Reject synchronizing vCPU state to its associated VMSA if the vCPU has
already been launched, i.e. if the VMSA has already been encrypted.  On a
host with SNP enabled, accessing guest-private memory generates an RMP #PF
and panics the host.

  BUG: unable to handle page fault for address: ff1276cbfdf36000
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x80000003) - RMP violation
  PGD 5a31801067 P4D 5a31802067 PUD 40ccfb5063 PMD 40e5954063 PTE 80000040fdf36163
  SEV-SNP: PFN 0x40fdf36, RMP entry: [0x6010fffffffff001 - 0x000000000000001f]
  Oops: Oops: 0003 [#1] SMP NOPTI
  CPU: 33 UID: 0 PID: 996180 Comm: qemu-system-x86 Tainted: G           OE
  Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
  Hardware name: Dell Inc. PowerEdge R7625/0H1TJT, BIOS 1.5.8 07/21/2023
  RIP: 0010:sev_es_sync_vmsa+0x54/0x4c0 [kvm_amd]
  Call Trace:
   <TASK>
   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]
   kvm_vm_ioctl+0x3fd/0xcc0 [kvm]
   __x64_sys_ioctl+0xa3/0x100
   x64_sys_call+0xfe0/0x2350
   do_syscall_64+0x81/0x10f0
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7ffff673287d
   </TASK>

Note, the KVM flaw has been present since commit ad73109ae7ec ("KVM: SVM:
Provide support to launch and run an SEV-ES guest"), but has only been
actively dangerous for the host since SNP support was added.  With SEV-ES,
KVM would "just" clobber guest state, which is totally fine from a host
kernel perspective since userspace can clobber guest state any time before
sev_launch_update_vmsa().

Fixes: ad27ce155566 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command")
Reported-by: Jethro Beekman <jethro@fortanix.com>
Closes: https://lore.kernel.org/all/d98692e2-d96b-4c36-8089-4bc1e5cc3d57@fortanix.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 3f9c1aa39a0a..fa319a66938c 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;
-- 
2.53.0.473.g4a7958ca14-goog