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>

diff --git a/fs/buffer.c b/fs/buffer.c
index c6f4660f92df816282919bb1fcb86e7821e1d847..fd53b806ab7eb2ffeef0b4387b15b05cf69ed3c8 100644 (file)
--- 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;