[PATCH] buffer: fix NULL dereference of bh->b_folio in __bh_submit()

[Joseph Qi <joseph.qi@linux.alibaba.com>]

Commit a2c924c240e7 ("buffer: set BIO_COMPLETE_IN_TASK for dropbehind
writeback") added an unconditional folio_test_dropbehind(bh->b_folio) in
__bh_submit().  But jbd2 shadow buffers have a NULL b_folio since commit
5febcba29792 ("jbd2: point the shadow buffer at the frozen data
directly") made them point b_data at the kmalloced frozen data rather
than a folio.  Submitting such a buffer during journal commit oopses:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  RIP: 0010:__bh_submit.constprop.0+0x87/0x120
  Call Trace:
   jbd2_journal_commit_transaction+0x932/0x1b10
   kjournald2+0xb2/0x250

Hit by the ocfs2-testsuite fill_verify_holes test running with
data=writeback.

Dropbehind only applies to buffers backed by a folio, so skip the check
when b_folio is NULL.

Fixes: 5febcba29792 ("jbd2: point the shadow buffer at the frozen data directly")
Tested-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
Tested-by: Luca Weiss <luca.weiss@fairphone.com> # sm7225-fairphone-fp4
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
 fs/buffer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 427d8a817cd5..f46fa6413032 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1106,7 +1106,8 @@ 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))
-- 
2.39.3