[PATCH] fs/buffer: check for NULL pointer before folio_test_dropbehind()

[Zhaoyu Liu]

Add a NULL check for bh->b_folio before folio_test_dropbehind()
to avoid panic for non-pagecache buffer heads from jbd2.

Sample crash report:

BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP PTI
CPU: 2 UID: 0 PID: 332 Comm: jbd2/vda1-8 Not tainted
7.3.0-rc2-next-20260907+ #3 PREEMPT (lazy)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
BIOS 2024.05-2ubuntu0.3 05/07/2025
RIP: 0010:__bh_submit+0xa2/0x240
RSP: 0018:ffffce590064bc48 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8ab102861960 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffce590064bc90 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000009801
R13: 0000000000000000 R14: ffff8ab10ab90600 R15: 0000000000000000
FS:  0000000000000000 GS:ffff8ab2e2571000   knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000109364000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 ? __pfx_journal_end_buffer_io_sync+0x10/0x10
 bh_submit+0x15/0x30
 jbd2_journal_commit_transaction+0x6c3/0x1840
 ? psi_group_change+0x201/0x4e0
 kjournald2+0xa9/0x250
 ? __pfx_autoremove_wake_function+0x10/0x10
 ? __pfx_kjournald2+0x10/0x10
 kthread+0xfd/0x140
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x2a4/0x320
 ? __pfx_kthread+0x10/0x10
 ret_from_fork_asm+0x1a/0x30

Fixes: a2c924c240e7 ("buffer: set BIO_COMPLETE_IN_TASK for dropbehind writeback")

Signed-off-by: Zhaoyu Liu <zackary.liu.pro@xxxxxxxxx>
---
 fs/buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 427d8a817cd5..4aea0bb08609 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1106,7 +1106,7 @@ static void __bh_submit(struct buffer_head *bh, blk_opf_t opf,
 
 	bio = bio_alloc(bh->b_bdev, 1, opf, GFP_NOIO);
 
-	if (folio_test_dropbehind(bh->b_folio) && op_is_write(opf))
+	if (bh->b_folio && folio_test_dropbehind(bh->b_folio) && op_is_write(opf))
 		bio_set_flag(bio, BIO_COMPLETE_IN_TASK);
 
 	if (IS_ENABLED(CONFIG_FS_ENCRYPTION))
-- 
2.45.2