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fs/buffer: add alert in try_to_free_buffers() for folios without buffers

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>

authored by

Deepakkumar Karn and committed by
Christian Brauner b68f91ef 961b2ad1

+4
+4
fs/buffer.c
··· 2948 2948 if (folio_test_writeback(folio)) 2949 2949 return false; 2950 2950 2951 + /* Misconfigured folio check */ 2952 + if (WARN_ON_ONCE(!folio_buffers(folio))) 2953 + return true; 2954 + 2951 2955 if (mapping == NULL) { /* can this still happen? */ 2952 2956 ret = drop_buffers(folio, &buffers_to_free); 2953 2957 goto out;