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

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 25, 2026, Jethro Beekman wrote:
> 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 it does fix that case, correct?  I don't want to complicate one fix just
because there are other bugs that are similar but yet distinct.

> but I don't believe this addresses the issues in snp_launch_update() and

Do you mean snp_launch_update_vmsa() here?  Or am I missing an interaction with
vCPUs in snp_launch_update()?

> sev_vcpu_create().

There are a pile of SEV lifecycle and locking issues, i.e. this is just one of
several flaws.  Fixing the locking has been on my todo list for a few months (we
found some "fun" bugs with an internal run of syzkaller), and I'm finally getting
to it.  Hopefully I'll post a series early next week.

Somewhat off the cuff, but I think the easiest way to close the race between
KVM_CREATE_VCPU and KVM_SEV_SNP_LAUNCH_FINISH is to reject KVM_SEV_SNP_LAUNCH_FINISH
if a vCPU is being created.  Or did I misunderstand the race you're pointing out?

Though unless there's a strong reason not to, I'd prefer to get greedy and block
all of sev_mem_enc_ioctl(), e.g.

11:23:23 ✔ ~/go/src/kernel.org/linux $ gdd
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index ea515cf41168..2b1033c0ec54 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2047,8 +2047,8 @@ static int sev_check_source_vcpus(struct kvm *dst, struct kvm *src)
        struct kvm_vcpu *src_vcpu;
        unsigned long i;
 
-       if (src->created_vcpus != atomic_read(&src->online_vcpus) ||
-           dst->created_vcpus != atomic_read(&dst->online_vcpus))
+       if (kvm_is_vcpu_creation_in_progress(src) ||
+           kvm_is_vcpu_creation_in_progress(dst))
                return -EBUSY;
 
        if (!sev_es_guest(src))
@@ -2596,6 +2596,11 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
                goto out;
        }
 
+       if (kvm_is_vcpu_creation_in_progress(kvm)) {
+               r = -EBUSY;
+               goto out;
+       }
+
        /*
         * Once KVM_SEV_INIT2 initializes a KVM instance as an SNP guest, only
         * allow the use of SNP-specific commands.
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2c7d76262898..60ca5222e1e5 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1032,6 +1032,13 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
        return NULL;
 }
 
+static inline bool kvm_is_vcpu_creation_in_progress(struct kvm *kvm)
+{
+       lockdep_assert_held(&kvm->lock);
+
+       return kvm->created_vcpus != atomic_read(&kvm->online_vcpus);
+}
+
 void kvm_destroy_vcpus(struct kvm *kvm);
 
 int kvm_trylock_all_vcpus(struct kvm *kvm);