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

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Message-ID: <CANRm+Cz04gsXJLkZn4++XBB8i481bOuEpk5_BCfWgzxCHaHRHg@mail.gmail.com>
Date:   Tue, 5 Dec 2017 08:53:11 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Jim Mattson <jmattson@...gle.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

2017-12-05 8:28 GMT+08:00 Jim Mattson <jmattson@...gle.com>:
> 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?

Yeah, it can happen. Which do you prefer, ioctl fails or |
X86_EFLAGS_FIXED unconditionally in the ioctl handler in kvm?

Regards,
Wanpeng Li

>
> 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