KVM: SEV: Save the SEV policy if and only if LAUNCH_START succeeds

[PATCH] KVM: SEV: Save the SEV policy if and only if LAUNCH_START succeeds

Posted by Sean Christopherson 1 month, 1 week ago

Wait until LAUNCH_START fully succeeds to set a VM's SEV/SNP policy so
that KVM doesn't keep a potentially stale policy.  In practice, the issue
is benign as the policy is only used to detect if the VMSA can be
decrypted, and the VMSA only needs to be decrypted if LAUNCH_UPDATE and
thus LAUNCH_START succeeded.

Fixes: 962e2b6152ef ("KVM: SVM: Decrypt SEV VMSA in dump_vmcb() if debugging is enabled")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f4381878a9e5..65b59939754c 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -583,8 +583,6 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (copy_from_user(&params, u64_to_user_ptr(argp->data), sizeof(params)))
 		return -EFAULT;
 
-	sev->policy = params.policy;
-
 	memset(&start, 0, sizeof(start));
 
 	dh_blob = NULL;
@@ -632,6 +630,7 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		goto e_free_session;
 	}
 
+	sev->policy = params.policy;
 	sev->handle = start.handle;
 	sev->fd = argp->sev_fd;
 
@@ -2201,8 +2200,6 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		start.desired_tsc_khz = kvm->arch.default_tsc_khz;
 	}
 
-	sev->policy = params.policy;
-
 	sev->snp_context = snp_context_create(kvm, argp);
 	if (!sev->snp_context)
 		return -ENOTTY;
@@ -2218,6 +2215,7 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		goto e_free_context;
 	}
 
+	sev->policy = params.policy;
 	sev->fd = argp->sev_fd;
 	rc = snp_bind_asid(kvm, &argp->error);
 	if (rc) {

base-commit: ecbcc2461839e848970468b44db32282e5059925
-- 
2.51.0.261.g7ce5a0a67e-goog

Re: [PATCH] KVM: SEV: Save the SEV policy if and only if LAUNCH_START succeeds

Posted by Sean Christopherson 2 weeks, 3 days ago

On Thu, 21 Aug 2025 14:38:41 -0700, Sean Christopherson wrote:
> Wait until LAUNCH_START fully succeeds to set a VM's SEV/SNP policy so
> that KVM doesn't keep a potentially stale policy.  In practice, the issue
> is benign as the policy is only used to detect if the VMSA can be
> decrypted, and the VMSA only needs to be decrypted if LAUNCH_UPDATE and
> thus LAUNCH_START succeeded.
> 
> 
> [...]

Applied to kvm-x86 svm, thanks!

[1/1] KVM: SEV: Save the SEV policy if and only if LAUNCH_START succeeds
      https://github.com/kvm-x86/linux/commit/2f5f8fb9de09

--
https://github.com/kvm-x86/linux/tree/next

Re: [PATCH] KVM: SEV: Save the SEV policy if and only if LAUNCH_START succeeds

Posted by Tom Lendacky 1 month, 1 week ago

On 8/21/25 16:38, Sean Christopherson wrote:
> Wait until LAUNCH_START fully succeeds to set a VM's SEV/SNP policy so
> that KVM doesn't keep a potentially stale policy.  In practice, the issue
> is benign as the policy is only used to detect if the VMSA can be
> decrypted, and the VMSA only needs to be decrypted if LAUNCH_UPDATE and
> thus LAUNCH_START succeeded.
> 
> Fixes: 962e2b6152ef ("KVM: SVM: Decrypt SEV VMSA in dump_vmcb() if debugging is enabled")
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Kim Phillips <kim.phillips@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  arch/x86/kvm/svm/sev.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index f4381878a9e5..65b59939754c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -583,8 +583,6 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  	if (copy_from_user(&params, u64_to_user_ptr(argp->data), sizeof(params)))
>  		return -EFAULT;
>  
> -	sev->policy = params.policy;
> -
>  	memset(&start, 0, sizeof(start));
>  
>  	dh_blob = NULL;
> @@ -632,6 +630,7 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  		goto e_free_session;
>  	}
>  
> +	sev->policy = params.policy;
>  	sev->handle = start.handle;
>  	sev->fd = argp->sev_fd;
>  
> @@ -2201,8 +2200,6 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  		start.desired_tsc_khz = kvm->arch.default_tsc_khz;
>  	}
>  
> -	sev->policy = params.policy;
> -
>  	sev->snp_context = snp_context_create(kvm, argp);
>  	if (!sev->snp_context)
>  		return -ENOTTY;
> @@ -2218,6 +2215,7 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  		goto e_free_context;
>  	}
>  
> +	sev->policy = params.policy;
>  	sev->fd = argp->sev_fd;
>  	rc = snp_bind_asid(kvm, &argp->error);
>  	if (rc) {
> 
> base-commit: ecbcc2461839e848970468b44db32282e5059925