Date:   Wed, 8 May 2019 20:18:50 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+8d9bb6157e7b379f740e@...kaller.appspotmail.com>,
        KVM list <kvm@...r.kernel.org>, adrian.hunter@...el.com,
        David Miller <davem@...emloft.net>,
        Artem Bityutskiy <dedekind1@...il.com>, jbaron@...hat.com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mtd@...ts.infradead.org, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Rik van Riel <riel@...riel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: BUG: soft lockup in kvm_vm_ioctl

On Wed, May 08, 2019 at 06:58:28PM +0200, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> From: Dmitry Vyukov <dvyukov@...gle.com>
> Date: Wed, May 8, 2019 at 1:25 PM
> To: Eric Biggers
> Cc: syzbot, KVM list, <adrian.hunter@...el.com>, David Miller, Artem
> Bityutskiy, <jbaron@...hat.com>, Josh Poimboeuf, LKML,
> <linux-mtd@...ts.infradead.org>, Andy Lutomirski, Ingo Molnar, Peter
> Zijlstra, Richard Weinberger, Rik van Riel, Steven Rostedt,
> syzkaller-bugs, Thomas Gleixner
> 
> > From: Eric Biggers <ebiggers@...nel.org>
> > Date: Thu, May 2, 2019 at 4:34 AM
> > To: syzbot, Dmitry Vyukov, <kvm@...r.kernel.org>
> > Cc: <adrian.hunter@...el.com>, <davem@...emloft.net>,
> > <dedekind1@...il.com>, <jbaron@...hat.com>, <jpoimboe@...hat.com>,
> > <linux-kernel@...r.kernel.org>, <linux-mtd@...ts.infradead.org>,
> > <luto@...nel.org>, <mingo@...nel.org>, <peterz@...radead.org>,
> > <richard@....at>, <riel@...riel.com>, <rostedt@...dmis.org>,
> > <syzkaller-bugs@...glegroups.com>, <tglx@...utronix.de>
> >
> > > On Wed, May 01, 2019 at 07:36:05AM -0700, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot found the following crash on:
> > > >
> > > > HEAD commit:    baf76f0c slip: make slhc_free() silently accept an error p..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1407f57f200000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=a42d110b47dd6b36
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=8d9bb6157e7b379f740e
> > > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1266a588a00000
> > > >
> > > > The bug was bisected to:
> > > >
> > > > commit 252153ba518ac0bcde6b7152c63380d4415bfe5d
> > > > Author: Eric Biggers <ebiggers@...gle.com>
> > > > Date:   Wed Nov 29 20:43:17 2017 +0000
> > > >
> > > >     ubifs: switch to fscrypt_prepare_setattr()
> > > >
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1448f588a00000
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1648f588a00000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1248f588a00000
> > > >
> > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > Reported-by: syzbot+8d9bb6157e7b379f740e@...kaller.appspotmail.com
> > > > Fixes: 252153ba518a ("ubifs: switch to fscrypt_prepare_setattr()")
> > > >
> > > > watchdog: BUG: soft lockup - CPU#0 stuck for 123s! [syz-executor.3:22023]
> > > > Modules linked in:
> > > > irq event stamp: 26556
> > > > hardirqs last  enabled at (26555): [<ffffffff81006673>]
> > > > trace_hardirqs_on_thunk+0x1a/0x1c
> > > > hardirqs last disabled at (26556): [<ffffffff8100668f>]
> > > > trace_hardirqs_off_thunk+0x1a/0x1c
> > > > softirqs last  enabled at (596): [<ffffffff87400662>]
> > > > __do_softirq+0x662/0x95a kernel/softirq.c:320
> > > > softirqs last disabled at (517): [<ffffffff8144e4e0>] invoke_softirq
> > > > kernel/softirq.c:374 [inline]
> > > > softirqs last disabled at (517): [<ffffffff8144e4e0>] irq_exit+0x180/0x1d0
> > > > kernel/softirq.c:414
> > > > CPU: 0 PID: 22023 Comm: syz-executor.3 Not tainted 5.1.0-rc6+ #89
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > > Google 01/01/2011
> > > > RIP: 0010:csd_lock_wait kernel/smp.c:108 [inline]
> > > > RIP: 0010:smp_call_function_single+0x13e/0x420 kernel/smp.c:302
> > > > Code: 00 48 8b 4c 24 08 48 8b 54 24 10 48 8d 74 24 40 8b 7c 24 1c e8 23 fa
> > > > ff ff 41 89 c5 eb 07 e8 e9 87 0a 00 f3 90 44 8b 64 24 58 <31> ff 41 83 e4 01
> > > > 44 89 e6 e8 54 89 0a 00 45 85 e4 75 e1 e8 ca 87
> > > > RSP: 0018:ffff88809277f3e0 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
> > > > RAX: ffff8880a8bfc040 RBX: 1ffff110124efe80 RCX: ffffffff8166051c
> > > > RDX: 0000000000000000 RSI: ffffffff81660507 RDI: 0000000000000005
> > > > RBP: ffff88809277f4b8 R08: ffff8880a8bfc040 R09: ffffed1015d25be9
> > > > R10: ffffed1015d25be8 R11: ffff8880ae92df47 R12: 0000000000000003
> > > > R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000000
> > > > FS:  00007fd569980700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 00007fd56997e178 CR3: 00000000a4fd2000 CR4: 00000000001426f0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  smp_call_function_many+0x750/0x8c0 kernel/smp.c:434
> > > >  smp_call_function+0x42/0x90 kernel/smp.c:492
> > > >  on_each_cpu+0x31/0x200 kernel/smp.c:602
> > > >  text_poke_bp+0x107/0x19b arch/x86/kernel/alternative.c:821
> > > >  __jump_label_transform+0x263/0x330 arch/x86/kernel/jump_label.c:91
> > > >  arch_jump_label_transform+0x2b/0x40 arch/x86/kernel/jump_label.c:99
> > > >  __jump_label_update+0x16a/0x210 kernel/jump_label.c:389
> > > >  jump_label_update kernel/jump_label.c:752 [inline]
> > > >  jump_label_update+0x1ce/0x3d0 kernel/jump_label.c:731
> > > >  static_key_slow_inc_cpuslocked+0x1c1/0x250 kernel/jump_label.c:129
> > > >  static_key_slow_inc+0x1b/0x30 kernel/jump_label.c:144
> > > >  kvm_arch_vcpu_init+0x6b7/0x870 arch/x86/kvm/x86.c:9068
> > > >  kvm_vcpu_init+0x272/0x370 arch/x86/kvm/../../../virt/kvm/kvm_main.c:320
> > > >  vmx_create_vcpu+0x191/0x2540 arch/x86/kvm/vmx/vmx.c:6577
> > > >  kvm_arch_vcpu_create+0x80/0x120 arch/x86/kvm/x86.c:8755
> > > >  kvm_vm_ioctl_create_vcpu arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
> > > > [inline]
> > > >  kvm_vm_ioctl+0x5ce/0x19c0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3105
> > > >  vfs_ioctl fs/ioctl.c:46 [inline]
> > > >  file_ioctl fs/ioctl.c:509 [inline]
> > > >  do_vfs_ioctl+0xd6e/0x1390 fs/ioctl.c:696
> > > >  ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
> > > >  __do_sys_ioctl fs/ioctl.c:720 [inline]
> > > >  __se_sys_ioctl fs/ioctl.c:718 [inline]
> > > >  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
> > > >  do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
> > > >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > > RIP: 0033:0x458da9
> > > > Code: ad b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
> > > > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff
> > > > 0f 83 7b b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > > > RSP: 002b:00007fd56997fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000458da9
> > > > RDX: 0000000000000000 RSI: 000000000000ae41 RDI: 0000000000000005
> > > > RBP: 000000000073bfa0 R08: 0000000000000000 R09: 0000000000000000
> > > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007fd5699806d4
> > > > R13: 00000000004c1905 R14: 00000000004d40d0 R15: 00000000ffffffff
> > > > Sending NMI from CPU 0 to CPUs 1:
> >
> > > Can the KVM maintainers take a look at this?  This doesn't have anything to do
> > > with my commit that syzbot bisected it to.
> > >
> > > +Dmitry, statistics lession: if a crash occurs only 1 in 10 times, as was the
> > > case here, then often it will happen 0 in 10 times by chance.  syzbot needs to
> > > run the reproducer more times if it isn't working reliably.  Otherwise it ends
> > > up blaming some random commit.
> >
> > Added a note to https://github.com/google/syzkaller/issues/1051
> > Thanks
> 
> As we increase number of instances, we increase chances of hitting
> unrelated bugs. E.g. take a look at the bisection log for:
> https://syzkaller.appspot.com/bug?extid=f14868630901fc6151d3
> What is the optimum number of tests is a good question. I suspect that
> the current 10 instances is close to optimum. If we use significantly
> more we may break every other bisection on unrelated bugs...
> 

Only because syzbot is being super dumb in how it does the bisection.  AFAICS,
in the example you linked to, buggy kernels reliably crashed 10 out of 10 times
with the original crash signature, "WARNING in cgroup_exit".  Then at some point
it tested some kernel without the bug and got a different crash just 1 in 10
times, "WARNING: ODEBUG bug in netdev_freemem".

The facts that the crash frequency was very different, and the crash signature
was different, should be taken as a very strong signal that it's not the bug
being bisected for.  And this is something easily checked for in code.

BTW, I hope you're treating fixing this as a high priority, given that syzbot is
now sending bug reports to kernel developers literally selected at random.  This
is a great way to teach people to ignore syzbot reports.  (When I suggested
bisection originally, I had assumed you'd implement some basic sanity checks so
that only bisection results likely to be reliable would be mailed out.)

- Eric

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