[PATCH] fs/buffer: fix a NULL pointer dereference in drop_buffers()

[Liu Shixin]

syzbot found a null-ptr-deref by KASAN:

 BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
 BUG: KASAN: null-ptr-deref in atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
 BUG: KASAN: null-ptr-deref in buffer_busy fs/buffer.c:2856 [inline]
 BUG: KASAN: null-ptr-deref in drop_buffers+0x61/0x2f0 fs/buffer.c:2868
 Read of size 4 at addr 0000000000000060 by task syz-executor.5/24786

 CPU: 0 PID: 24786 Comm: syz-executor.5 Not tainted 6.0.0-syzkaller-09589-g55be6084c8e0 #0
 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022
 Call Trace:
  <TASK>
  __dump_stack lib/dump_stack.c:88 [inline]
  dump_stack_lvl+0x1e3/0x2cb lib/dump_stack.c:106
  print_report+0xf1/0x220 mm/kasan/report.c:436
  kasan_report+0xfb/0x130 mm/kasan/report.c:495
  kasan_check_range+0x2a7/0x2e0 mm/kasan/generic.c:189
  instrument_atomic_read include/linux/instrumented.h:71 [inline]
  atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
  buffer_busy fs/buffer.c:2856 [inline]
  drop_buffers+0x61/0x2f0 fs/buffer.c:2868
  try_to_free_buffers+0x2b1/0x640 fs/buffer.c:2898
[...]

We use folio_has_private() to decide whether call filemap_release_folio(),
which may call try_to_free_buffers() then. folio_has_private() return true
for both PG_private and PG_private_2. We should only call try_to_free_buffers()
for case PG_private. So we should recheck PG_private in try_to_free_buffers().

Reported-by: syzbot+fbdb4ec578ebdcfb9ed2@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 266cf658efcf ("FS-Cache: Recruit a page flags for cache management")
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
 fs/buffer.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index d9c6d1fbb6dd..c302b578e437 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2822,6 +2822,9 @@ bool try_to_free_buffers(struct folio *folio)
 	if (folio_test_writeback(folio))
 		return false;
 
+	if (!folio_test_private(folio))
+		return false;
+
 	if (mapping == NULL) {		/* can this still happen? */
 		ret = drop_buffers(folio, &buffers_to_free);
 		goto out;
-- 
2.25.1