Re: [PATCH] x86/kvm: move kvm_load/put_guest_xcr0 into atomic context

[Paolo Bonzini]

On 12/04/19 09:55, WANG Chao wrote:
> guest xcr0 could leak into host when MCE happens in guest mode. Because
> do_machine_check() could schedule out at a few places.
> 
> For example:
> 
> kvm_load_guest_xcr0
> ...
> kvm_x86_ops->run(vcpu) {
>   vmx_vcpu_run
>     vmx_complete_atomic_exit
>       kvm_machine_check
>         do_machine_check
>           do_memory_failure
>             memory_failure
>               lock_page
> 
> In this case, host_xcr0 is 0x2ff, guest vcpu xcr0 is 0xff. After schedule
> out, host cpu has guest xcr0 loaded (0xff).
> 
> In __switch_to {
>      switch_fpu_finish
>        copy_kernel_to_fpregs
>          XRSTORS
> 
> If any bit i in XSTATE_BV[i] == 1 and xcr0[i] == 0, XRSTORS will
> generate #GP (In this case, bit 9). Then ex_handler_fprestore kicks in
> and tries to reinitialize fpu by restoring init fpu state. Same story as
> last #GP, except we get DOUBLE FAULT this time.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: WANG Chao <chao.wang@xxxxxxxxx>

Thanks for the detailed commit message.  Patch queued!.

Paolo

> ---
>  arch/x86/kvm/svm.c     |  2 ++
>  arch/x86/kvm/vmx/vmx.c |  4 ++++
>  arch/x86/kvm/x86.c     | 10 ++++------
>  arch/x86/kvm/x86.h     |  2 ++
>  4 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index e0a791c3d4fc..2bf73076de7f 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5621,6 +5621,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	svm->vmcb->save.cr2 = vcpu->arch.cr2;
>  
>  	clgi();
> +	kvm_load_guest_xcr0(vcpu);
>  
>  	/*
>  	 * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> @@ -5766,6 +5767,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  	if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
>  		kvm_before_interrupt(&svm->vcpu);
>  
> +	kvm_put_guest_xcr0(vcpu);
>  	stgi();
>  
>  	/* Any pending NMI will happen here */
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ab432a930ae8..3157598c52f1 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6410,6 +6410,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
>  		vmx_set_interrupt_shadow(vcpu, 0);
>  
> +	kvm_load_guest_xcr0(vcpu);
> +
>  	if (static_cpu_has(X86_FEATURE_PKU) &&
>  	    kvm_read_cr4_bits(vcpu, X86_CR4_PKE) &&
>  	    vcpu->arch.pkru != vmx->host_pkru)
> @@ -6506,6 +6508,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  			__write_pkru(vmx->host_pkru);
>  	}
>  
> +	kvm_put_guest_xcr0(vcpu);
> +
>  	vmx->nested.nested_run_pending = 0;
>  	vmx->idt_vectoring_info = 0;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 099b851dabaf..22f66e9a7dc5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -800,7 +800,7 @@ void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw)
>  }
>  EXPORT_SYMBOL_GPL(kvm_lmsw);
>  
> -static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
> +void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
>  {
>  	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
>  			!vcpu->guest_xcr0_loaded) {
> @@ -810,8 +810,9 @@ static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
>  		vcpu->guest_xcr0_loaded = 1;
>  	}
>  }
> +EXPORT_SYMBOL_GPL(kvm_load_guest_xcr0);
>  
> -static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
> +void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
>  {
>  	if (vcpu->guest_xcr0_loaded) {
>  		if (vcpu->arch.xcr0 != host_xcr0)
> @@ -819,6 +820,7 @@ static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
>  		vcpu->guest_xcr0_loaded = 0;
>  	}
>  }
> +EXPORT_SYMBOL_GPL(kvm_put_guest_xcr0);
>  
>  static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
>  {
> @@ -7865,8 +7867,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		goto cancel_injection;
>  	}
>  
> -	kvm_load_guest_xcr0(vcpu);
> -
>  	if (req_immediate_exit) {
>  		kvm_make_request(KVM_REQ_EVENT, vcpu);
>  		kvm_x86_ops->request_immediate_exit(vcpu);
> @@ -7919,8 +7919,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	vcpu->mode = OUTSIDE_GUEST_MODE;
>  	smp_wmb();
>  
> -	kvm_put_guest_xcr0(vcpu);
> -
>  	kvm_before_interrupt(vcpu);
>  	kvm_x86_ops->handle_external_intr(vcpu);
>  	kvm_after_interrupt(vcpu);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 28406aa1136d..aedc5d0d4989 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -347,4 +347,6 @@ static inline void kvm_after_interrupt(struct kvm_vcpu *vcpu)
>  	__this_cpu_write(current_vcpu, NULL);
>  }
>  
> +void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu);
> +void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu);
>  #endif
>