KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal

Explicitly filter out 64-bit exit codes when invoking exit handlers, as
svm_exit_handlers[] will never be sized with entries that use bits 63:32.

Processing the non-failing exit code as a 32-bit value will allow tracking
exit_code as a single 64-bit value (which it is, architecturally).  This
will also allow hardening KVM against Spectre-like attacks without needing
to do silly things to avoid build failures on 32-bit kernels
(array_index_nospec() rightly asserts that the index fits in an "unsigned
long").

Omit the check when running as a VM, as KVM has historically failed to set
bits 63:32 appropriately when synthesizing VM-Exits, i.e. KVM could get
false positives when running as a VM on an older, broken KVM/kernel.  From
a functional perspective, omitting the check is "fine", as any unwanted
collision between e.g. VMEXIT_INVALID and a 32-bit exit code will be
fatal to KVM-on-KVM regardless of what KVM-as-L1 does.

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230211347.4099600-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -3467,8 +3467,22 @@ static void dump_vmcb(struct kvm_vcpu *vcpu)
 		sev_free_decrypted_vmsa(vcpu, save);
 }
 
-int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code)
+int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 __exit_code)
 {
+	u32 exit_code = __exit_code;
+
+	/*
+	 * SVM uses negative values, i.e. 64-bit values, to indicate that VMRUN
+	 * failed.  Report all such errors to userspace (note, VMEXIT_INVALID,
+	 * a.k.a. SVM_EXIT_ERR, is special cased by svm_handle_exit()).  Skip
+	 * the check when running as a VM, as KVM has historically left garbage
+	 * in bits 63:32, i.e. running KVM-on-KVM would hit false positives if
+	 * the underlying kernel is buggy.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR) &&
+	    (u64)exit_code != __exit_code)
+		goto unexpected_vmexit;
+
 #ifdef CONFIG_MITIGATION_RETPOLINE
 	if (exit_code == SVM_EXIT_MSR)
 		return msr_interception(vcpu);
@@ -3495,7 +3509,7 @@ int svm_invoke_exit_handler(struct kvm_vcpu *vcpu, u64 exit_code)
 
 unexpected_vmexit:
 	dump_vmcb(vcpu);
-	kvm_prepare_unexpected_reason_exit(vcpu, exit_code);
+	kvm_prepare_unexpected_reason_exit(vcpu, __exit_code);
 	return 0;
 }