diff --git a/arch/powerpc/kvm/book3s_rtas.c b/arch/powerpc/kvm/book3s_rtas.c
index 26b25994c969..41137ec72762 100644
--- a/arch/powerpc/kvm/book3s_rtas.c
+++ b/arch/powerpc/kvm/book3s_rtas.c
@@ -240,6 +240,17 @@ int kvmppc_rtas_hcall(struct kvm_vcpu *vcpu)
 	 * value so we can restore it on the way out.
 	 */
 	orig_rets = args.rets;
+	if (be32_to_cpu(args.nargs) >= ARRAY_SIZE(args.args)) {
+		/*
+		 * Don't overflow our args array: ensure there is room for
+		 * at least rets[0] (even if the call specifies 0 nret).
+		 *
+		 * Each handler must then check for the correct nargs and nret
+		 * values, but they may always return failure in rets[0].
+		 */
+		rc = -EINVAL;
+		goto fail;
+	}
 	args.rets = &args.args[be32_to_cpu(args.nargs)];
 
 	mutex_lock(&vcpu->kvm->arch.rtas_token_lock);
@@ -267,9 +278,17 @@ int kvmppc_rtas_hcall(struct kvm_vcpu *vcpu)
 fail:
 	/*
 	 * We only get here if the guest has called RTAS with a bogus
-	 * args pointer. That means we can't get to the args, and so we
-	 * can't fail the RTAS call. So fail right out to userspace,
-	 * which should kill the guest.
+	 * args pointer or nargs/nret values that would overflow the
+	 * array. That means we can't get to the args, and so we can't
+	 * fail the RTAS call. So fail right out to userspace, which
+	 * should kill the guest.
+	 *
+	 * SLOF should actually pass the hcall return value from the
+	 * rtas handler call in r3, so enter_rtas could be modified to
+	 * return a failure indication in r3 and we could return such
+	 * errors to the guest rather than failing to host userspace.
+	 * However old guests that don't test for failure could then
+	 * continue silently after errors, so for now we won't do this.
 	 */
 	return rc;
 }