linux-kernel - [PATCH] fs/buffer: add alert in try_to_free

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Message-ID: <20251210180228.211655-1-dkarn@redhat.com>
Date: Wed, 10 Dec 2025 23:32:28 +0530
From: Deepakkumar Karn <dkarn@...hat.com>
To: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Deepakkumar Karn <dkarn@...hat.com>
Subject: [PATCH] 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@...hat.com>
---
 fs/buffer.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index 838c0c571022..b229baa77055 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 false;
+
 	if (mapping == NULL) {		/* can this still happen? */
 		ret = drop_buffers(folio, &buffers_to_free);
 		goto out;
-- 
2.52.0