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 09:57:05 +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 v2] KVM: VMX: Fix invalid guest state detection after
 task-switch emulation

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?