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From: Jens Axboe <axboe@kernel.dk>
To: Jan Kara <jack@suse.cz>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	linux-block@vger.kernel.org
Subject: Re: [PATCH] loop: Don't change loop device under exclusive opener
Date: Mon, 27 May 2019 07:34:28 -0600
Message-ID: <b0f27980-be75-bded-3e74-bce14fc7ea47@kernel.dk> (raw)
In-Reply-To: <20190527122915.GB9998@quack2.suse.cz>

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!

-- 
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 [this message]
2019-07-18  8:15       ` Kai-Heng Feng
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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