KVM: SVM: fix missing sev_decommission in sev_receive_start

DECOMMISSION the current SEV context if binding an ASID fails after
RECEIVE_START.  Per AMD's SEV API, RECEIVE_START generates a new guest
context and thus needs to be paired with DECOMMISSION:

     The RECEIVE_START command is the only command other than the LAUNCH_START
     command that generates a new guest context and guest handle.

The missing DECOMMISSION can result in subsequent SEV launch failures,
as the firmware leaks memory and might not able to allocate more SEV
guest contexts in the future.

Note, LAUNCH_START suffered the same bug, but was previously fixed by
commit 934002cd ("KVM: SVM: Call SEV Guest Decommission if ASID
binding fails").

Cc: Alper Gun <alpergun@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: David Rienjes <rientjes@google.com>
Cc: Marc Orr <marcorr@google.com>
Cc: John Allen <john.allen@amd.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Vipin Sharma <vipinsh@google.com>
Cc: stable@vger.kernel.org
Reviewed-by: NMarc Orr <marcorr@google.com>
Acked-by: NBrijesh Singh <brijesh.singh@amd.com>
Fixes: af43cbbf ("KVM: SVM: Add support for KVM_SEV_RECEIVE_START command")
Signed-off-by: NMingwei Zhang <mizhang@google.com>
Reviewed-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210912181815.3899316-1-mizhang@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -1404,8 +1404,10 @@ static int sev_receive_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Bind ASID to this guest */
 	ret = sev_bind_asid(kvm, start.handle, error);
-	if (ret)
+	if (ret) {
+		sev_decommission(start.handle);
 		goto e_free_session;
+	}
 
 	params.handle = start.handle;
 	if (copy_to_user((void __user *)(uintptr_t)argp->data,