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. :)
Thanks
Jianchao
