Re: [PATCH] KVM: SEV: Track SNP launch state and disallow invalid userspace interactions

[Jethro Beekman]

On 2026-02-25 12:21, Sean Christopherson wrote:
> On Wed, Feb 25, 2026, Jethro Beekman wrote:
>> On 2026-02-25 12:05, Sean Christopherson wrote:
>>> On Mon, Jan 19, 2026, Jethro Beekman wrote:
>>>> Calling any of the SNP_LAUNCH_ ioctls after SNP_LAUNCH_FINISH results in a
>>>> kernel page fault due to RMP violation. Track SNP launch state and exit early.
>>>
>>> What exactly trips the RMP #PF?  A backtrace would be especially helpful for
>>> posterity.
>>
>> Here's a backtrace for calling ioctl(KVM_SEV_SNP_LAUNCH_FINISH) twice. Note this is with a modified version of QEMU.
> 
>> RIP: 0010:sev_es_sync_vmsa+0x54/0x4c0 [kvm_amd]
>>  snp_launch_update_vmsa+0x19d/0x290 [kvm_amd]
>>  snp_launch_finish+0xb6/0x380 [kvm_amd]
>>  sev_mem_enc_ioctl+0x14e/0x720 [kvm_amd]
>>  kvm_arch_vm_ioctl+0x837/0xcf0 [kvm]
> 
> Ah, it's the VMSA that's being accessed.  Can't we just do?
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 723f4452302a..1e40ae592c93 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -882,6 +882,9 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>         u8 *d;
>         int i;
>  
> +       if (vcpu->arch.guest_state_protected)
> +               return -EINVAL;
> +
>         /* Check some debug related fields before encrypting the VMSA */
>         if (svm->vcpu.guest_debug || (svm->vmcb->save.dr7 & ~DR7_FIXED_1))
>                 return -EINVAL;

I tried relying on guest_state_protected instead of creating new state but I don't think it's sufficient. In particular, your proposal may fix snp_launch_finish() but I don't believe this addresses the issues in snp_launch_update() and sev_vcpu_create().

-- 
Jethro Beekman | CTO | Fortanix