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

[Yalagada Pavan Kumar]

On Wed, Sep 02, 2026 at 09:33:57AM +0800, Joseph Qi wrote:
> 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.
>
Hi,

I was working on a fix for this syzbot report [1] and didn't realize that you were
already working on it. I noticed your patch on the mailing list, so i won't
send a duplicate patch.
 
> Fixes: 5febcba29792 ("jbd2: point the shadow buffer at the frozen data directly")

Could you please add the Reported-by: and Closes: tags from the syzbot report
to your patch? This will help syzbot associate the patch with the reported
issue and track the fix.

[1]: https://syzkaller.appspot.com/bug?extid=41453ea05ab61c075f1f

Thank you,
Pavan

> Tested-by: Srikanth Aithal <Srikanth.Aithal@xxxxxxx>
> Tested-by: Luca Weiss <luca.weiss@xxxxxxxxxxxxx> # sm7225-fairphone-fp4
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> ---
>  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
>