linux-ext4 - Re: [PATCH 3/3] ext4: add rcu to prevent use after free when umount filesystem

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Date:   Thu, 8 Apr 2021 16:53:38 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, yukuai3@...wei.com
Subject: Re: [PATCH 3/3] ext4: add rcu to prevent use after free when umount
 filesystem

On Thu 08-04-21 22:38:08, Zhang Yi wrote:
> On 2021/4/8 21:56, Jan Kara wrote:
> > On Thu 08-04-21 19:36:18, Zhang Yi wrote:
> >> There is a race between bdev_try_to_free_page() and
> >> jbd2_journal_destroy() that could end up triggering a use after free
> >> issue about journal.
> >>
> >> drop cache                           umount filesystem
> >> bdev_try_to_free_page()
> >>  get journal
> >>  jbd2_journal_try_to_free_buffers()  ext4_put_super()
> >>                                       kfree(journal)
> >>    access journal <-- lead to UAF
> >>
> >> The above race also could happens between the bdev_try_to_free_page()
> >> and the error path of ext4_fill_super(). This patch avoid this race by
> >> add rcu protection around accessing sbi->s_journal in
> >> bdev_try_to_free_page() and destroy the journal after an rcu grace
> >> period.
> >>
> >> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> > 
> > OK, I see the problem. But cannot the use-after-free happen even for the
> > superblock itself (e.g., EXT4_SB(sb)->s_journal dereference)? I don't see
> > anything preventing that as blkdev_releasepage() just shamelessly does:
> > 
> > super = BDEV_I(page->mapping->host)->bdev.bd_super
> > 
> Hi, Jan.
> 
> I checked the superblock. In theory, the bdev_try_to_free_page() is invoked
> with page locked, the umount process will wait the page unlock on
> kill_block_super()->..->kill_bdev()->truncate_inode_pages_range() before free
> superblock, so I guess the use-after-free problem couldn't happen in general.
> But I think it's fragile and may invalidate if the bdev has more than one
> operners(__blkdev_put() call kill_bdev only if bd_openers becomes zero)?

Yes, kill_bdev() is only called when bd_openers drops to 0 but there can be
other processes having the bdev open (non-exclusively).

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR