FAILED: patch "[PATCH] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 318e8c339c9a0891c389298bb328ed0762a9935e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025021717-pelt-wick-0392@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 318e8c339c9a0891c389298bb328ed0762a9935e Mon Sep 17 00:00:00 2001
From: Patrick Bellasi <[email protected]>
Date: Wed, 5 Feb 2025 14:04:41 +0000
Subject: [PATCH] x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit

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.

index 87198d957e2f..be2c311f5118 100644
@@ -2599,7 +2599,8 @@ config MITIGATION_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 MITIGATION_IBRS_ENTRY
 	bool "Enable IBRS on kernel entry"

index 5a505aa65489..a5d0998d7604 100644
@@ -1115,6 +1115,8 @@ 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

@@ -1124,9 +1126,6 @@ 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,

@@ -2646,6 +2645,7 @@ static void __init srso_select_mitigation(void)
 		if (IS_ENABLED(CONFIG_MITIGATION_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;
 
 				/*

@@ -2655,6 +2655,13 @@ 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 MITIGATION_IBPB_ENTRY.\n");

@@ -2663,8 +2670,8 @@ static void __init srso_select_mitigation(void)
 
 ibpb_on_vmexit:
 	case SRSO_CMD_IBPB_ON_VMEXIT:
-		if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
-			if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
+		if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
+			if (has_microcode) {
 				setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
 				srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
 

@@ -2676,8 +2683,8 @@ 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 MITIGATION_IBPB_ENTRY.\n");
+		}
 		break;
 	default:
 		break;