[PATCH v2 2/2] cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n

[Sean Christopherson <seanjc@google.com>]

Explicitly disallow enabling mitigations at runtime for kernels that were
built with CONFIG_CPU_MITIGATIONS=n, as some architectures may omit code
entirely if mitigations are disabled at compile time.

E.g. on x86, a large pile of Kconfigs are buried behind CPU_MITIGATIONS,
and trying to provide sane behavior for retroactively enabling mitigations
is extremely difficult, bordering on impossible.  E.g. page table isolation
and call depth tracking requrie build-time support, BHI mitigations will
still be off without additional kernel parameters, etc.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/x86/Kconfig                                |  8 ++++++--
 include/linux/cpu.h                             | 11 +++++++++++
 kernel/cpu.c                                    | 13 ++++++++++---
 4 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 902ecd92a29f..213d0719e2b7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3423,6 +3423,9 @@
 			arch-independent options, each of which is an
 			aggregation of existing arch-specific options.
 
+			Note, "mitigations" is supported if and only if the
+			kernel was built with CPU_MITIGATIONS=y.
+
 			off
 				Disable all optional CPU mitigations.  This
 				improves system performance, but it may also
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 619a04d5c131..928820e61cb5 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2495,9 +2495,13 @@ menuconfig CPU_MITIGATIONS
 	help
 	  Say Y here to enable options which enable mitigations for hardware
 	  vulnerabilities (usually related to speculative execution).
+	  Mitigations can be disabled or restricted to SMT systems at runtime
+	  via the "mitigations" kernel parameter.
 
-	  If you say N, all mitigations will be disabled. You really
-	  should know what you are doing to say so.
+	  If you say N, all mitigations will be disabled.  This CANNOT be
+	  overridden at runtime.
+
+	  Say 'Y', unless you really know what you are doing.
 
 if CPU_MITIGATIONS
 
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 272e4e79e15c..ee0a3b4e0769 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -221,7 +221,18 @@ void cpuhp_report_idle_dead(void);
 static inline void cpuhp_report_idle_dead(void) { }
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
+#ifdef CONFIG_CPU_MITIGATIONS
 extern bool cpu_mitigations_off(void);
 extern bool cpu_mitigations_auto_nosmt(void);
+#else
+static inline bool cpu_mitigations_off(void)
+{
+	return false;
+}
+static inline bool cpu_mitigations_auto_nosmt(void)
+{
+	return false;
+}
+#endif
 
 #endif /* _LINUX_CPU_H_ */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index bb0ff275fb46..24235c1d6e82 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3196,6 +3196,7 @@ void __init boot_cpu_hotplug_init(void)
 	this_cpu_write(cpuhp_state.target, CPUHP_ONLINE);
 }
 
+#ifdef CONFIG_CPU_MITIGATIONS
 /*
  * These are used for a global "mitigations=" cmdline option for toggling
  * optional CPU mitigations.
@@ -3207,8 +3208,7 @@ enum cpu_mitigations {
 };
 
 static enum cpu_mitigations cpu_mitigations __ro_after_init =
-	IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
-					     CPU_MITIGATIONS_OFF;
+	CPU_MITIGATIONS_AUTO;
 
 static int __init mitigations_parse_cmdline(char *arg)
 {
@@ -3224,7 +3224,6 @@ static int __init mitigations_parse_cmdline(char *arg)
 
 	return 0;
 }
-early_param("mitigations", mitigations_parse_cmdline);
 
 /* mitigations=off */
 bool cpu_mitigations_off(void)
@@ -3239,3 +3238,11 @@ bool cpu_mitigations_auto_nosmt(void)
 	return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
 }
 EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt);
+#else
+static int __init mitigations_parse_cmdline(char *arg)
+{
+	pr_crit("Kernel compiled without mitigations, ignoring 'mitigations'; system may still be vulnerable\n");
+	return 0;
+}
+#endif
+early_param("mitigations", mitigations_parse_cmdline);
-- 
2.44.0.769.g3c40516874-goog