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From: Zhang Yi <>
To: Christoph Hellwig <>
Cc: <>, <>,
	<>, <>, <>,
Subject: Re: [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super()
Date: Fri, 16 Apr 2021 16:00:48 +0800
Message-ID: <> (raw)
In-Reply-To: <>

Hi, Christoph

On 2021/4/15 22:52, Christoph Hellwig wrote:
> On Wed, Apr 14, 2021 at 09:47:37PM +0800, Zhang Yi wrote:
>> There still exist a use after free issue when accessing the journal
>> structure and ext4_sb_info structure on freeing bdev buffers in
>> bdev_try_to_free_page(). The problem is bdev_try_to_free_page() could be
>> raced by ext4_put_super(), it dose freeing sb->s_fs_info and
>> sbi->s_journal while release page progress are still accessing them.
>> So it could end up trigger use-after-free or NULL pointer dereference.
> I think the right fix is to not even call into ->bdev_try_to_free_page
> unless the superblock is active.
> .
Thanks for your suggestions.

Now, we use already use "if (bdev->bd_super)" to prevent call into
->bdev_try_to_free_page unless the super is alive, and the problem is
bd_super becomes NULL concurrently after this check. So, IIUC, I think it's
the same to switch to check the superblock is active or not. The acvive
flag also could becomes inactive (raced by umount) after we call into

In order to close this race, One solution is introduce a lock to synchronize
the active state between kill_block_super() and blkdev_releasepage(), but
the releasing page process have to try to acquire this lock in
blkdev_releasepage() for each page, and the umount process still need to wait
until the page release if some one invoke into ->bdev_try_to_free_page().
I think this solution may affect performace and is not a good way.
Think about it in depth, use percpu refcount seems have the smallest
performance effect on blkdev_releasepage().

If you don't like the refcount, maybe we could add synchronize_rcu_expedited()
in ext4_put_super(), it also could prevent this race. Any suggestions?


  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-14 13:47 [RFC PATCH v2 0/7] ext4, jbd2: fix 3 issues about bdev_try_to_free_page() Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 1/7] jbd2: remove the out label in __jbd2_journal_remove_checkpoint() Zhang Yi
2021-04-21 10:01   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 2/7] jbd2: ensure abort the journal if detect IO error when writing original buffer back Zhang Yi
2021-04-21 13:20   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 3/7] jbd2: don't abort the journal when freeing buffers Zhang Yi
2021-04-21 13:23   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 4/7] jbd2: do not free buffers in jbd2_journal_try_to_free_buffers() Zhang Yi
2021-04-15 14:46   ` Christoph Hellwig
2021-04-14 13:47 ` [RFC PATCH v2 5/7] ext4: use RCU to protect accessing superblock in blkdev_releasepage() Zhang Yi
2021-04-15 14:48   ` Christoph Hellwig
2021-04-14 13:47 ` [RFC PATCH v2 6/7] fs: introduce a usage count into the superblock Zhang Yi
2021-04-15 14:40   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super() Zhang Yi
2021-04-15 14:52   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi [this message]
2021-04-20 13:08       ` Christoph Hellwig
2021-04-21 13:46         ` Jan Kara
2021-04-21 16:57           ` Theodore Ts'o
2021-04-22  9:04             ` Jan Kara
2021-04-23 11:39               ` Zhang Yi
2021-04-23 16:06                 ` Jan Kara
2021-04-23 14:40               ` Theodore Ts'o

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