linux-kernel - Re: [PATCH 0/2] Fix out-of-bound access if pagecache of udf device is corrupted

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Date:   Tue, 13 Jun 2023 09:43:23 +0800
From:   "haowenchao (C)" <haowenchao2@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     Jan Kara <jack@...e.com>, <linux-kernel@...r.kernel.org>,
        <linfeilong@...wei.com>
Subject: Re: [PATCH 0/2] Fix out-of-bound access if pagecache of udf device is
 corrupted

On 2023/6/12 22:40, Jan Kara wrote:
> On Tue 13-06-23 11:22:52, Wenchao Hao wrote:
>> Following steps would cause out-of-bound access and even cause kernel
>> panic when using udf:
>>
>> dd if=/dev/zero of=udf.img bs=1M count=512
>> mkfs.udf udf.img
>> mount -o loop -t udf udf.img /mnt
>> dd if=/dev/random of=/dev/loop0 bs=512 count=1 seek=128
>> umount /mnt
>>
>> [if /mnt is mounted on /dev/loop0]
>>
>> It is because we did not check if udf_sb_info->s_lvid_bh is valid in
>> udf_sb_lvidiu().
>>
>> Although it's illegal to write backend device since filesystem has been
>> mounted, but we should avoid kernel panic if it happened.
> 
> No, it is perfectly valid to crash the kernel if someone writes the buffer
> cache of the device while the device is mounted (which your example above
> does). There is no practical protection against this because someone could
> overwrite the buffer just after the moment you verify its validity. The
> only protection would be to lock the buffer for each access and fully
> verify validity of the data after each locking but the performance and
> maintenance overhead of this is too high to justify. So I'm sorry but I
> will not take any patches that try to "fix" situations when someone writes
> buffer cache while the filesystem is mounted.
> 
> I guess your work is motivated by some syzbot reproducer which was doing
> this. Let me work on a kernel option which syzbot can use to not report
> these issues.
> 
> 
> 								Honza

Yes, the issue is discovered by syzbot. Looking forward you patches.