linux-kernel - Re: kvm: WARNING in em_jmp

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Date:   Tue, 15 Nov 2016 06:30:53 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzkaller <syzkaller@...glegroups.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steve Rutherford <srutherford@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: kvm: WARNING in em_jmp_far

On Tue, Nov 15, 2016 at 6:24 AM, Nadav Amit <nadav.amit@...il.com> wrote:
>
>> On Nov 14, 2016, at 9:06 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>
>> Hello,
>>
>> The following program triggers WARNING in em_jmp_far:
>> https://gist.githubusercontent.com/dvyukov/16bfd3d68fa7d5461101ef74e07796e4/raw/e6d663980681f2c5838ff6cd361cede7d3204838/gistfile1.txt
>>
>>
>> WARNING: CPU: 1 PID: 15748 at arch/x86/kvm/emulate.c:2128 em_jmp_far+0x4a7/0x530
>
> I don’t know how to “read” the test, but it seems that this warning
> can be triggered if CS base/limit cause a #GP exception when EIP
> is loaded.
>
> I think it safe to remove this warning (which I introduced) as well as
> the redundant “return rc” that follows it. The code should handle the
> emulation correctly regardless of the warning.

There was also a similar WARNING in em_ret_far:
https://groups.google.com/forum/#!msg/syzkaller/o5ZftARBhrs/r1ivQ-HtBgAJ

Please mail a fix and add a test.

Thanks