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

[Wanpeng Li]

2017-07-20 6:53 GMT+08:00 Nadav Amit <nadav.amit@xxxxxxxxx>:
> Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
>> 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?
>
> No. The problem is not with old_rflags now, but with rflags. If vm86_active,
> then rflags is changed and you donât compare the guest-visible rflags
> anymore.

Ah, I see. So we should compare the old_flags with the
rmode->save_rflags(guest-visible rflags) instead of the rflags (shadow
rflags), right?

Regards,
Wanpeng Li