linux-kernel - Re: [PATCH] KVM: VMX: Fix invalid guest state detection after task-switch emulation

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Date:   Wed, 12 Jul 2017 21:50:52 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] KVM: VMX: Fix invalid guest state detection after
 task-switch emulation

2017-07-12 0:09 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 11/07/2017 17:54, Radim Krčmář wrote:
>> 2017-07-11 00:13-0700, Wanpeng Li:
>>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>>
>>> This can be reproduced by EPT=1, unrestricted_guest=N, emulate_invalid_state=Y
>>> or EPT=0, the trace of kvm-unit-tests/taskswitch2.flat is like below, it tries
>>> to emulate invalid guest state task-switch:
>>>
>>> kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
>>> kvm_emulate_insn: 42000:0:0f 0b (0x2)
>>> kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
>>> kvm_inj_exception: #UD (0x0)
>>> kvm_entry: vcpu 0
>>> kvm_exit: reason TASK_SWITCH rip 0x0 info 40000058 0
>>> kvm_emulate_insn: 42000:0:0f 0b (0x2)
>>> kvm_emulate_insn: 42000:0:0f 0b (0x2) failed
>>> kvm_inj_exception: #UD (0x0)
>>> ......................
>>>
>>> It appears that the task-switch emulation updates rflags (and vm86
>>> flag) only after the segments are loaded, causing vmx->emulation_required
>>> to be set, when in fact invalid guest state emulation is not needed.
>>>
>>> This patch fixes it by updating vmx->emulation_required after the
>>> rflags (and vm86 flag) is updated in task-switch emulation.
>>>
>>> Suggested-by: Nadav Amit <nadav.amit@...il.com>
>>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>> Cc: Nadav Amit <nadav.amit@...il.com>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>> ---
>>>  arch/x86/kvm/vmx.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index f50cbfd..70270a2 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -6255,6 +6255,7 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
>>>       * TODO: What about debug traps on tss switch?
>>>       *       Are we supposed to inject them and update dr6?
>>>       */
>>> +    vmx->emulation_required = emulation_required(vcpu);
>>
>> Hm, so the problem happened because changes to rflags can flip the value
>> of emulation_required().  I would add this line to vmx_set_rflags() to
>> make sure that we fixed everything, thanks.
>
> Note that there is some extra complication, because emulation_required
> is expensive and you'll want to run it only when EFLAGS.VM changes.

Agreed.

Regards,
Wanpeng Li