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Date:   Tue, 22 Nov 2022 15:35:04 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     syzbot <syzbot+1d8c82e66f2e76b6b427@syzkaller.appspotmail.com>
Cc:     djwong@kernel.org, linux-kernel@vger.kernel.org,
        linux-xfs@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] kernel BUG in assfail
Message-ID: <20221122043504.GR3600936@dread.disaster.area>
References: <00000000000063536805ee0769d8@google.com>
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On Mon, Nov 21, 2022 at 08:14:43PM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    eb7081409f94 Linux 6.1-rc6
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12e5bb0d880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8cdf448d3b35234
> dashboard link: https://syzkaller.appspot.com/bug?extid=1d8c82e66f2e76b6b427
> compiler:       Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=169e86fd880000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1691470d880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/4a019f55c517/disk-eb708140.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/eb36e890aa8b/vmlinux-eb708140.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/feee2c23ec64/bzImage-eb708140.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/c5c0a12afa51/mount_2.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+1d8c82e66f2e76b6b427@syzkaller.appspotmail.com
> 
> loop0: detected capacity change from 0 to 32768
> XFS (loop0): Mounting V5 Filesystem
> XFS (loop0): Log inconsistent (didn't find previous header)
> XFS: Assertion failed: 0, file: fs/xfs/xfs_log_recover.c, line: 429

Not a bug. Not even close to a bug.

The kernel was clearly ompiled with CONFIG_XFS_DEBUG=y, which will
trigger debug asserts when things go wrong in ways that should not
ever happen in normal circumstances.

Turn off CONFIG_XFS_DEBUG, and this failure will return
-EFSCORRUPTED as expected because syzbot fed it a corrupt log. THe
mount will simply fail as you'd expect given the malicious
corruption that syzbot has performed.

If syzbot is going to maliciously corrupt XFS filesytsems and then
try to abuse them, then syzbot has two choices. Either:

1. do not enable CONFIG_XFS_DEBUG; or
2. do not report the inevitable stream of ASSERT failures that will
   be encountered to the XFS list.

-Dave.
-- 
Dave Chinner
david@fromorbit.com