x86/svm: Add code to clobber the RSB on VM exit

CVE-2017-5753
CVE-2017-5715

Add code to overwrite the local CPU RSB entries from the previous less
privileged mode.
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(cherry picked from commit 3dc0cf238b89fb023fd5ee6cdf2dbff5ffd4046c)
Signed-off-by: Andy Whitcroft <apw@canonical.com>

arch/x86/kvm/svm.c

@@ -3932,6 +3932,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 			wrmsrl(MSR_IA32_SPEC_CTRL, FEATURE_ENABLE_IBRS);
 	}
 
+	stuff_RSB();
+
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
 #else