Re: [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super()

[Zhang Yi <yi.zhang@huawei.com>]

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
bdev_try_to_free_page().

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?

Thanks,
Yi.