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

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Message-Id: <DA8734AE-E457-4BA7-A066-2A87E10223A6@gmail.com>
Date:   Wed, 19 Jul 2017 09:25:06 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        "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>
Subject: Re: [PATCH v2] KVM: VMX: Fix invalid guest state detection after
 task-switch emulation

Radim Krčmář <rkrcmar@...hat.com> wrote:

> 2017-07-19 08:14-0700, Nadav Amit:
>> Radim Krčmář <rkrcmar@...hat.com> 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, but…
> 
> 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@...il.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> [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 Krčmář <rkrcmar@...hat.com>
> ---
> 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 “massaged” it. So the value you compare
with is not what the guest “sees”.