[PATCH] ext4: fix semaphore unlocking order

[PATCH] ext4: fix semaphore unlocking order

[Mikhail Ukhin]

Fuzzing reports a possible deadlock in jbd2_log_wait_commit.

The problem occurs in ext4_ind_migrate due to an incorrect order of
unlocking of the journal and write semaphores - the order of unlocking
must be the reverse of the order of locking.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Signed-off-by: Artem Sadovnikov <ancowi69@gmail.com>
---
 fs/ext4/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index d98ac2af8199..a5e1492bbaaa 100644
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -663,8 +663,8 @@ int ext4_ind_migrate(struct inode *inode)
 	if (unlikely(ret2 && !ret))
 		ret = ret2;
 errout:
-	ext4_journal_stop(handle);
 	up_write(&EXT4_I(inode)->i_data_sem);
+	ext4_journal_stop(handle);
 out_unlock:
 	ext4_writepages_up_write(inode->i_sb, alloc_ctx);
 	return ret;
-- 
2.25.1

Re: [PATCH] ext4: fix semaphore unlocking order

[Ritesh Harjani]

Mikhail Ukhin <mish.uxin2012@yandex.ru> writes:

> Fuzzing reports a possible deadlock in jbd2_log_wait_commit.
>
> The problem occurs in ext4_ind_migrate due to an incorrect order of
> unlocking of the journal and write semaphores - the order of unlocking
> must be the reverse of the order of locking.
>

Maybe we should update the subject msg to:
      "ext4: "fix i_data_sem unlock order in ext4_ind_migrate()"

and also should add:
    CC: stable@vger.kernel.org


I think this should have been fixed in patch [1], but looks like it
forgot to fix the unlock order.

[1]: https://lore.kernel.org/all/1364801462-13120-1-git-send-email-dmonakhov@openvz.org/


> Found by Linux Verification Center (linuxtesting.org) with syzkaller.

I am not sure if there is an easy reproducer. If yes, we should consider
adding it in fstests.

>
> Signed-off-by: Artem Sadovnikov <ancowi69@gmail.com>

Thanks for fixing it. With above changes to subject and CC tag, this
looks good to me. Feel free to add - 

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

> ---
>  fs/ext4/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
> index d98ac2af8199..a5e1492bbaaa 100644
> --- a/fs/ext4/migrate.c
> +++ b/fs/ext4/migrate.c
> @@ -663,8 +663,8 @@ int ext4_ind_migrate(struct inode *inode)
>  	if (unlikely(ret2 && !ret))
>  		ret = ret2;
>  errout:
> -	ext4_journal_stop(handle);
>  	up_write(&EXT4_I(inode)->i_data_sem);
> +	ext4_journal_stop(handle);
>  out_unlock:
>  	ext4_writepages_up_write(inode->i_sb, alloc_ctx);
>  	return ret;
> -- 
> 2.25.1

Re: [PATCH] ext4: fix semaphore unlocking order

[Ritesh Harjani]

Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes:

> Mikhail Ukhin <mish.uxin2012@yandex.ru> writes:
>
>> Fuzzing reports a possible deadlock in jbd2_log_wait_commit.

I think I agree with what Jan hinted to me in the call, that
how can an unlock order mismatch be a deadlock.

But yes, a wrong unlock order can increase the locking times of
thread-2 waiting on lock B; for e.g. if a premption happens between
unlock of lock A & B by thread-1.

So it is always good to fix the unlock order too.

>>
>> The problem occurs in ext4_ind_migrate due to an incorrect order of
>> unlocking of the journal and write semaphores - the order of unlocking
>> must be the reverse of the order of locking.
>>
>
> Maybe we should update the subject msg to:
>       "ext4: "fix i_data_sem unlock order in ext4_ind_migrate()"
>
> and also should add:
>     CC: stable@vger.kernel.org

In that case, I am not really sure, if this requires a cc'd stable.
So, I will leave this upto Ted.

>
>
> I think this should have been fixed in patch [1], but looks like it
> forgot to fix the unlock order.
>
> [1]: https://lore.kernel.org/all/1364801462-13120-1-git-send-email-dmonakhov@openvz.org/
>
>
>> Found by Linux Verification Center (linuxtesting.org) with syzkaller.

It will be good to know what was the test which identified this though?

-ritesh

Re: [PATCH] ext4: fix i_data_sem unlock order in ext4_ind_migrate()

[Mikhail Ukhin]

On 04.04.2024 19:02, Ritesh Harjani (IBM) wrote:
> It will be good to know what was the test which identified this though?
>
> -ritesh


Unfortunately syzkaller was not able to generate a reproducer for this 
issue.


- Ukhin Mikhail.