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From: Kai-Heng Feng <kaihengfeng@me.com>
To: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	linux-block@vger.kernel.org,
	John Lenton <john.lenton@canonical.com>,
	jean-baptiste.lallement@canonical.com
Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener
Date: Thu, 18 Jul 2019 16:15:42 +0800
Message-ID: <894DDAA8-2ADD-467C-8E4F-4DE6B9A50625@me.com> (raw)
In-Reply-To: <b0f27980-be75-bded-3e74-bce14fc7ea47@kernel.dk>

Hi Jan,

at 21:34, Jens Axboe <axboe@kernel.dk> wrote:

> On 5/27/19 6:29 AM, Jan Kara wrote:
>> On Thu 16-05-19 14:44:07, Jens Axboe wrote:
>>> On 5/16/19 8:01 AM, Jan Kara wrote:
>>>> Loop module allows calling LOOP_SET_FD while there are other openers of
>>>> the loop device. Even exclusive ones. This can lead to weird
>>>> consequences such as kernel deadlocks like:
>>>>
>>>> mount_bdev()				lo_ioctl()
>>>>    udf_fill_super()
>>>>      udf_load_vrs()
>>>>        sb_set_blocksize() - sets desired block size B
>>>>        udf_tread()
>>>>          sb_bread()
>>>>            __bread_gfp(bdev, block, B)
>>>> 					  loop_set_fd()
>>>> 					    set_blocksize()
>>>>              - now __getblk_slow() indefinitely loops because B != bdev
>>>>                block size
>>>>
>>>> Fix the problem by disallowing LOOP_SET_FD ioctl when there are
>>>> exclusive openers of a loop device.
>>>>
>>>> [Deliberately chosen not to CC stable as a user with priviledges to
>>>> trigger this race has other means of taking the system down and this
>>>> has a potential of breaking some weird userspace setup]
>>>>
>>>> Reported-and-tested-by:  
>>>> syzbot+10007d66ca02b08f0e60@syzkaller.appspotmail.com
>>>> Signed-off-by: Jan Kara <jack@suse.cz>
>>>> ---
>>>>   drivers/block/loop.c | 18 +++++++++++++++++-
>>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>>
>>>> Hi Jens!
>>>>
>>>> What do you think about this patch? It fixes the problem but it also
>>>> changes user visible behavior so there are chances it breaks some
>>>> existing setup (although I have hard time coming up with a realistic
>>>> scenario where it would matter).
>>>
>>> I also have a hard time thinking about valid cases where this would be a
>>> problem. I think, in the end, that fixing the issue is more important
>>> than a potentially hypothetical use case.
>>>
>>>> Alternatively we could change getblk() code handle changing block
>>>> size. That would fix the particular issue syzkaller found as well but
>>>> I'm not sure what else is broken when block device changes while fs
>>>> driver is working with it.
>>>
>>> I think your solution here is saner.
>>
>> Will you pick up the patch please? I cannot find it in your tree...  
>> Thanks!
>
> Done!

This patch introduced a regression [1].
A reproducer can be found at [2].

[1] https://bugs.launchpad.net/bugs/1836914
[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1836914/comments/4

Kai-Heng

>
> -- 
> Jens Axboe



  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 14:01 Jan Kara
2019-05-16 20:44 ` Jens Axboe
2019-05-27 12:29   ` Jan Kara
2019-05-27 13:34     ` Jens Axboe
2019-07-18  8:15       ` Kai-Heng Feng [this message]
2019-07-30  9:29         ` Jan Kara
2019-07-30  9:36           ` John Lenton
2019-07-30 10:16             ` Jan Kara
2019-07-30 13:36               ` Jan Kara
2019-07-30 17:59                 ` Kai-Heng Feng
2019-07-30 19:17                 ` Jens Axboe
2019-07-30 21:11                   ` John Lenton
2019-07-31  8:56                   ` Jan Kara
2019-08-05 16:41                 ` Bart Van Assche
2019-08-05 21:01                   ` Tetsuo Handa
2019-08-07  9:45                   ` Jan Kara
2019-08-07 18:45                     ` Bart Van Assche
2019-08-08 13:37                     ` Jens Axboe
2019-07-30 10:16           ` Tetsuo Handa

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