[PATCH v5] mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()

[David Carlier <devnexen@gmail.com>]

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 can lead to incorrect folio handling or a kernel crash.

Pass the caller's ops into mfill_copy_folio_retry() and compare against
the current vma_uffd_ops() after re-acquiring the lock. Return -EAGAIN
if they differ so the operation can be retried.

Fixes: 59da5c32ffa3 ("userfaultfd: mfill_atomic(): remove retry logic")
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 mm/userfaultfd.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 481ec7eb4442..214923a411c1 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -443,7 +443,9 @@ 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,
+				  const struct vm_uffd_ops *ops,
+				  struct folio *folio)
 {
 	unsigned long src_addr = state->src_addr;
 	void *kaddr;
@@ -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) != ops)
+		return -EAGAIN;
+
 	err = mfill_establish_pmd(state);
 	if (err)
 		return err;
@@ -495,7 +505,7 @@ static int __mfill_atomic_pte(struct mfill_state *state,
 		 * will take care of unlocking if needed.
 		 */
 		if (unlikely(ret)) {
-			ret = mfill_copy_folio_retry(state, folio);
+			ret = mfill_copy_folio_retry(state, ops, folio);
 			if (ret)
 				goto err_folio_put;
 		}
-- 
2.53.0