linux-kernel - Re: [PATCH 6/9] KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal

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Message-ID: <60f7c9b3-312f-41e2-ab47-c4361df1d825@redhat.com>
Date: Sat, 15 Nov 2025 00:32:04 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Vitaly Kuznetsov <vkuznets@...hat.com>, "K. Y. Srinivasan"
 <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
 Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
Cc: kvm@...r.kernel.org, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
 Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH 6/9] KVM: SVM: Filter out 64-bit exit codes when invoking
 exit handlers on bare metal

On 11/13/25 23:56, Sean Christopherson wrote:
> Explicitly filter out 64-bit exit codes when invoking exit handlers, as
> svm_exit_handlers[] will never be sized with entries that use bits 63:32.
> 
> Processing the non-failing exit code as a 32-bit value will allow tracking
> exit_code as a single 64-bit value (which it is, architecturally).  This
> will also allow hardening KVM against Spectre-like attacks without needing
> to do silly things to avoid build failures on 32-bit kernels
> (array_index_nospec() rightly asserts that the index fits in an "unsigned
> long").
> 
> Omit the check when running as a VM, as KVM has historically failed to set
> bits 63:32 appropriately when synthesizing VM-Exits, i.e. KVM could get
> false positives when running as a VM on an older, broken KVM/kernel.  From
> a functional perspective, omitting the check is "fine", as any unwanted
> collision between e.g. VMEXIT_INVALID and a 32-bit exit code will be
> fatal to KVM-on-KVM regardless of what KVM-as-L1 does.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/svm/svm.c | 18 ++++++++++++++++--
>   1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 202a4d8088a2..3b05476296d0 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3433,8 +3433,22 @@ static void dump_vmcb(struct kvm_vcpu *vcpu)
>   		sev_free_decrypted_vmsa(vcpu, save);
>   }
>   
> -int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code)
> +int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 __exit_code)
>   {
> +	u32 exit_code = __exit_code;
> +
> +	/*
> +	 * SVM uses negative values, i.e. 64-bit values, to indicate that VMRUN
> +	 * failed.  Report all such errors to userspace (note, VMEXIT_INVALID,
> +	 * a.k.a. SVM_EXIT_ERR, is special cased by svm_handle_exit()).  Skip
> +	 * the check when running as a VM, as KVM has historically left garbage
> +	 * in bits 63:32, i.e. running KVM-on-KVM would hit false positives if
> +	 * the underlying kernel is buggy.
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR) &&
> +	    (u64)exit_code != __exit_code)
> +		goto unexpected_vmexit;

I reviewed the series and it looks good, but with respect to this patch 
and patch 8, is it really worth it?  While there is a possibility that 
code 0x00000000ffffffff is used, or that any high 32-bit values other 
than all-zeros or all-ones are used, they'd be presumably enabled by 
some control bits in the VMCB or some paravirt thing in the hypervisor.

What really matters is that SEV-ES's kvm_get_cached_sw_exit_code() is 
reading the full 64 bits and discarding invalid codes before reaching 
svm_invoke_exit_handler().

I totally agree, of course, with passing __exit_code as u64 and adding a 
comment explaining what's going on with "u32 exit_code == (u32)__exit_code".

Paolo

>   #ifdef CONFIG_MITIGATION_RETPOLINE
>   	if (exit_code == SVM_EXIT_MSR)
>   		return msr_interception(vcpu);
> @@ -3461,7 +3475,7 @@ int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code)
>   
>   unexpected_vmexit:
>   	dump_vmcb(vcpu);
> -	kvm_prepare_unexpected_reason_exit(vcpu, exit_code);
> +	kvm_prepare_unexpected_reason_exit(vcpu, __exit_code);
>   	return 0;
>   }
>