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
[off-list ref] wrote:

quoted

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