linux-kernel - Re: [PATCH] KVM: VMX: downgrade warning on unexpected exit code

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Date:   Fri, 10 Mar 2017 19:56:49 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steve Rutherford <srutherford@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH] KVM: VMX: downgrade warning on unexpected exit code

On Sun, Jan 15, 2017 at 8:18 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> 2017-01-02 11:23+0100, Dmitry Vyukov:
>>> Hello,
>>>
>>> I've got the following warning while running syzkaller fuzzer:
>>>
>>> WARNING: CPU: 2 PID: 13257 at arch/x86/kvm/vmx.c:8633
>>> vmx_handle_exit+0x262b/0x38b0 arch/x86/kvm/vmx.c:8633
>>> vmx: unexpected exit reason 0xb
>>> CPU: 2 PID: 13257 Comm: syz-executor7 Not tainted 4.10.0-rc1+ #118
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>> Call Trace:
>>>  __dump_stack lib/dump_stack.c:15 [inline]
>>>  dump_stack+0x292/0x3a2 lib/dump_stack.c:51
>>>  panic+0x1cb/0x3a9 kernel/panic.c:179
>>>  __warn+0x1c4/0x1e0 kernel/panic.c:539
>>>  warn_slowpath_fmt+0xc5/0x100 kernel/panic.c:562
>>>  vmx_handle_exit+0x262b/0x38b0 arch/x86/kvm/vmx.c:8633
>>>  vcpu_enter_guest arch/x86/kvm/x86.c:6884 [inline]
>>>  vcpu_run arch/x86/kvm/x86.c:6943 [inline]
>>>  kvm_arch_vcpu_ioctl_run+0xf3d/0x45f0 arch/x86/kvm/x86.c:7101
>>>  kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
>>>  vfs_ioctl fs/ioctl.c:43 [inline]
>>>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>>>  SYSC_ioctl fs/ioctl.c:698 [inline]
>>>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>>>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>>> RIP: 0033:0x4438a9
>>> RSP: 002b:00007f935aa2fb58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
>>> RAX: ffffffffffffffda RBX: 0000000000000015 RCX: 00000000004438a9
>>> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000015
>>> RBP: 00000000006ddb30 R08: 0000000000000000 R09: 0000000000000000
>>> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700000
>>> R13: 0000000000000006 R14: 0000000000000015 R15: 0000000020014000
>>> Dumping ftrace buffer:
>>>    (ftrace buffer empty)
>>> Kernel Offset: disabled
>>>
>>> Exit reason 0xb is GETSEC instruction. This does not look harmful as
>>> it is handled as #UD. But I think we should print a single line
>>> message regarding non-emulated instruction as in other cases, just to
>>> not scare cloud admins and to make syzkaller ignore it.
>>>
>>> FTR, a raw reproducer is here:
>>> https://gist.githubusercontent.com/dvyukov/c762f6ea04ebbba49cdee0a6caca31b4/raw/f21deb04cdc70ae74100c12447d71bb0cd2025c7/gistfile1.txt
>>
>> I can't reproduce -- maybe a nested bug, which will take a while to
>> figure out.  Still, host dump at that point is useless, so the change
>> makes sense.
>>
>> A guest dump would be useful (e.g. I think that GETSEC should not exit
>
> Thanks!
>
> I had to disable dump_vmcs locally because it constantly produces tons
> of output even when no bugs happen.
> If you really-really need it, I can enable dump_vmcs and re-reproduce the bug.
>
>
>> if guest CR4.SMX is disabled), but just giving the error is about as
>> good and we want something short and rate-limited if the message can be
>> trigerred by a guest in production ...
>> ---8<---
>> We never needed the call trace and we better rate-limit if it can be
>> triggered by a guest.
>>
>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
>> ---
>>  arch/x86/kvm/vmx.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index a236decb81e4..7cd606daa01e 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8595,7 +8595,8 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
>>             && kvm_vmx_exit_handlers[exit_reason])
>>                 return kvm_vmx_exit_handlers[exit_reason](vcpu);
>>         else {
>> -               WARN_ONCE(1, "vmx: unexpected exit reason 0x%x\n", exit_reason);
>> +               vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
>> +                               exit_reason);
>>                 kvm_queue_exception(vcpu, UD_VECTOR);
>>                 return 1;
>>         }
>> --
>> 2.11.0
>>

Was this ever merged? Still happens for me.