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.

Fixes: 864bcaa ("x86/cpu/kvm: Provide UNTRAIN_RET_VM") [1]
Fixes: d893832 ("x86/srso: Add IBPB on VMEXIT") [2]
Reported-by: Yosry Ahmed <[email protected]>
Signed-off-by: Patrick Bellasi <[email protected]>
Reviewed-by: Borislav Petkov (AMD) <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: derkling

arch/x86/Kconfig

@@ -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"

arch/x86/kernel/cpu/bugs.c

@@ -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;