On 12/20/18 12:48 PM, Jens Axboe wrote:
> On 12/19/18 9:32 PM, jianchao.wang wrote:
>>
>>
>> On 12/20/18 12:19 PM, Jens Axboe wrote:
>>> On 12/19/18 8:24 PM, jianchao.wang wrote:
>>>>
>>>>
>>>> On 12/20/18 11:17 AM, Jens Axboe wrote:
>>>>> On 12/19/18 5:16 PM, Bart Van Assche wrote:
>>>>>> On Wed, 2018-12-19 at 16:27 -0700, Jens Axboe wrote:
>>>>>>> On 12/19/18 4:24 PM, Bart Van Assche wrote:
>>>>>>>> Hello,
>>>>>>>>
>>>>>>>> If I run the srp blktests in a loop then I see the below call stack appearing
>>>>>>>> sporadically. I have not yet had the time to analyze this but I'm reporting
>>>>>>>> this here in case someone else would already have had a look at this.
>>>>>>>>
>>>>>>>> Bart.
>>>>>>>>
>>>>>>>> ==================================================================
>>>>>>>> BUG: KASAN: use-after-free in bt_iter+0x86/0xf0
>>>>>>>> Read of size 8 at addr ffff88803b335240 by task fio/21412
>>>>>>>>
>>>>>>>> CPU: 0 PID: 21412 Comm: fio Tainted: G W 4.20.0-rc6-dbg+ #3
>>>>>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
>>>>>>>> Call Trace:
>>>>>>>> dump_stack+0x86/0xca
>>>>>>>> print_address_description+0x71/0x239
>>>>>>>> kasan_report.cold.5+0x242/0x301
>>>>>>>> __asan_load8+0x54/0x90
>>>>>>>> bt_iter+0x86/0xf0
>>>>>>>> blk_mq_queue_tag_busy_iter+0x373/0x5e0
>>>>>>>> blk_mq_in_flight+0x96/0xb0
>>>>>>>> part_in_flight+0x40/0x140
>>>>>>>> part_round_stats+0x18e/0x370
>>>>>>>> blk_account_io_start+0x3d7/0x670
>>>>>>>> blk_mq_bio_to_request+0x19c/0x3a0
>>>>>>>> blk_mq_make_request+0x7a9/0xcb0
>>>>>>>> generic_make_request+0x41d/0x960
>>>>>>>> submit_bio+0x9b/0x250
>>>>>>>> do_blockdev_direct_IO+0x435c/0x4c70
>>>>>>>> __blockdev_direct_IO+0x79/0x88
>>>>>>>> ext4_direct_IO+0x46c/0xc00
>>>>>>>> generic_file_direct_write+0x119/0x210
>>>>>>>> __generic_file_write_iter+0x11c/0x280
>>>>>>>> ext4_file_write_iter+0x1b8/0x6f0
>>>>>>>> aio_write+0x204/0x310
>>>>>>>> io_submit_one+0x9d3/0xe80
>>>>>>>> __x64_sys_io_submit+0x115/0x340
>>>>>>>> do_syscall_64+0x71/0x210
>>>>>>>> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>>>>> RIP: 0033:0x7f02cf043219
>>>>>>>
>>>>>>> I've seen this one before as well, it's not a new thing. As far as I can
>>>>>>> tell, it's a false positive. There should be no possibility for a
>>>>>>> use-after-free iterating the static tags/requests.
>>>>>>
>>>>>> Are you sure this is a false positive?
>>>>>
>>>>> No I'm not, but the few times I have seen it, I haven't been able to
>>>>> make much sense of it. It goes back quite a bit.
>>>>>
>>>>> I have not yet encountered any false
>>>>>> positive KASAN complaints. According to the following gdb output this complaint
>>>>>> refers to reading rq->q:
>>>>>>
>>>>>> (gdb) list *(bt_iter+0x86)
>>>>>> 0xffffffff816b9346 is in bt_iter (block/blk-mq-tag.c:237).
>>>>>> 232
>>>>>> 233 /*
>>>>>> 234 * We can hit rq == NULL here, because the tagging functions
>>>>>> 235 * test and set the bit before assigning ->rqs[].
>>>>>> 236 */
>>>>>> 237 if (rq && rq->q == hctx->queue)
>>>>>> 238 iter_data->fn(hctx, rq, iter_data->data, reserved);
>>>>>> 239 return true;
>>>>>> 240 }
>>>>>> 241
>>>>>>
>>>>>> From the disassembly output:
>>>>>>
>>>>>> 232
>>>>>> 233 /*
>>>>>> 234 * We can hit rq == NULL here, because the tagging functions
>>>>>> 235 * test and set the bit before assigning ->rqs[].
>>>>>> 236 */
>>>>>> 237 if (rq && rq->q == hctx->queue)
>>>>>> 0xffffffff816b9339 <+121>: test %r12,%r12
>>>>>> 0xffffffff816b933c <+124>: je 0xffffffff816b935f <bt_iter+159>
>>>>>> 0xffffffff816b933e <+126>: mov %r12,%rdi
>>>>>> 0xffffffff816b9341 <+129>: callq 0xffffffff813bd3e0 <__asan_load8>
>>>>>> 0xffffffff816b9346 <+134>: lea 0x138(%r13),%rdi
>>>>>> 0xffffffff816b934d <+141>: mov (%r12),%r14
>>>>>> 0xffffffff816b9351 <+145>: callq 0xffffffff813bd3e0 <__asan_load8>
>>>>>> 0xffffffff816b9356 <+150>: cmp 0x138(%r13),%r14
>>>>>> 0xffffffff816b935d <+157>: je 0xffffffff816b936f <bt_iter+175>
>>>>>>
>>>>>> BTW, rq may but does not have to refer to tags->static_rqs[...]. It may also
>>>>>> refer to hctx->fq.flush_rq.
>>>>>
>>>>> But even those are persistent for the lifetime of the queue... But since
>>>>> kasan complains it belongs to a specific page, I'm guessing it's one
>>>>> of the regular requests since those are out of a chopped up page. Which
>>>>> means it makes even less sense.
>>>>>
>>>>> Is this happening while devices are being actively torn down? And
>>>>> are you using shared tags? That's the only way I could see this
>>>>> triggering.
>>>>>
>>>>
>>>> Or could it be caused by the stale request in hctx->tags->rqs[] slot ?
>>>> We don't clear it after free the requests.
>>>>
>>>> And there could be a scenario like,
>>>> There used to be a io scheduler attached.
>>>> After some workload, the io scheduler is detached.
>>>> So there could be rqs allocated by the io scheduler left in hctx->tags->rqs.
>>>>
>>>> blk_mq_get_request blk_mq_queue_tag_busy_iter
>>>> -> blk_mq_get_tag
>>>> -> bt_for_each
>>>> -> bt_iter
>>>> -> rq = taags->rqs[]
>>>> -> rq->q
>>>> -> blk_mq_rq_ctx_init
>>>> -> data->hctx->tags->rqs[rq->tag] = rq;
>>>>
>>>> If the scenario is possible, maybe we could fix it as following.
>>>
>>> Ah yes, good point, I bet that's what it is. But we just had this exact
>>> discussion in another thread, and my point there was that we should
>>> clear these when they go away, not inline. So how about clearing entries
>>> when the sched tags go away?
>>>
>> I guess it should be OK. :)
>
> Something like this. Totally untested... And I wonder if there's a more
> efficient way to do this, not that it matters THAT much. But still.
>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 2de972857496..341cb8b9cfb7 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2025,16 +2025,21 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> {
> struct page *page;
>
> - if (tags->rqs && set->ops->exit_request) {
> - int i;
> + if (tags->rqs) {
> + int i, j;
>
> for (i = 0; i < tags->nr_tags; i++) {
> struct request *rq = tags->static_rqs[i];
>
> if (!rq)
> continue;
> - set->ops->exit_request(set, rq, hctx_idx);
> + if (set->ops->exit_request)
> + set->ops->exit_request(set, rq, hctx_idx);
> tags->static_rqs[i] = NULL;
> +
> + for (j = 0; j < tags->nr_tags; j++)
> + if (tags->rqs[j] == rq)
> + tags->rqs[j] = NULL;
> }
> }
>
>
I'm afraid this cannot work.
The 'tags' here could be the hctx->sched_tags, but what we need to clear is hctx->tags->rqs[].
Thanks
Jianchao
