linux-kernel - Re: [GIT PULL] Block pull request for- 4.11-rc1

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Date:   Fri, 24 Feb 2017 13:00:39 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Bart Van Assche <Bart.VanAssche@...disk.com>
Cc:     "hch@....de" <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "snitzer@...hat.com" <snitzer@...hat.com>
Subject: Re: [GIT PULL] Block pull request for- 4.11-rc1

On 02/24/2017 12:43 PM, Linus Torvalds wrote:
> On Fri, Feb 24, 2017 at 9:39 AM, Bart Van Assche
> <Bart.VanAssche@...disk.com> wrote:
>>
>> So the crash is caused by an attempt to dereference address 0x6b6b6b6b6b6b6b6b
>> at offset 0x270. I think this means the crash is caused by a use-after-free.
> 
> Yeah, that's POISON_FREE, and that might explain why you see crashes
> that others don't - you obviously have SLAB poisoning enabled. Jens
> may not have.
> 
> %rdi is "struct mapped_device *md", which came from dm_softirq_done() doing
> 
>         struct dm_rq_target_io *tio = tio_from_request(rq);
>         struct request *clone = tio->clone;
>         int rw;
> 
>         if (!clone) {
>                 rq_end_stats(tio->md, rq);
>                 rw = rq_data_dir(rq);
>                 if (!rq->q->mq_ops)
>                         blk_end_request_all(rq, tio->error);
>                 else
>                         blk_mq_end_request(rq, tio->error);
>                 rq_completed(tio->md, rw, false);
>                 return;
>         }
> 
> so it's the 'tio' pointer that has been free'd. But it's worth noting
> that we did apparently successfully dereference "tio" earlier in that
> dm_softirq_done() *without* getting the poison value, so what I think
> might be going on is that the 'tio' thing gets free'd when the code
> does the blk_end_request_all()/blk_mq_end_request() call.
> 
> Which makes sense - that ends the lifetime of the request, which in
> turn also ends the lifetime of the "tio_from_request()", no?
> 
> So the fix may be as simple as just doing
> 
>         if (!clone) {
>                 struct mapped_device *md = tio->md;
> 
>                 rq_end_stats(md, rq);
>                 ...
>                 rq_completed(md, rw, false);
>                 return;
>         }
> 
> because the 'mapped_device' pointer hopefully is still valid, it's
> just 'tio' that has been freed.
> 
> Jens? Bart? Christoph? Somebody who knows this code should
> double-check my thinking above. I don't actually know the tio
> lifetimes, I'm just going by looking at how earlier accesses seemed to
> be fine (eg that "tio->clone" got us NULL, not a POISON_FREE pointer,
> for example).

I think that is spot on. With the request changes for CDBs, for non
blk-mq, we know also carry the payload after the request. But since
blk-mq never frees the request, the above use-after-free with poison
will only happen for !mq. Caching 'md' and avoiding a dereference of
'tio' after calling blk_end_request_all() will likely fix it.

Bart, can you test that?

-- 
Jens Axboe