bfq: Avoid merging queues with different parents

In the Linux kernel, the following vulnerability has been resolved:

bfq: Avoid merging queues with different parents

It can happen that the parent of a bfqq changes between the moment we

decide two queues are worth to merge (and set bic->stable_merge_bfqq)

and the moment bfq_setup_merge() is called. This can happen e.g. because

the process submitted IO for a different cgroup and thus bfqq got

reparented. It can even happen that the bfqq we are merging with has

parent cgroup that is already offline and going to be destroyed in which

case the merge can lead to use-after-free issues such as:

BUG: KASAN: use-after-free in __bfq_deactivate_entity+0x9cb/0xa50

Read of size 8 at addr ffff88800693c0c0 by task runc:[2:INIT]/10544

CPU: 0 PID: 10544 Comm: runc:[2:INIT] Tainted: G E 5.15.2-0.g5fb85fd-default #1 openSUSE Tumbleweed (unreleased) f1f3b891c72369aebecd2e43e4641a6358867c70

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014

Call Trace:

<IRQ>

dump_stack_lvl+0x46/0x5a

print_address_description.constprop.0+0x1f/0x140

? __bfq_deactivate_entity+0x9cb/0xa50

kasan_report.cold+0x7f/0x11b

? __bfq_deactivate_entity+0x9cb/0xa50

__bfq_deactivate_entity+0x9cb/0xa50

? update_curr+0x32f/0x5d0

bfq_deactivate_entity+0xa0/0x1d0

bfq_del_bfqq_busy+0x28a/0x420

? resched_curr+0x116/0x1d0

? bfq_requeue_bfqq+0x70/0x70

? check_preempt_wakeup+0x52b/0xbc0

__bfq_bfqq_expire+0x1a2/0x270

bfq_bfqq_expire+0xd16/0x2160

? try_to_wake_up+0x4ee/0x1260

? bfq_end_wr_async_queues+0xe0/0xe0

? _raw_write_unlock_bh+0x60/0x60

? _raw_spin_lock_irq+0x81/0xe0

bfq_idle_slice_timer+0x109/0x280

? bfq_dispatch_request+0x4870/0x4870

__hrtimer_run_queues+0x37d/0x700

? enqueue_hrtimer+0x1b0/0x1b0

? kvm_clock_get_cycles+0xd/0x10

? ktime_get_update_offsets_now+0x6f/0x280

hrtimer_interrupt+0x2c8/0x740

Fix the problem by checking that the parent of the two bfqqs we are

merging in bfq_setup_merge() is the same.