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From: Wanpeng Li <kernellwp@gmail.com>
Date: Fri, 8 Dec 2017 17:27:11 +0800
Message-ID: <CANRm+Cxn1pzHmT6sqm=MGSkrEMBzzrrZ3dwNz_m6J9bDY3jFDQ@mail.gmail.com>
Subject: Re: WARNING in x86_emulate_insn
To: Tianyu Lan <lantianyu1986@gmail.com>
Cc: Jim Mattson <jmattson@google.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>
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2017-12-08 16:28 GMT+08:00 Tianyu Lan <lantianyu1986@gmail.com>:
> 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 can observe ctxt->exception.vector == 0xff which triggers Dmitry's
report. Do you figure out the reason?

Regards,
Wanpeng Li

>
> 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);