x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit

commit 318e8c339c9a0891c389298bb328ed0762a9935e upstream.

In [1] the meaning of the synthetic IBPB flags has been redefined for a better separation of concerns:
- ENTRY_IBPB -- issue IBPB on entry only
- IBPB_ON_VMEXIT -- issue IBPB on VM-Exit only
and the Retbleed mitigations have been updated to match this new semantics.

Commit [2] was merged shortly before [1], and their interaction was not handled properly. This resulted in IBPB not being triggered on VM-Exit in all SRSO mitigation configs requesting an IBPB there.

Specifically, an IBPB on VM-Exit is triggered only when X86_FEATURE_IBPB_ON_VMEXIT is set. However:

- X86_FEATURE_IBPB_ON_VMEXIT is not set for "spec_rstack_overflow=ibpb", because before [1] having X86_FEATURE_ENTRY_IBPB was enough. Hence, an IBPB is triggered on entry but the expected IBPB on VM-exit is not.

- X86_FEATURE_IBPB_ON_VMEXIT is not set also when "spec_rstack_overflow=ibpb-vmexit" if X86_FEATURE_ENTRY_IBPB is already set.

That's because before [1] this was effectively redundant. Hence, e.g. a "retbleed=ibpb spec_rstack_overflow=bpb-vmexit" config mistakenly reports the machine still vulnerable to SRSO, despite an IBPB being triggered both on entry and VM-Exit, because of the Retbleed selected mitigation config.

- UNTRAIN_RET_VM won't still actually do anything unless CONFIG_MITIGATION_IBPB_ENTRY is set.

For "spec_rstack_overflow=ibpb", enable IBPB on both entry and VM-Exit and clear X86_FEATURE_RSB_VMEXIT which is made superfluous by X86_FEATURE_IBPB_ON_VMEXIT. This effectively makes this mitigation option similar to the one for 'retbleed=ibpb', thus re-order the code for the RETBLEED_MITIGATION_IBPB option to be less confusing by having all features enabling before the disabling of the not needed ones.

For "spec_rstack_overflow=ibpb-vmexit", guard this mitigation setting with CONFIG_MITIGATION_IBPB_ENTRY to ensure UNTRAIN_RET_VM sequence is effectively compiled in. Drop instead the CONFIG_MITIGATION_SRSO guard, since none of the SRSO compile cruft is required in this configuration. Also, check only that the required microcode is present to effectively enabled the IBPB on VM-Exit.

Finally, update the KConfig description for CONFIG_MITIGATION_IBPB_ENTRY to list also all SRSO config settings enabled by this guard.

---
 arch/x86/Kconfig           |  3 ++-
 arch/x86/kernel/cpu/bugs.c | 21 ++++++++++++++-------
 2 files changed, 16 insertions(+), 8 deletions(-)

index 05c82fd5d0f60..989d432b58345 100644
@@ -2514,7 +2514,8 @@ config CPU_IBPB_ENTRY
 	depends on CPU_SUP_AMD && X86_64
 	default y
 	help
-	  Compile the kernel with support for the retbleed=ibpb mitigation.
+	  Compile the kernel with support for the retbleed=ibpb and
+	  spec_rstack_overflow={ibpb,ibpb-vmexit} mitigations.
 
 config CPU_IBRS_ENTRY
 	bool "Enable IBRS on kernel entry"

index 7b5ba5b8592a2..7df458a6553eb 100644
@@ -1113,6 +1113,8 @@ static void __init retbleed_select_mitigation(void)
 
 	case RETBLEED_MITIGATION_IBPB:
 		setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+		setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
+		mitigate_smt = true;
 
 		/*
 		 * IBPB on entry already obviates the need for

@@ -1122,9 +1124,6 @@ static void __init retbleed_select_mitigation(void)
 		setup_clear_cpu_cap(X86_FEATURE_UNRET);
 		setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
 
-		setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
-		mitigate_smt = true;
-
 		/*
 		 * There is no need for RSB filling: entry_ibpb() ensures
 		 * all predictions, including the RSB, are invalidated,

@@ -2626,6 +2625,7 @@ static void __init srso_select_mitigation(void)
 		if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
 			if (has_microcode) {
 				setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+				setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
 				srso_mitigation = SRSO_MITIGATION_IBPB;
 
 				/*

@@ -2635,6 +2635,13 @@ static void __init srso_select_mitigation(void)
 				 */
 				setup_clear_cpu_cap(X86_FEATURE_UNRET);
 				setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
+
+				/*
+				 * There is no need for RSB filling: entry_ibpb() ensures
+				 * all predictions, including the RSB, are invalidated,
+				 * regardless of IBPB implementation.
+				 */
+				setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 			}
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");

@@ -2643,8 +2650,8 @@ static void __init srso_select_mitigation(void)
 		break;
 
 	case SRSO_CMD_IBPB_ON_VMEXIT:
-		if (IS_ENABLED(CONFIG_CPU_SRSO)) {
-			if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
+		if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+			if (has_microcode) {
 				setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
 				srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
 

@@ -2656,9 +2663,9 @@ static void __init srso_select_mitigation(void)
 				setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
 			}
 		} else {
-			pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
+			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
 			goto pred_cmd;
-                }
+		}
 		break;
 
 	default:

2.48.1.601.g30ceb7b040-goog

All 4 backports by Patrick:

Feb 21           Patrick Bellasi ( : 148|) [PATCH 6.6] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
Feb 21           Patrick Bellasi ( : 147|) [PATCH 6.1] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
Feb 21           Patrick Bellasi ( : 147|) [PATCH 5.15] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
Feb 21           Patrick Bellasi ( : 147|) [PATCH 5.10] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit

LGTM.

Thanks!

Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected. No action required from the submitter.

The upstream commit SHA1 provided is correct: 318e8c339c9a0891c389298bb328ed0762a9935e

Note: The patch differs from the upstream commit:
---
1: 318e8c339c9a0 ! 1: 7e78323cfe696 x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit
@@ Metadata
## Commit message ##
x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit

+ commit 318e8c339c9a0891c389298bb328ed0762a9935e upstream.
+
In [1] the meaning of the synthetic IBPB flags has been redefined for a better separation of concerns:
- ENTRY_IBPB -- issue IBPB on entry only
@@ Commit message

## arch/x86/Kconfig ##
-@@ arch/x86/Kconfig: config MITIGATION_IBPB_ENTRY +@@ arch/x86/Kconfig: config CPU_IBPB_ENTRY
depends on CPU_SUP_AMD && X86_64 default y
help
@@ arch/x86/Kconfig: config MITIGATION_IBPB_ENTRY
+ Compile the kernel with support for the retbleed=ibpb and
+ spec_rstack_overflow={ibpb,ibpb-vmexit} mitigations.

- config MITIGATION_IBRS_ENTRY
+ config CPU_IBRS_ENTRY
bool "Enable IBRS on kernel entry"

## arch/x86/kernel/cpu/bugs.c ##
@@ arch/x86/kernel/cpu/bugs.c: static void __init retbleed_select_mitigation(void)
* There is no need for RSB filling: entry_ibpb() ensures
* all predictions, including the RSB, are invalidated,
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
- if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
+ if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+ setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
+ setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
- pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
+ pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
+ break;

- ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
-- if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
+- if (IS_ENABLED(CONFIG_CPU_SRSO)) {
- if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
-+ if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
++ if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+ if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT); srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
@@ arch/x86/kernel/cpu/bugs.c: static void __init srso_select_mitigation(void)
setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
-- pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
+- pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
++ pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
+ goto pred_cmd;
- }
-+ pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
+ }
break;
+
default:
- break;
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-6.6.y        |  Success    |  Success   |