linux-kernel - Re: [PATCH v2] KVM: VMX: Fix rflags cache during vCPU reset

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Message-ID: <CALMp9eQNhLcStivpk37DkSoY0qQHPobXAXsT8YKbP3Uwg0Gamg@mail.gmail.com>
Date:   Mon, 4 Dec 2017 16:28:52 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2] KVM: VMX: Fix rflags cache during vCPU reset

That seems like a convoluted path to produce an illegal RFLAGS value.
What's to prevent syzkaller from simply clearing bit 1 of RFLAGS with
the KVM_SET_REGS ioctl?

On Mon, Nov 20, 2017 at 4:34 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> 2017-11-21 7:09 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>> On 20/11/2017 23:52, Wanpeng Li wrote:
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index b348920..131fa1c 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -5590,6 +5590,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>>               vmcs_write64(GUEST_IA32_DEBUGCTL, 0);
>>>       }
>>>
>>> +     kvm_set_rflags(vcpu, 2);
>>>       vmcs_writel(GUEST_RFLAGS, 0x02);
>>
>> I think the vmcs_writel can go, kvm_set_rflags ends up calling
>> vmx_set_rflags.
>
> Agreed.
>
>>
>>>       kvm_rip_write(vcpu, 0xfff0);
>>>
>>>
>>
>> Beautified testcase:
>>
>>     #include <unistd.h>
>>     #include <sys/syscall.h>
>>     #include <string.h>
>>     #include <stdint.h>
>>     #include <linux/kvm.h>
>>     #include <fcntl.h>
>>     #include <sys/ioctl.h>
>>
>>     long r[5];
>>     int main()
>>     {
>>         struct kvm_debugregs dr = { 0 };
>>
>>         r[2] = open("/dev/kvm", O_RDONLY);
>>         r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
>>         r[4] = ioctl(r[3], KVM_CREATE_VCPU, 7);
>>         struct kvm_guest_debug debug = {
>>                 .control = 0xf0403,
>>                 .arch = {
>>                         .debugreg[6] = 0x2,
>>                         .debugreg[7] = 0x2
>>                 }
>>         };
>>         ioctl(r[4], KVM_SET_GUEST_DEBUG, &debug);
>>         ioctl(r[4], KVM_RUN, 0);
>>     }
>>
>> No need to do anything, I'll handle this patch after -rc1.
>
> Thanks for that. :)
>
> Regards,
> Wanpeng Li