x86/srso: Add IBPB on VMEXIT

Upstream commit: d893832d0e1ef41c72cdae444268c1d64a2be8ad

Add the option to flush IBPB only on VMEXIT in order to protect from
malicious guests but one otherwise trusts the software that runs on the
hypervisor.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index a8c2733c816cdf01e7da3f08986589f45e488909..e721b8426c245bf6517b7de038e241e4d534ca7e 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -310,6 +310,7 @@
 
 #define X86_FEATURE_SRSO               (11*32+24) /* "" AMD BTB untrain RETs */
 #define X86_FEATURE_SRSO_ALIAS         (11*32+25) /* "" AMD BTB untrain RETs through aliasing */
+#define X86_FEATURE_IBPB_ON_VMEXIT     (11*32+26) /* "" Issue an IBPB only on VMEXIT */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX_VNNI           (12*32+ 4) /* AVX VNNI instructions */

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 82417ce310cc5c5e1ff59f394a2f9586077b914d..af612778a328aa360f260c2416083ce651342a7f 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2316,6 +2316,7 @@ enum srso_mitigation {
        SRSO_MITIGATION_MICROCODE,
        SRSO_MITIGATION_SAFE_RET,
        SRSO_MITIGATION_IBPB,
+       SRSO_MITIGATION_IBPB_ON_VMEXIT,
 };
 
 enum srso_mitigation_cmd {
@@ -2323,6 +2324,7 @@ enum srso_mitigation_cmd {
        SRSO_CMD_MICROCODE,
        SRSO_CMD_SAFE_RET,
        SRSO_CMD_IBPB,
+       SRSO_CMD_IBPB_ON_VMEXIT,
 };
 
 static const char * const srso_strings[] = {
@@ -2330,6 +2332,7 @@ static const char * const srso_strings[] = {
        [SRSO_MITIGATION_MICROCODE]      = "Mitigation: microcode",
        [SRSO_MITIGATION_SAFE_RET]       = "Mitigation: safe RET",
        [SRSO_MITIGATION_IBPB]           = "Mitigation: IBPB",
+       [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only"
 };
 
 static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
@@ -2348,6 +2351,8 @@ static int __init srso_parse_cmdline(char *str)
                srso_cmd = SRSO_CMD_SAFE_RET;
        else if (!strcmp(str, "ibpb"))
                srso_cmd = SRSO_CMD_IBPB;
+       else if (!strcmp(str, "ibpb-vmexit"))
+               srso_cmd = SRSO_CMD_IBPB_ON_VMEXIT;
        else
                pr_err("Ignoring unknown SRSO option (%s).", str);
 
@@ -2431,6 +2436,20 @@ static void __init srso_select_mitigation(void)
                        pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
                        goto pred_cmd;
                }
+               break;
+
+       case SRSO_CMD_IBPB_ON_VMEXIT:
+               if (IS_ENABLED(CONFIG_CPU_SRSO)) {
+                       if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
+                               setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
+                               srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
+                       }
+               } else {
+                       pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
+                       goto pred_cmd;
+                }
+               break;
+
        default:
                break;
        }

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0a212fe2cd3985dd449d46a5c8e4bf01097adf05..fdb6007f2eb865b98699b26263ae97cb51e504ab 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1485,7 +1485,9 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
        if (sd->current_vmcb != svm->vmcb) {
                sd->current_vmcb = svm->vmcb;
-               indirect_branch_prediction_barrier();
+
+               if (!cpu_feature_enabled(X86_FEATURE_IBPB_ON_VMEXIT))
+                       indirect_branch_prediction_barrier();
        }
        if (kvm_vcpu_apicv_active(vcpu))
                avic_vcpu_load(vcpu, cpu);

diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index 34367dc203f217dd9f87e45831ef9ce0ecc871b3..5be9a63f09fff2a6ac9e23f61758377924aa572b 100644 (file)
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -223,6 +223,9 @@ SYM_FUNC_START(__svm_vcpu_run)
         */
        UNTRAIN_RET
 
+       /* SRSO */
+       ALTERNATIVE "", "call entry_ibpb", X86_FEATURE_IBPB_ON_VMEXIT
+
        /*
         * Clear all general purpose registers except RSP and RAX to prevent
         * speculative use of the guest's values, even those that are reloaded