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

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Date:   Tue, 11 Jul 2017 17:54:58 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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

2017-07-11 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.