Re: [PATCH v2] jbd2: skip reading super block if it has been verified

[Jan Kara <jack@suse.cz>]

On Fri 16-06-23 09:55:47, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> We got a NULL pointer dereference issue below while running generic/475
> I/O failure pressure test.
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000000
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  PGD 0 P4D 0
>  Oops: 0002 [#1] PREEMPT SMP PTI
>  CPU: 1 PID: 15600 Comm: fsstress Not tainted 6.4.0-rc5-xfstests-00055-gd3ab1bca26b4 #190
>  RIP: 0010:jbd2_journal_set_features+0x13d/0x430
>  ...
>  Call Trace:
>   <TASK>
>   ? __die+0x23/0x60
>   ? page_fault_oops+0xa4/0x170
>   ? exc_page_fault+0x67/0x170
>   ? asm_exc_page_fault+0x26/0x30
>   ? jbd2_journal_set_features+0x13d/0x430
>   jbd2_journal_revoke+0x47/0x1e0
>   __ext4_forget+0xc3/0x1b0
>   ext4_free_blocks+0x214/0x2f0
>   ext4_free_branches+0xeb/0x270
>   ext4_ind_truncate+0x2bf/0x320
>   ext4_truncate+0x1e4/0x490
>   ext4_handle_inode_extension+0x1bd/0x2a0
>   ? iomap_dio_complete+0xaf/0x1d0
> 
> The root cause is the journal super block had been failed to write out
> due to I/O fault injection, it's uptodate bit was cleared by
> end_buffer_write_sync() and didn't reset yet in jbd2_write_superblock().
> And it raced by journal_get_superblock()->bh_read(), unfortunately, the
> read IO is also failed, so the error handling in
> journal_fail_superblock() unexpectedly clear the journal->j_sb_buffer,
> finally lead to above NULL pointer dereference issue.
> 
> If the journal super block had been read and verified, there is no need
> to call bh_read() read it again even if it has been failed to written
> out. So the fix could be simply move buffer_verified(bh) in front of
> bh_read(). Also remove a stale comment left in
> jbd2_journal_check_used_features().
> 
> Fixes: 51bacdba23d8 ("jbd2: factor out journal initialization from journal_get_superblock()")
> Reported-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

This works as a workaround. It is a bit kludgy but for now I guess it is
good enough. Thanks for the fix and feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza


> ---
> v1->v2:
>  - Remove a stale comment left in jbd2_journal_check_used_features().
> 
>  fs/jbd2/journal.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index b5e57735ab3f..559242df0f9a 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1919,6 +1919,9 @@ static int journal_get_superblock(journal_t *journal)
>  	bh = journal->j_sb_buffer;
>  
>  	J_ASSERT(bh != NULL);
> +	if (buffer_verified(bh))
> +		return 0;
> +
>  	err = bh_read(bh, 0);
>  	if (err < 0) {
>  		printk(KERN_ERR
> @@ -1926,9 +1929,6 @@ static int journal_get_superblock(journal_t *journal)
>  		goto out;
>  	}
>  
> -	if (buffer_verified(bh))
> -		return 0;
> -
>  	sb = journal->j_superblock;
>  
>  	err = -EINVAL;
> @@ -2229,7 +2229,6 @@ int jbd2_journal_check_used_features(journal_t *journal, unsigned long compat,
>  
>  	if (!compat && !ro && !incompat)
>  		return 1;
> -	/* Load journal superblock if it is not loaded yet. */
>  	if (journal_get_superblock(journal))
>  		return 0;
>  	if (!jbd2_format_support_feature(journal))
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR