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

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Date:   Thu, 20 Jul 2017 16:04:16 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v2] KVM: VMX: Fix invalid guest state detection after
 task-switch emulation

2017-07-20 15:57 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-07-19 16:40-0700, 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.
>>
>> Thanks Radim for moving the update to vmx__set_flags and adding Paolo's
>> suggestion for the check.
>>
>> 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>
>> ---
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>  static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>>  {
>> +     unsigned long old_rflags = vmx_get_rflags(vcpu);
>> +     unsigned long save_rflags = rflags;
>> +
>>       __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>>       to_vmx(vcpu)->rflags = rflags;
>>       if (to_vmx(vcpu)->rmode.vm86_active) {
>> @@ -2370,6 +2378,9 @@ 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 ^ save_rflags) & X86_EFLAGS_VM)
>
> Looks good, thanks to both.
>
> The desired rflags are saved in to_vmx(vcpu)->rflags, so we don't need
> the temporary variable -- would using that or vmx_get_rflags(vcpu)
> generate worse code?

Yeah, I will send out a newer version.

Regards,
Wanpeng Li