cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n

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 require build-time support, BHI mitigations will
still be off without additional kernel parameters, etc.

  [ bp: Touchups. ]

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240420000556.2645001-3-seanjc@google.com

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 902ecd92a29fbe83df18d32d1a8fe652c8277132..213d0719e2b7a68e4daa12482a4c60361e19e3c4 100644 (file)
--- 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 619a04d5c1310ddd22b512f19ecec0b387242813..928820e61cb5020a1eeb3bcc99b0a72227ae6c82 100644 (file)
--- 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 272e4e79e15c48db487feba3b7e6103e71b21e8d..861c3bfc5f1700db69d90168c239a935ebad76ca 100644 (file)
--- 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 true;
+}
+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 bb0ff275fb467d736cf798c24ab5b850001fefa1..63447eb85dab6bd0fa2ab23c25bffdf788a2b9c1 100644 (file)
--- 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.
@@ -3206,9 +3207,7 @@ enum cpu_mitigations {
        CPU_MITIGATIONS_AUTO_NOSMT,
 };
 
-static enum cpu_mitigations cpu_mitigations __ro_after_init =
-       IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
-                                            CPU_MITIGATIONS_OFF;
+static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
 
 static int __init mitigations_parse_cmdline(char *arg)
 {
@@ -3224,7 +3223,6 @@ 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 +3237,11 @@ 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);