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Message-ID: <e3172770-9b39-4105-966f-faf64a6b6515@huaweicloud.com>
Date: Fri, 7 Mar 2025 20:36:08 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: Jan Kara <jack@...e.cz>, Baokun Li <libaokun1@...wei.com>,
linux-kernel@...r.kernel.org, Mahesh Kumar <maheshkumar657g@...il.com>,
linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v2 2/3] ext4: avoid journaling sb update on error if
journal is destroying
On 2025/3/7 18:27, Ojaswin Mujoo wrote:
> On Fri, Mar 07, 2025 at 04:43:24PM +0800, Zhang Yi wrote:
>> On 2025/3/7 16:13, Ojaswin Mujoo wrote:
>>> On Fri, Mar 07, 2025 at 12:04:26PM +0530, Ojaswin Mujoo wrote:
>>>> On Fri, Mar 07, 2025 at 10:49:28AM +0800, Zhang Yi wrote:
>>>>> On 2025/3/6 22:28, Ojaswin Mujoo wrote:
>>>>>> Presently we always BUG_ON if trying to start a transaction on a journal marked
>>>>>> with JBD2_UNMOUNT, since this should never happen. However, while ltp running
>>>>>> stress tests, it was observed that in case of some error handling paths, it is
>>>>>> possible for update_super_work to start a transaction after the journal is
>>>>>> destroyed eg:
>>>>>>
>>>>>> (umount)
>>>>>> ext4_kill_sb
>>>>>> kill_block_super
>>>>>> generic_shutdown_super
>>>>>> sync_filesystem /* commits all txns */
>>>>>> evict_inodes
>>>>>> /* might start a new txn */
>>>>>> ext4_put_super
>>>>>> flush_work(&sbi->s_sb_upd_work) /* flush the workqueue */
>>>>>> jbd2_journal_destroy
>>>>>> journal_kill_thread
>>>>>> journal->j_flags |= JBD2_UNMOUNT;
>>>>>> jbd2_journal_commit_transaction
>>>>>> jbd2_journal_get_descriptor_buffer
>>>>>> jbd2_journal_bmap
>>>>>> ext4_journal_bmap
>>>>>> ext4_map_blocks
>>>>>> ...
>>>>>> ext4_inode_error
>>>>>> ext4_handle_error
>>>>>> schedule_work(&sbi->s_sb_upd_work)
>>>>>>
>>>>>> /* work queue kicks in */
>>>>>> update_super_work
>>>>>> jbd2_journal_start
>>>>>> start_this_handle
>>>>>> BUG_ON(journal->j_flags &
>>>>>> JBD2_UNMOUNT)
>>>>>>
>>>>>> Hence, introduce a new sbi flag s_journal_destroying to indicate journal is
>>>>>> destroying only do a journaled (and deferred) update of sb if this flag is not
>>>>>> set. Otherwise, just fallback to an un-journaled commit.
>>>>>>
>>>>>> We set sbi->s_journal_destroying = true only after all the FS updates are done
>>>>>> during ext4_put_super() (except a running transaction that will get commited
>>>>>> during jbd2_journal_destroy()). After this point, it is safe to commit the sb
>>>>>> outside the journal as it won't race with a journaled update (refer
>>>>>> 2d01ddc86606).
>>>>>>
>>>>>> Also, we don't need a similar check in ext4_grp_locked_error since it is only
>>>>>> called from mballoc and AFAICT it would be always valid to schedule work here.
>>>>>>
>>>>>> Fixes: 2d01ddc86606 ("ext4: save error info to sb through journal if available")
>>>>>> Reported-by: Mahesh Kumar <maheshkumar657g@...il.com>
>>>>>> Suggested-by: Jan Kara <jack@...e.cz>
>>>>>> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
>>>>>> ---
>>>>>> fs/ext4/ext4.h | 2 ++
>>>>>> fs/ext4/ext4_jbd2.h | 8 ++++++++
>>>>>> fs/ext4/super.c | 4 +++-
>>>>>> 3 files changed, 13 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>>>>>> index 2b7d781bfcad..d48e93bd5690 100644
>>>>>> --- a/fs/ext4/ext4.h
>>>>>> +++ b/fs/ext4/ext4.h
>>>>>> @@ -1728,6 +1728,8 @@ struct ext4_sb_info {
>>>>>> */
>>>>>> struct work_struct s_sb_upd_work;
>>>>>>
>>>>>> + bool s_journal_destorying;
>>>>>> +
>>>>>> /* Atomic write unit values in bytes */
>>>>>> unsigned int s_awu_min;
>>>>>> unsigned int s_awu_max;
>>>>>> diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
>>>>>> index 9b3c9df02a39..6bd3ca84410d 100644
>>>>>> --- a/fs/ext4/ext4_jbd2.h
>>>>>> +++ b/fs/ext4/ext4_jbd2.h
>>>>>> @@ -437,6 +437,14 @@ static inline int ext4_journal_destroy(struct ext4_sb_info *sbi, journal_t *jour
>>>>>> {
>>>>>> int err = 0;
>>>>>>
>>>>>> + /*
>>>>>> + * At this point all pending FS updates should be done except a possible
>>>>>> + * running transaction (which will commit in jbd2_journal_destroy). It
>>>>>> + * is now safe for any new errors to directly commit superblock rather
>>>>>> + * than going via journal.
>>>>>> + */
>>>>>> + sbi->s_journal_destorying = true;
>>>>>> +
>>>>>
>>>>> Hi, Ojaswin!
>>>>>
>>>>> I'm afraid you still need to flush the superblock update work here,
>>>>> otherwise I guess the race condition you mentioned in v1 could still
>>>>> occur.
>>>>>
>>>>> ext4_put_super()
>>>>> flush_work(&sbi->s_sb_upd_work)
>>>>>
>>>>> **kjournald2**
>>>>> jbd2_journal_commit_transaction()
>>>>> ...
>>>>> ext4_inode_error()
>>>>> /* JBD2_UNMOUNT not set */
>>>>> schedule_work(s_sb_upd_work)
>>>>>
>>>>> **workqueue**
>>>>> update_super_work
>>>>> /* s_journal_destorying is not set */
>>>>> if (journal && !s_journal_destorying)
>>>>>
>>>>> ext4_journal_destroy()
>>>>> /* set s_journal_destorying */
>>>>> sbi->s_journal_destorying = true;
>>>>> jbd2_journal_destroy()
>>>>> journal->j_flags |= JBD2_UNMOUNT;
>>>>>
>>>>> jbd2_journal_start()
>>>>> start_this_handle()
>>>>> BUG_ON(JBD2_UNMOUNT)
>>>>>
>>>>> Thanks,
>>>>> Yi.
>>>> Hi Yi,
>>>>
>>>> Yes you are right, somehow missed this edge case :(
>>>>
>>>> Alright then, we have to move out sbi->s_journal_destroying outside the
>>>> helper. Just wondering if I should still let it be in
>>>> ext4_journal_destroy and just add an extra s_journal_destroying = false
>>>> before schedule_work(s_sb_upd_work), because it makes sense.
>>>>
>>>> Okay let me give it some thought but thanks for pointing this out!
>>>>
>>>> Regards,
>>>> ojaswin
>>>
>>> Okay so thinking about it a bit more, I see you also suggested to flush
>>> the work after marking sbi->s_journal_destroying. But will that solve
>>> it?
>>>
>>> ext4_put_super()
>>> flush_work(&sbi->s_sb_upd_work)
>>>
>>> **kjournald2**
>>> jbd2_journal_commit_transaction()
>>> ...
>>> ext4_inode_error()
>>> /* JBD2_UNMOUNT not set */
>>> schedule_work(s_sb_upd_work)
>>>
>>> **workqueue**
>>> update_super_work
>>> /* s_journal_destorying is not set */
>>> if (journal && !s_journal_destorying)
>>>
>>> ext4_journal_destroy()
>>> /* set s_journal_destorying */
>>> sbi->s_journal_destorying = true;
>>> flush_work(&sbi->s_sb_upd_work)
>>> schedule_work()
>> ^^^^^^^^^^^^^^^
>> where does this come from?
>>
>> After this flush_work, we can guarantee that the running s_sb_upd_work
>> finishes before we set JBD2_UNMOUNT. Additionally, the journal will
>> not commit transaction or call schedule_work() again because it has
>> been aborted due to the previous error. Am I missing something?
>>
>> Thanks,
>> Yi.
>
> Hmm, so I am thinking of a corner case in ext4_handle_error() where
>
> if(journal && !is_journal_destroying)
>
> is computed but schedule_work() not called yet, which is possible cause
> the cmp followed by jump is not atomic in nature. If the schedule_work
> is only called after we have done the flush then we end up with this:
>
> if (journal && !s_journal_destorying)
> ext4_journal_destroy()
> /* set s_journal_destorying */
> sbi->s_journal_destorying = true;
> flush_work(&sbi->s_sb_upd_work)
> schedule_work()
>
> Which is possible IMO, although the window is tiny.
Yeah, right!
Sorry for misread the location where you add the "!s_journal_destorying"
check, the graph I provided was in update_super_work(), which was wrong.
The right one should be:
ext4_put_super()
flush_work(&sbi->s_sb_upd_work)
**kjournald2**
jbd2_journal_commit_transaction()
...
ext4_inode_error()
/* s_journal_destorying is not set */
if (journal && !s_journal_destorying)
(schedule_work(s_sb_upd_work)) //can be here
ext4_journal_destroy()
/* set s_journal_destorying */
sbi->s_journal_destorying = true;
jbd2_journal_destroy()
journal->j_flags |= JBD2_UNMOUNT;
(schedule_work(s_sb_upd_work)) //also can be here
**workqueue**
update_super_work()
journal = sbi->s_journal //get journal
kfree(journal)
jbd2_journal_start(journal) //journal UAF
start_this_handle()
BUG_ON(JBD2_UNMOUNT) //bugon here
So there are two problems here, the first one is the 'journal' UAF,
the second one is triggering JBD2_UNMOUNT flag BUGON.
>>>
>>> As for the fix, how about we do something like this:
>>>
>>> ext4_put_super()
>>>
>>> flush_work(&sbi->s_sb_upd_work)
>>> destroy_workqueue(sbi->rsv_conversion_wq);
>>>
>>> ext4_journal_destroy()
>>> /* set s_journal_destorying */
>>> sbi->s_journal_destorying = true;
>>>
>>> /* trigger a commit and wait for it to complete */
>>>
>>> flush_work(&sbi->s_sb_upd_work)
>>>
>>> jbd2_journal_destroy()
>>> journal->j_flags |= JBD2_UNMOUNT;
>>>
>>> jbd2_journal_start()
>>> start_this_handle()
>>> BUG_ON(JBD2_UNMOUNT)
>>>
>>> Still giving this codepath some thought but seems like this might just
>>> be enough to fix the race. Thoughts on this?
>>>
I think this solution should work, the forced commit and flush_work()
should ensure that the last transaction is committed and that the
potential work is done.
Besides, the s_journal_destorying flag is set and check concurrently
now, so we need WRITE_ONCE() and READ_ONCE() for it. Besides, what
about adding a new flag into sbi->s_mount_state instead of adding
new s_journal_destorying?
Thanks,
Yi.
>>>
>>>>>
>>>>>> err = jbd2_journal_destroy(journal);
>>>>>> sbi->s_journal = NULL;
>>>>>>
>>>>>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>>>>>> index 8ad664d47806..31552cf0519a 100644
>>>>>> --- a/fs/ext4/super.c
>>>>>> +++ b/fs/ext4/super.c
>>>>>> @@ -706,7 +706,7 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
>>>>>> * constraints, it may not be safe to do it right here so we
>>>>>> * defer superblock flushing to a workqueue.
>>>>>> */
>>>>>> - if (continue_fs && journal)
>>>>>> + if (continue_fs && journal && !EXT4_SB(sb)->s_journal_destorying)
>>>>>> schedule_work(&EXT4_SB(sb)->s_sb_upd_work);
>>>>>> else
>>>>>> ext4_commit_super(sb);
>>>>>> @@ -5311,6 +5311,8 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
>>>>>> spin_lock_init(&sbi->s_error_lock);
>>>>>> INIT_WORK(&sbi->s_sb_upd_work, update_super_work);
>>>>>>
>>>>>> + sbi->s_journal_destorying = false;
>>>>>> +
>>>>>> err = ext4_group_desc_init(sb, es, logical_sb_block, &first_not_zeroed);
>>>>>> if (err)
>>>>>> goto failed_mount3;
>>>>>
>>>
>>
>
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