Messages in this thread | | | Subject | Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs | From | John Garry <> | Date | Mon, 21 Dec 2020 12:06:35 +0000 |
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On 18/12/2020 22:43, Bart Van Assche wrote: > On 12/17/20 3:07 AM, John Garry wrote: >> References to old IO sched requests are currently cleared from the >> tagset when freeing those requests; switching elevator or changing >> request queue depth is such a scenario in which this occurs. >> >> However, this does not stop the potentially racy behaviour of freeing >> and clearing a request reference between a tagset iterator getting a >> reference to a request and actually dereferencing that request. >> >> Such a use-after-free can be triggered, as follows: >> >> ================================================================== >> BUG: KASAN: use-after-free in bt_iter+0xa0/0x120 >> Read of size 8 at addr ffff00108d589300 by task fio/3052 >> >> CPU: 32 PID: 3052 Comm: fio Tainted: GW >> 5.10.0-rc4-64839-g2dcf1ee5054f #693 >> Hardware name: Huawei Taishan 2280 /D05, BIOS Hisilicon >> D05 IT21 Nemo 2.0 RC0 04/18/2018 >> Call trace: >> dump_backtrace+0x0/0x2d0 >> show_stack+0x18/0x68 >> dump_stack+0x100/0x16c >> print_address_description.constprop.12+0x6c/0x4e8 >> kasan_report+0x130/0x200 >> __asan_load8+0x9c/0xd8 >> bt_iter+0xa0/0x120 >> blk_mq_queue_tag_busy_iter+0x2d8/0x540 >> blk_mq_in_flight+0x80/0xb8 >> part_stat_show+0xd8/0x238 >> dev_attr_show+0x44/0x90 >> sysfs_kf_seq_show+0x128/0x1c8 >> kernfs_seq_show+0xa0/0xb8 >> seq_read_iter+0x1ec/0x6a0 >> seq_read+0x1d0/0x250 >> kernfs_fop_read+0x70/0x330 >> vfs_read+0xe4/0x250 >> ksys_read+0xc8/0x178 >> __arm64_sys_read+0x44/0x58 >> el0_svc_common.constprop.2+0xc4/0x1e8 >> do_el0_svc+0x90/0xa0 >> el0_sync_handler+0x128/0x178 >> el0_sync+0x158/0x180 >> >> This is found experimentally by running fio on 2x SCSI disks - 1x disk >> holds the root partition. Userspace is constantly triggering the tagset >> iter from reading the root (gen)disk partition info. And so if the IO >> sched is constantly changed on the other disk, eventually the UAF occurs, >> as described above. > > Hi John,
Hi Bart,
> > Something is not clear to me. The above call stack includes > blk_mq_queue_tag_busy_iter(). That function starts with > percpu_ref_tryget(&q->q_usage_counter) and ends with calling > percpu_ref_put(&q->q_usage_counter). So it will only iterate over a tag set > if q->q_usage_counter is live. However, both blk_mq_update_nr_requests() > and elevator_switch() start with freezing the request queue. > blk_mq_freeze_queue() starts with killing q->q_usage_counter and waits > until that counter has dropped to zero. In other words, > blk_mq_queue_tag_busy_iter() should not iterate over a tag set while a tag > set is being freed or reallocated.
Right, this is what I thought, but Ming reminded me "blk_mq_queue_tag_busy_iter() can be run on another request queue just between one driver tag is allocated and updating the request map, so one extra request reference still can be grabbed."
> Does this mean that we do not yet have > a full explanation about why the above call stack can be triggered?
We understand it, and I'll describe my experiment in detail: a. fio runs on 2x disks, sda (root partition disk) and sdb. b. for sda, userpace triggers blk_mq_queue_tag_busy_iter(), as in stackframe above. Since its request queue is not frozen, it will iter the busy tags. c. on sdb, I continuously change the IO scheduler.
So sdb request queue gets frozen as we switch IO sched, but we could have this sequence of events: - blk_mq_queue_tag_busy_iter() on sda takes reference to a sdb request - Getting a tag and updating ->rqs[] in tagset is not atomic - requests for sdb cleared in tagset and request memory is freed - blk_mq_queue_tag_busy_iter() on sda still holds reference to sdb request and dereferences it -> UAF
Hope it's clear. It is a bit unlikely, I will admit, but it still can happen and UAF is never good. So please let me know if other idea to solve.
Thanks, John
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