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From: Tianyu Lan <lantianyu1986@gmail.com>
Date: Fri, 8 Dec 2017 16:48:17 +0800
Message-ID: <CAOLK0pyeBa7XXn7ajJJ8SCeHsM_BwWFXT_p+r-o8aTzthJPV3w@mail.gmail.com>
Subject: Re: WARNING in x86_emulate_insn
To: Ingo Molnar <mingo@kernel.org>
Cc: Jim Mattson <jmattson@google.com>, Wanpeng Li <kernellwp@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        syzbot 
        <bot+75375385991b4f8c599704a10849863c586ea284@syzkaller.appspotmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, kvm <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>, syzkaller-bugs@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andrew Jones <drjones@redhat.com>
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2017-12-08 16:44 GMT+08:00 Ingo Molnar <mingo@kernel.org>:
>
> * Tianyu Lan <lantianyu1986@gmail.com> wrote:
>
>> Hi Jim&Wanpeng:
>>          Thanks for your help.
>>
>> 2017-12-08 5:25 GMT+08:00 Jim Mattson <jmattson@google.com>:
>> > Try disabling the module parameter, "unrestricted_guest." Make sure
>> > that the module parameter, "emulate_invalid_guest_state" is enabled.
>> > This combination allows userspace to feed invalid guest state into the
>> > in-kernel emulator.
>>
>> Yes, you are right. I need to disable unrestricted_guest to reproduce the issue.
>>
>> I find this is pop instruction emulation issue. According "SDM VOL2,
>> chapter INSTRUCTION
>> SET REFERENCE. POP—Pop a Value from the Stack"
>>
>> Protected Mode Exceptions
>> #GP(0) If attempt is made to load SS register with NULL segment selector.
>>
>> This test case hits it but current code doesn't check such case.
>> The following patch can fix the issue.
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index abe74f7..e2ac5cc 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -1844,6 +1844,9 @@ static int emulate_pop(struct x86_emulate_ctxt *ctxt,
>>         int rc;
>>         struct segmented_address addr;
>>
>> +       if ( !get_segment_selector(ctxt, VCPU_SREG_SS))
>> +               return emulate_gp(ctxt, 0);
>> +
>>         addr.ea = reg_read(ctxt, VCPU_REGS_RSP) & stack_mask(ctxt);
>>         addr.seg = VCPU_SREG_SS;
>>         rc = segmented_read(ctxt, addr, dest, len);
>
> s/if ( !get_segment_selector
>  /if (!get_segment_selector

Sorry. I mixed xen and kernel code style...

>
> I think it would also be nice to convert the syzkaller testcase to a new KVM unit
> test:

Sure. I will add it.

>
>   git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git
>
> There's a test_pop() function in kvm-unit-tests/x86/emulator.c.
>
> Thanks,
>
>         Ingo


-- 
Best regards
Tianyu Lan