acf21..d32eef58d154 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -96,9 +96,36 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 						    NULL,
 						    gfp_mask);
 		if (folio) {
+			u32 hash;
+
+			/*
+			 * Zero the folio to prevent information leaks to userspace.
+			 * Use folio_zero_user() which is optimized for huge/gigantic
+			 * pages. Pass 0 as addr_hint since this is not a faulting path
+			 *  and we don't have a user virtual address yet.
+			 */
+			folio_zero_user(folio, 0);
+
+			/*
+			 * Mark the folio uptodate before adding to page cache,
+			 * as required by filemap.c and other hugetlb paths.
+			 */
+			__folio_mark_uptodate(folio);
+
+			/*
+			 * Serialize hugepage allocation and instantiation to prevent
+			 * races with concurrent allocations, as required by all other
+			 * callers of hugetlb_add_to_page_cache().
+			 */
+			hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
+			mutex_lock(&hugetlb_fault_mutex_table[hash]);
+
 			err = hugetlb_add_to_page_cache(folio,
 							memfd->f_mapping,
 							idx);
+
+			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+
 			if (err) {
 				folio_put(folio);
 				goto err_unresv;
-- 
2.43.0

Re: [PATCH v2] mm/memfd: fix information leak in hugetlb folios

[David Hildenbrand (Red Hat)]

On 12.11.25 15:50, Deepanshu Kartikey wrote:
> When allocating hugetlb folios for memfd, three initialization steps
> are missing:
> 
> 1. Folios are not zeroed, leading to kernel memory disclosure to userspace
> 2. Folios are not marked uptodate before adding to page cache
> 3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()
> 
> The memfd allocation path bypasses the normal page fault handler
> (hugetlb_no_page) which would handle all of these initialization steps.
> This is problematic especially for udmabuf use cases where folios are
> pinned and directly accessed by userspace via DMA.
> 
> Fix by matching the initialization pattern used in hugetlb_no_page():
> - Zero the folio using folio_zero_user() which is optimized for huge pages
> - Mark it uptodate with folio_mark_uptodate()
> - Take hugetlb_fault_mutex before adding to page cache to prevent races
> 
> The folio_zero_user() change also fixes a potential security issue where
> uninitialized kernel memory could be disclosed to userspace through
> read() or mmap() operations on the memfd.
> 
> Reported-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@gmail.com/ [v1]
> Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
> Cc: stable@vger.kernel.org
> Suggested-by: Oscar Salvador <osalvador@suse.de>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Tested-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
> 
> v1 -> v2:
> - Use folio_zero_user() instead of folio_zero_range() (optimized for huge pages)
> - Add folio_mark_uptodate() before adding to page cache
> - Add hugetlb_fault_mutex locking around hugetlb_add_to_page_cache()
> - Add Fixes: tag and Cc: stable for backporting
> - Add Suggested-by: tags for Oscar and David
> ---
>   mm/memfd.c | 27 +++++++++++++++++++++++++++
>   1 file changed, 27 insertions(+)
> 
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 1d109c1acf21..d32eef58d154 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -96,9 +96,36 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
>   						    NULL,
>   						    gfp_mask);
>   		if (folio) {
> +			u32 hash;
> +
> +			/*
> +			 * Zero the folio to prevent information leaks to userspace.
> +			 * Use folio_zero_user() which is optimized for huge/gigantic
> +			 * pages. Pass 0 as addr_hint since this is not a faulting path
> +			 *  and we don't have a user virtual address yet.
> +			 */
> +			folio_zero_user(folio, 0);

Staring at hugetlbfs_fallocate(), we see, to pass the offset within the 
file.

I think it shouldn't make a difference here (I don't see how the offset 
in the file would be better than 0: it's in both cases not the user 
address).

> +
> +			/*
> +			 * Mark the folio uptodate before adding to page cache,
> +			 * as required by filemap.c and other hugetlb paths.
> +			 */
> +			__folio_mark_uptodate(folio);

Personally, I'd drop this comment as it is really just doing what we do 
everywhere else :)

Hoping we can factor that out into hugetlb code properly.

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David

Re: [PATCH v2] mm/memfd: fix information leak in hugetlb folios

[Oscar Salvador]

On Wed, Nov 12, 2025 at 08:20:34PM +0530, Deepanshu Kartikey wrote:
> When allocating hugetlb folios for memfd, three initialization steps
> are missing:
> 
> 1. Folios are not zeroed, leading to kernel memory disclosure to userspace
> 2. Folios are not marked uptodate before adding to page cache
> 3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()
> 
> The memfd allocation path bypasses the normal page fault handler
> (hugetlb_no_page) which would handle all of these initialization steps.
> This is problematic especially for udmabuf use cases where folios are
> pinned and directly accessed by userspace via DMA.
> 
> Fix by matching the initialization pattern used in hugetlb_no_page():
> - Zero the folio using folio_zero_user() which is optimized for huge pages
> - Mark it uptodate with folio_mark_uptodate()
> - Take hugetlb_fault_mutex before adding to page cache to prevent races
> 
> The folio_zero_user() change also fixes a potential security issue where
> uninitialized kernel memory could be disclosed to userspace through
> read() or mmap() operations on the memfd.
> 
> Reported-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@gmail.com/ [v1]
> Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
> Cc: stable@vger.kernel.org
> Suggested-by: Oscar Salvador <osalvador@suse.de>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Tested-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

As David mentioned, we can drop the comment wrt. __folio_mark_uptodate.
As for the addr_hint in folio_zero_user, I do not think it makes a
difference in here.
AFAIK, it serves the purpose that subpages belong to the addr_hint will
be zeroed the latest to keep them in cache, but here it does not really
apply, so '0' should just work?

Acked-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs

Re: [PATCH v2] mm/memfd: fix information leak in hugetlb folios

[Hugh Dickins]

On Wed, 12 Nov 2025, Deepanshu Kartikey wrote:

> When allocating hugetlb folios for memfd, three initialization steps
> are missing:
> 
> 1. Folios are not zeroed, leading to kernel memory disclosure to userspace
> 2. Folios are not marked uptodate before adding to page cache
> 3. hugetlb_fault_mutex is not taken before hugetlb_add_to_page_cache()
> 
> The memfd allocation path bypasses the normal page fault handler
> (hugetlb_no_page) which would handle all of these initialization steps.
> This is problematic especially for udmabuf use cases where folios are
> pinned and directly accessed by userspace via DMA.
> 
> Fix by matching the initialization pattern used in hugetlb_no_page():
> - Zero the folio using folio_zero_user() which is optimized for huge pages
> - Mark it uptodate with folio_mark_uptodate()
> - Take hugetlb_fault_mutex before adding to page cache to prevent races
> 
> The folio_zero_user() change also fixes a potential security issue where
> uninitialized kernel memory could be disclosed to userspace through
> read() or mmap() operations on the memfd.
> 
> Reported-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/all/20251112031631.2315651-1-kartikey406@gmail.com/ [v1]
> Closes: https://syzkaller.appspot.com/bug?extid=f64019ba229e3a5c411b
> Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
> Cc: stable@vger.kernel.org
> Suggested-by: Oscar Salvador <osalvador@suse.de>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Tested-by: syzbot+f64019ba229e3a5c411b@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

Acked-by: Hugh Dickins <hughd@google.com>

Sorry if you were all waiting on a Ack from me.  We're agreed that
the comment above __folio_mark_uptodate() could be deleted, and that
it would be much better if this code can be moved to a shared home
in hugetlb later on; but for now it's more urgent to get this patch
into hotfixes and on to Linus - please Andrew.

Thanks!
Hugh

> ---
> 
> v1 -> v2:
> - Use folio_zero_user() instead of folio_zero_range() (optimized for huge pages)
> - Add folio_mark_uptodate() before adding to page cache
> - Add hugetlb_fault_mutex locking around hugetlb_add_to_page_cache()
> - Add Fixes: tag and Cc: stable for backporting
> - Add Suggested-by: tags for Oscar and David
> ---
>  mm/memfd.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 1d109c1acf21..d32eef58d154 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -96,9 +96,36 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
>  						    NULL,
>  						    gfp_mask);
>  		if (folio) {
> +			u32 hash;
> +
> +			/*
> +			 * Zero the folio to prevent information leaks to userspace.
> +			 * Use folio_zero_user() which is optimized for huge/gigantic
> +			 * pages. Pass 0 as addr_hint since this is not a faulting path
> +			 *  and we don't have a user virtual address yet.
> +			 */
> +			folio_zero_user(folio, 0);
> +
> +			/*
> +			 * Mark the folio uptodate before adding to page cache,
> +			 * as required by filemap.c and other hugetlb paths.
> +			 */
> +			__folio_mark_uptodate(folio);
> +
> +			/*
> +			 * Serialize hugepage allocation and instantiation to prevent
> +			 * races with concurrent allocations, as required by all other
> +			 * callers of hugetlb_add_to_page_cache().
> +			 */
> +			hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
> +			mutex_lock(&hugetlb_fault_mutex_table[hash]);
> +
>  			err = hugetlb_add_to_page_cache(folio,
>  							memfd->f_mapping,
>  							idx);
> +
> +			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> +
>  			if (err) {
>  				folio_put(folio);
>  				goto err_unresv;
> -- 
> 2.43.0

Re: [PATCH v2] mm/memfd: fix information leak in hugetlb folios

[Hugh Dickins]

On Sat, 22 Nov 2025, Hugh Dickins wrote:
> 
> Acked-by: Hugh Dickins <hughd@google.com>
> 
> Sorry if you were all waiting on a Ack from me.  We're agreed that
> the comment above __folio_mark_uptodate() could be deleted, and that
> it would be much better if this code can be moved to a shared home
> in hugetlb later on; but for now it's more urgent to get this patch
> into hotfixes and on to Linus - please Andrew.

Ah, it is already there in mm-hotfixes-unstable, thanks;
sorry, I somehow just never saw the usual mm-commits mail.

Hugh