Re: [PATCH v2] KVM: VMX: Fix invalid guest state detection after task-switch emulation

[Wanpeng Li]

2017-07-20 0:25 GMT+08:00 Nadav Amit <nadav.amit@xxxxxxxxx>:
> Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:
>
>> 2017-07-19 08:14-0700, Nadav Amit:
>>> Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:
>>>> @@ -2363,6 +2368,8 @@ static unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>>>>
>>>> static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>>>> {
>>>> +   unsigned long old_rflags = to_vmx(vcpu)->rflags;
>>>
>>> It assumes rflags was decached from the VMCS before. Probably it is true, butâ
>>
>> Right, it's better to use accessors everywhere, thanks.
>> The line should read:
>>
>> +     unsigned long old_rflags = vmx_get_rflags(vcpu);
>>
>> ---8<---
>> 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.
>>
>> Suggested-by: Nadav Amit <nadav.amit@xxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> [Wanpeng wrote the commit message with initial patch and Radim moved the
>> update to vmx_set_rflags and added Paolo's suggestion for the check.]
>> Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> ---
>> arch/x86/kvm/vmx.c | 15 ++++++++++-----
>> 1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 84e62acf2dd8..a776aea0043a 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2326,6 +2326,11 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
>>       __vmx_load_host_state(to_vmx(vcpu));
>> }
>>
>> +static bool emulation_required(struct kvm_vcpu *vcpu)
>> +{
>> +     return emulate_invalid_guest_state && !guest_state_valid(vcpu);
>> +}
>> +
>> static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu);
>>
>> /*
>> @@ -2363,6 +2368,8 @@ static unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>>
>> static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>> {
>> +     unsigned long old_rflags = vmx_get_rflags(vcpu);
>> +
>>       __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>>       to_vmx(vcpu)->rflags = rflags;
>>       if (to_vmx(vcpu)->rmode.vm86_active) {
>> @@ -2370,6 +2377,9 @@ static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>>               rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
>>       }
>>       vmcs_writel(GUEST_RFLAGS, rflags);
>> +
>> +     if ((old_rflags ^ rflags) & X86_EFLAGS_VM)
>> +             to_vmx(vcpu)->emulation_required = emulation_required(vcpu);
>
> Sorry for not pointing it before, but here you compare the old_rflags with
> the new rflags but after you already âmassagedâ it. So the value you compare
> with is not what the guest âseesâ.

So you mean we should use unsigned long old_rflags =
vmcs_readl(GUEST_RFLAGS); right?

Regards,
Wanpeng Li