On Fri, Feb 24, 2017 at 9:39 AM, Bart Van Assche
<Bart.VanAssche@xxxxxxxxxxx> 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).
Linus
