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Date: Wed, 27 Oct 2021 15:29:16 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: Linux kernel: powerpc: KVM guest can trigger host crash on Power8
To: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
 mpe@ellerman.id.au
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 <05b88724-90b6-a38a-bb3b-7392f85c1934@physik.fu-berlin.de>
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Excerpts from John Paul Adrian Glaubitz's message of October 26, 2021 6:48 =
pm:
> Hi Michael!
>=20
>> The Linux kernel for powerpc since v5.2 has a bug which allows a
>> malicious KVM guest to crash the host, when the host is running on
>> Power8.
>>=20
>> Only machines using Linux as the hypervisor, aka. KVM, powernv or bare
>> metal, are affected by the bug. Machines running PowerVM are not
>> affected.
>>=20
>> The bug was introduced in:
>>=20
>>     10d91611f426 ("powerpc/64s: Reimplement book3s idle code in C")
>>=20
>> Which was first released in v5.2.
>>=20
>> The upstream fix is:
>>=20
>>   cdeb5d7d890e ("KVM: PPC: Book3S HV: Make idle_kvm_start_guest() return=
 0 if it went to guest")
>>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/com=
mit/?id=3Dcdeb5d7d890e14f3b70e8087e745c4a6a7d9f337
>>=20
>> Which will be included in the v5.16 release.
>=20
> I have tested these patches against 5.14 but it seems the problem [1] sti=
ll remains for me
> for big-endian guests. I built a patched kernel yesterday, rebooted the K=
VM server and let
> the build daemons do their work over night.
>=20
> When I got up this morning, I noticed the machine was down, so I checked =
the serial console
> via IPMI and saw the same messages again as reported in [1]:
>=20
> [41483.963562] watchdog: BUG: soft lockup - CPU#104 stuck for 25521s! [mi=
gration/104:175]
> [41507.963307] watchdog: BUG: soft lockup - CPU#104 stuck for 25544s! [mi=
gration/104:175]
> [41518.311200] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41518.311216] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2729959=20
> [41547.962882] watchdog: BUG: soft lockup - CPU#104 stuck for 25581s! [mi=
gration/104:175]
> [41571.962627] watchdog: BUG: soft lockup - CPU#104 stuck for 25603s! [mi=
gration/104:175]
> [41581.330530] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41581.330546] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2736378=20
> [41611.962202] watchdog: BUG: soft lockup - CPU#104 stuck for 25641s! [mi=
gration/104:175]
> [41635.961947] watchdog: BUG: soft lockup - CPU#104 stuck for 25663s! [mi=
gration/104:175]
> [41644.349859] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41644.349876] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2742753=20
> [41671.961564] watchdog: BUG: soft lockup - CPU#104 stuck for 25697s! [mi=
gration/104:175]
> [41695.961309] watchdog: BUG: soft lockup - CPU#104 stuck for 25719s! [mi=
gration/104:175]
> [41707.369190] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41707.369206] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2749151=20
> [41735.960884] watchdog: BUG: soft lockup - CPU#104 stuck for 25756s! [mi=
gration/104:175]
> [41759.960629] watchdog: BUG: soft lockup - CPU#104 stuck for 25778s! [mi=
gration/104:175]
> [41770.388520] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41770.388548] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2755540=20
> [41776.076307] rcu: rcu_sched kthread timer wakeup didn't happen for 1423=
 jiffies! g49897 f0x0 RCU_GP_WAIT_FQS(5) ->state=3D0x402
> [41776.076327] rcu:     Possible timer handling issue on cpu=3D32 timer-s=
oftirq=3D1056014
> [41776.076336] rcu: rcu_sched kthread starved for 1424 jiffies! g49897 f0=
x0 RCU_GP_WAIT_FQS(5) ->state=3D0x402 ->cpu=3D32
> [41776.076350] rcu:     Unless rcu_sched kthread gets sufficient CPU time=
, OOM is now expected behavior.
> [41776.076360] rcu: RCU grace-period kthread stack dump:
> [41776.076434] rcu: Stack dump where RCU GP kthread last ran:
> [41783.960374] watchdog: BUG: soft lockup - CPU#104 stuck for 25801s! [mi=
gration/104:175]
> [41807.960119] watchdog: BUG: soft lockup - CPU#104 stuck for 25823s! [mi=
gration/104:175]
> [41831.959864] watchdog: BUG: soft lockup - CPU#104 stuck for 25846s! [mi=
gration/104:175]
> [41833.407851] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
> [41833.407868] rcu:     136-...0: (135 ticks this GP) idle=3D242/1/0x4000=
000000000000 softirq=3D32031/32033 fqs=3D2760381=20
> [41863.959524] watchdog: BUG: soft lockup - CPU#104 stuck for 25875s! [mi=
gration/104:175]

I don't suppose you were able to get any more of the log saved? (The=20
first error messages that happened might be interesting)

Thanks,
Nick