Commit 292411fd authored by David Carlier's avatar David Carlier Committed by Andrew Morton
Browse files

mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry() 

mfill_copy_folio_retry() drops mmap_lock for the copy_from_user() call.
During this window, the VMA can be replaced with a different type (e.g.
hugetlb), making the caller's ops pointer stale.  Subsequent use of the
stale ops would dispatch into the wrong per-vma handlers.

Capture the VMA's ops via vma_uffd_ops() before dropping the lock and
compare against the current vma_uffd_ops() after re-acquiring it.
Return -EAGAIN if they differ so the operation can be retried.  This
avoids comparing against the caller's ops which may have been
overridden to anon_uffd_ops for MAP_PRIVATE file-backed mappings.

Link: https://lore.kernel.org/20260424183638.196227-1-devnexen@gmail.com


Fixes: 6ab70303 ("userfaultfd: mfill_atomic(): remove retry logic")
Reported-by: default avatarUsama Arif <usama.arif@linux.dev>
Closes: https://lore.kernel.org/all/20260410114809.3592720-1-usama.arif@linux.dev/


Signed-off-by: default avatarDavid Carlier <devnexen@gmail.com>
Acked-by: default avatarMike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent ba13b28d

mm/userfaultfd.c

+11 −1
Original line number Diff line number Diff line
@@ -443,8 +443,10 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr) 
	return ret;

}



static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio)

static int mfill_copy_folio_retry(struct mfill_state *state,

				  struct folio *folio)

{

	const struct vm_uffd_ops *orig_ops = vma_uffd_ops(state->vma);

	unsigned long src_addr = state->src_addr;

	void *kaddr;

	int err;
@@ -465,6 +467,14 @@ static int mfill_copy_folio_retry(struct mfill_state *state, struct folio *folio 
	if (err)

		return err;



	/*

	 * The VMA type may have changed while the lock was dropped

	 * (e.g. replaced with a hugetlb mapping), making the caller's

	 * ops pointer stale.

	 */

	if (vma_uffd_ops(state->vma) != orig_ops)

		return -EAGAIN;



	err = mfill_establish_pmd(state);

	if (err)

		return err;