linux-kernel - Re: [syzbot] KASAN: use-after-free Read in jbd2_journal_wait

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Date:   Wed, 9 Feb 2022 15:26:47 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     Jan Kara <jack@...e.cz>
Cc:     syzbot <syzbot+afa2ca5171d93e44b348@...kaller.appspotmail.com>,
        jack@...e.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        tytso@....edu
Subject: Re: [syzbot] KASAN: use-after-free Read in jbd2_journal_wait_updates

On 22/02/08 05:14PM, Jan Kara wrote:
> On Tue 08-02-22 04:49:19, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    555f3d7be91a Merge tag '5.17-rc3-ksmbd-server-fixes' of gi..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17e55852700000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=88e0a6a3dbf057cf
> > dashboard link: https://syzkaller.appspot.com/bug?extid=afa2ca5171d93e44b348
> > compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13b03872700000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+afa2ca5171d93e44b348@...kaller.appspotmail.com
>
> So the syzbot reproducer looks bogus to me but the bug is real.
> jbd2_journal_wait_updates() looks at commit_transaction after dropping
> j_state_lock and sleeping which is certainly prone to use-after-free
> issues.

Yes, thanks for taking a look at it.

>
> Funnily, Ritesh's removal of t_handle_lock should "fix" the problem by
> removing this dereference. So Ritesh, please just add some reference to
> syzbot report and maybe a backport to stable would be warranted.
>

This actually looks like a regression because of commit [1].
So I am thinking of sending a separate patch [2] as a fix for this (after my
testing).

Not sure why fstests testing didn't pick this up (given it's a common race),
or is it because of my recent removal of CONFIG_KASAN from my testing :(

I will try a full "auto" test with CONFIG_KASAN enabled and see if we could hit
this in fstests or not. If not then I will work towards adding a targetted test
to capture such race.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=origin&id=4f98186848707f530669238d90e0562d92a78aab
[2]: https://github.com/riteshharjani/linux/commit/628648810011a22dfaba38ead49716720b27a31c

-ritesh