KVM: SVM: Harden exit_code against being used in Spectre-like attacks

Explicitly clamp the exit code used to index KVM's exit handlers to guard
against Spectre-like attacks, mainly to provide consistency between VMX
and SVM (VMX was given the same treatment by commit c926f2f ("KVM:
x86: Protect exit_reason from being used in Spectre-v1/L1TF attacks").

For normal VMs, it's _extremely_ unlikely the exit code could be used to
exploit a speculation vulnerability, as the exit code is set by hardware
and unexpected/unknown exit codes should be quite well bounded (as is/was
the case with VMX).  But with SEV-ES+, the exit code is guest-controlled
as it comes from the GHCB, not from hardware, i.e. an attack from the
guest is at least somewhat plausible.

Irrespective of SEV-ES+, hardening KVM is easy and inexpensive, and such
an attack is theoretically possible.

Link: https://patch.msgid.link/20251230211347.4099600-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -3501,6 +3501,7 @@ int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 __exit_code)
 	if (exit_code >= ARRAY_SIZE(svm_exit_handlers))
 		goto unexpected_vmexit;
 
+	exit_code = array_index_nospec(exit_code, ARRAY_SIZE(svm_exit_handlers));
 	if (!svm_exit_handlers[exit_code])
 		goto unexpected_vmexit;