On 23/12/2020 15:47, Bart Van Assche wrote:
On 12/23/20 3:40 AM, John Garry wrote:
Sorry, I got the 2x iter functions mixed up.
So if we use mutex to solve blk_mq_queue_tag_busy_iter() problem, then we
still have this issue in blk_mq_tagset_busy_iter() which I report previously
[0]:
[ 319.771745] BUG: KASAN: use-after-free in bt_tags_iter+0xe0/0x128
[ 319.777832] Read of size 4 at addr ffff0010b6bd27cc by task more/1866
[ 319.784262]
[ 319.785753] CPU: 61 PID: 1866 Comm: more Tainted: G W
5.10.0-rc4-18118-gaa7b9c30d8ff #1070
[ 319.795312] Hardware name: Huawei Taishan 2280 /D05, BIOS Hisilicon
D05 IT21 Nemo 2.0 RC0 04/18/2018
[ 319.804437] Call trace:
[ 319.806892] dump_backtrace+0x0/0x2d0
[ 319.810552] show_stack+0x18/0x68
[ 319.813865] dump_stack+0x100/0x16c
[ 319.817348] print_address_description.constprop.12+0x6c/0x4e8
[ 319.823176] kasan_report+0x130/0x200
[ 319.826831] __asan_load4+0x9c/0xd8
[ 319.830315] bt_tags_iter+0xe0/0x128
[ 319.833884] __blk_mq_all_tag_iter+0x320/0x3a8
[ 319.838320] blk_mq_tagset_busy_iter+0x8c/0xd8
[ 319.842760] scsi_host_busy+0x88/0xb8
[ 319.846418] show_host_busy+0x1c/0x48
[ 319.850079] dev_attr_show+0x44/0x90
[ 319.853655] sysfs_kf_seq_show+0x128/0x1c8
[ 319.857744] kernfs_seq_show+0xa0/0xb8
[ 319.861489] seq_read_iter+0x1ec/0x6a0
[ 319.865230] seq_read+0x1d0/0x250
[ 319.868539] kernfs_fop_read+0x70/0x330
[ 319.872369] vfs_read+0xe4/0x250
[ 319.875590] ksys_read+0xc8/0x178
[ 319.878898] __arm64_sys_read+0x44/0x58
[ 319.882730] el0_svc_common.constprop.2+0xc4/0x1e8
[ 319.887515] do_el0_svc+0x90/0xa0
[ 319.890824] el0_sync_handler+0x128/0x178
[ 319.894825] el0_sync+0x158/0x180
[ 319.898131]
[ 319.899614] The buggy address belongs to the page:
[ 319.904403] page:000000004e9e6864 refcount:0 mapcount:0
mapping:0000000000000000 index:0x0 pfn:0x10b6bd2
[ 319.913876] flags: 0xbfffc0000000000()
[ 319.917626] raw: 0bfffc0000000000 0000000000000000 fffffe0000000000
0000000000000000
[ 319.925363] raw: 0000000000000000 0000000000000000 00000000ffffffff
0000000000000000
[ 319.933096] page dumped because: kasan: bad access detected
[ 319.938658]
[ 319.940141] Memory state around the buggy address:
[ 319.944925] ffff0010b6bd2680: ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff
[ 319.952139] ffff0010b6bd2700: ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff
[ 319.959354] >ffff0010b6bd2780: ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff
[ 319.966566] ^
[ 319.972131] ffff0010b6bd2800: ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff
[ 319.979344] ffff0010b6bd2880: ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff
[ 319.986557]
==================================================================
[ 319.993770] Disabling lock debugging due to kernel taint
So to trigger this, I start fio on a disk, and then have one script
which constantly enables and disables an IO scheduler for that disk, and
another script which constantly reads /sys/class/scsi_host/host0/host_busy .
And in this problem, the driver tag we iterate may point to a stale IO sched
request.
Hi John,
Hi Bart,
Sorry for the slow reply, but I was on vacation since before you sent
this mail.
I propose to change the order in which blk_mq_sched_free_requests(q) and
blk_mq_debugfs_unregister(q) are called. Today blk_mq_sched_free_requests(q)
is called by blk_cleanup_queue() before blk_put_queue() is called.
blk_put_queue() calls blk_release_queue() if the last reference is dropped.
blk_release_queue() calls blk_mq_debugfs_unregister(). I prefer removing the
debugfs attributes earlier over modifying the tag iteration functions
because I think removing the debugfs attributes earlier is less risky.
But don't we already have this following path to remove the per-hctx
debugfs dir earlier than blk_mq_sched_free_requests() or
blk_release_queue():
blk_cleanup_queue() -> blk_mq_exit_queue() -> blk_mq_exit_hw_queues() ->
blk_mq_debugfs_unregister_hctx() ->
blk_mq_debugfs_unregister_hctx(hctx->debugfs_dir)
Having said that, I am not sure how this is related directly to the
problem I mentioned. In that problem, above, we trigger the
blk_mq_tagset_busy_iter() from the SCSI host sysfs file, and the
use-after-free comes about from disabling the elevator (and freeing the
sched requests) in parallel.
Although this will make it harder to debug lockups that happen while
removing a request queue, kernel developers who are analyzing such an issue
can undo this change in their development kernel tree.
Thanks,
John
