[PATCH v2 2/3] userfaultfd: refuse to __mfill_atomic_pte() for unsupported VMAs

[Mike Rapoport <rppt@kernel.org>]

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

__mfill_atomic_pte() unconditionally dereferences ops because there is an
assumption that VMAs that can undergo mfill_* operations are vetted on
registration and must have valid vm_uffd_ops.

Add a guard against potential bugs and make sure __mfill_atomic_pte() bails
out if ops is NULL.

Suggested-by: Lorenzo Stoakes <ljs@kernel.org>
Fixes: ad9ac3081332 ("userfaultfd: introduce vm_uffd_ops->alloc_folio()")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 mm/userfaultfd.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e5d2fb3ce2c1..2872c71bbf36 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -552,6 +552,11 @@ static int __mfill_atomic_pte(struct mfill_state *state,
 	struct folio *folio;
 	int ret;
 
+	if (!ops) {
+		VM_WARN_ONCE(1, "UFFDIO_COPY for unsupported VMA");
+		return -EOPNOTSUPP;
+	}
+
 	folio = ops->alloc_folio(state->vma, state->dst_addr);
 	if (!folio)
 		return -ENOMEM;
-- 
2.53.0