[PATCH 6.18 026/752] fs/buffer: add alert in try_to_free_buffers() for folios without buffers

Sasha Levin
2026-02-28T17:48:31Z

From: Deepakkumar Karn <dkarn@redhat.com>

[ Upstream commit b68f91ef3b3fe82ad78c417de71b675699a8467c ]

try_to_free_buffers() can be called on folios with no buffers attached
when filemap_release_folio() is invoked on a folio belonging to a mapping
with AS_RELEASE_ALWAYS set but no release_folio operation defined.

In such cases, folio_needs_release() returns true because of the
AS_RELEASE_ALWAYS flag, but the folio has no private buffer data. This
causes try_to_free_buffers() to call drop_buffers() on a folio with no
buffers, leading to a null pointer dereference.

Adding a check in try_to_free_buffers() to return early if the folio has no
buffers attached, with WARN_ON_ONCE() to alert about the misconfiguration.
This provides defensive hardening.

Signed-off-by: Deepakkumar Karn <dkarn@redhat.com>
Link: https://patch.msgid.link/20251211131211.308021-1-dkarn@redhat.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/buffer.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index 6a8752f7bbedb..b6b477ff7b75d 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2948,6 +2948,10 @@ bool try_to_free_buffers(struct folio *folio)
if (folio_test_writeback(folio))
return false;

+ /* Misconfigured folio check */
+ if (WARN_ON_ONCE(!folio_buffers(folio)))
+ return true;
+
if (mapping == NULL) { /* can this still happen? */
ret = drop_buffers(folio, &buffers_to_free);
goto out;
--
2.51.0