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;
On 12.11.25 15:50, Deepanshu Kartikey wrote:
When allocating hugetlb folios for memfd, three initialization steps are missing:
- Folios are not zeroed, leading to kernel memory disclosure to userspace
- Folios are not marked uptodate before adding to page cache
- 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
On Wed, Nov 12, 2025 at 08:20:34PM +0530, Deepanshu Kartikey wrote:
When allocating hugetlb folios for memfd, three initialization steps are missing:
- Folios are not zeroed, leading to kernel memory disclosure to userspace
- Folios are not marked uptodate before adding to page cache
- 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
On Wed, 12 Nov 2025, Deepanshu Kartikey wrote:
When allocating hugetlb folios for memfd, three initialization steps are missing:
- Folios are not zeroed, leading to kernel memory disclosure to userspace
- Folios are not marked uptodate before adding to page cache
- 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
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
linux-stable-mirror@lists.linaro.org
-
David Hildenbrand (Red Hat) -
Deepanshu Kartikey -
Hugh Dickins -
Oscar Salvador