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Date:   Mon, 11 Oct 2021 16:42:29 +0200
From:   Andrea Righi <andrea.righi@canonical.com>
To:     Marco Elver <elver@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: BUG: soft lockup in __kmalloc_node() with KFENCE enabled
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On Mon, Oct 11, 2021 at 12:03:52PM +0200, Marco Elver wrote:
> On Mon, 11 Oct 2021 at 11:53, Andrea Righi <andrea.righi@canonical.com> wrote:
> > On Mon, Oct 11, 2021 at 11:23:32AM +0200, Andrea Righi wrote:
> > ...
> > > > You seem to use the default 20s stall timeout. FWIW syzbot uses 160
> > > > secs timeout for TCG emulation to avoid false positive warnings:
> > > > https://github.com/google/syzkaller/blob/838e7e2cd9228583ca33c49a39aea4d863d3e36d/dashboard/config/linux/upstream-arm64-kasan.config#L509
> > > > There are a number of other timeouts raised as well, some as high as
> > > > 420 seconds.
> > >
> > > I see, I'll try with these settings and see if I can still hit the soft
> > > lockup messages.
> >
> > Still getting soft lockup messages even with the new timeout settings:
> >
> > [  462.663766] watchdog: BUG: soft lockup - CPU#2 stuck for 430s! [systemd-udevd:168]
> > [  462.755758] watchdog: BUG: soft lockup - CPU#3 stuck for 430s! [systemd-udevd:171]
> > [  924.663765] watchdog: BUG: soft lockup - CPU#2 stuck for 861s! [systemd-udevd:168]
> > [  924.755767] watchdog: BUG: soft lockup - CPU#3 stuck for 861s! [systemd-udevd:171]
> 
> The lockups are expected if you're hitting the TCG bug I linked. Try
> to pass '-enable-kvm' to the inner qemu instance (my bad if you
> already have), assuming that's somehow easy to do.

If I add '-enable-kvm' I can triggering other random panics (almost
immediately), like this one for example:

[21383.189976] BUG: kernel NULL pointer dereference, address: 0000000000000098
[21383.190633] #PF: supervisor read access in kernel mode
[21383.191072] #PF: error_code(0x0000) - not-present page
[21383.191529] PGD 0 P4D 0 
[21383.191771] Oops: 0000 [#1] SMP NOPTI
[21383.192113] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.15-rc4
[21383.192757] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
[21383.193414] RIP: 0010:wb_timer_fn+0x44/0x3c0
[21383.193855] Code: 41 8b 9c 24 98 00 00 00 41 8b 94 24 b8 00 00 00 41 8b 84 24 d8 00 00 00 4d 8b 74 24 28 01 d3 01 c3 49 8b 44 24 60 48 8b 40 78 <4c> 8b b8 98 00 00 00 4d 85 f6 0f 84 c4 00 00 00 49 83 7c 24 30 00
[21383.195366] RSP: 0018:ffffbcd140003e68 EFLAGS: 00010246
[21383.195842] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000004
[21383.196425] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9a3521f4fd80
[21383.197010] RBP: ffffbcd140003e90 R08: 0000000000000000 R09: 0000000000000000
[21383.197594] R10: 0000000000000004 R11: 000000000000000f R12: ffff9a34c75c4900
[21383.198178] R13: ffff9a34c3906de0 R14: 0000000000000000 R15: ffff9a353dc18c00
[21383.198763] FS:  0000000000000000(0000) GS:ffff9a353dc00000(0000) knlGS:0000000000000000
[21383.199558] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[21383.200212] CR2: 0000000000000098 CR3: 0000000005f54000 CR4: 00000000000006f0
[21383.200930] Call Trace:
[21383.201210]  <IRQ>
[21383.201461]  ? blk_stat_free_callback_rcu+0x30/0x30
[21383.202692]  blk_stat_timer_fn+0x138/0x140
[21383.203180]  call_timer_fn+0x2b/0x100
[21383.203666]  __run_timers.part.0+0x1d1/0x240
[21383.204227]  ? kvm_clock_get_cycles+0x11/0x20
[21383.204815]  ? ktime_get+0x3e/0xa0
[21383.205309]  ? native_apic_msr_write+0x2c/0x30
[21383.205914]  ? lapic_next_event+0x20/0x30
[21383.206412]  ? clockevents_program_event+0x94/0xf0
[21383.206873]  run_timer_softirq+0x2a/0x50
[21383.207260]  __do_softirq+0xcb/0x26f
[21383.207647]  irq_exit_rcu+0x8c/0xb0
[21383.208010]  sysvec_apic_timer_interrupt+0x7c/0x90
[21383.208464]  </IRQ>
[21383.208713]  asm_sysvec_apic_timer_interrupt+0x12/0x20

I think that systemd autotest used to use -enable-kvm, but then they
removed it, because it was introducing too many problems in the nested
KVM context. I'm not sure about the nature of those problems though, I
can investigate a bit and see if I can understand what they were
exactly.

-Andrea