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

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/07/2017 17:54, Radim Krčmář wrote:
> 2017-07-11 00:13-0700, Wanpeng Li:
>> From: Wanpeng Li <wanpeng.li@hotmail.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@gmail.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: Nadav Amit <nadav.amit@gmail.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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.

Paolo