Merge branch 'core/speculation' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip into for-next/mitigations

Pull in core support for the "mitigations=" cmdline option from Thomas
Gleixner via -tip, which we can build on top of when we expose our
mitigation state via sysfs.

Documentation/admin-guide/kernel-parameters.txt

···
 			in the "bleeding edge" mini2440 support kernel at
 			http://repo.or.cz/w/linux-2.6/mini2440.git
 
+	mitigations=
+			[X86,PPC,S390] Control optional mitigations for CPU
+			vulnerabilities.  This is a set of curated,
+			arch-independent options, each of which is an
+			aggregation of existing arch-specific options.
+
+			off
+				Disable all optional CPU mitigations.  This
+				improves system performance, but it may also
+				expose users to several CPU vulnerabilities.
+				Equivalent to: nopti [X86,PPC]
+					       nospectre_v1 [PPC]
+					       nobp=0 [S390]
+					       nospectre_v2 [X86,PPC,S390]
+					       spectre_v2_user=off [X86]
+					       spec_store_bypass_disable=off [X86,PPC]
+					       l1tf=off [X86]
+
+			auto (default)
+				Mitigate all CPU vulnerabilities, but leave SMT
+				enabled, even if it's vulnerable.  This is for
+				users who don't want to be surprised by SMT
+				getting disabled across kernel upgrades, or who
+				have other ways of avoiding SMT-based attacks.
+				Equivalent to: (default behavior)
+
+			auto,nosmt
+				Mitigate all CPU vulnerabilities, disabling SMT
+				if needed.  This is for users who always want to
+				be fully mitigated, even if it means losing SMT.
+				Equivalent to: l1tf=flush,nosmt [X86]
+
 	mminit_loglevel=
 			[KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
 			parameter allows control of the logging verbosity for

arch/powerpc/kernel/security.c

···
 	enable = security_ftr_enabled(SEC_FTR_FAVOUR_SECURITY) &&
 		 security_ftr_enabled(SEC_FTR_BNDS_CHK_SPEC_BAR);
 
-	if (!no_nospec)
+	if (!no_nospec && !cpu_mitigations_off())
 		enable_barrier_nospec(enable);
 }
 
···
 early_param("nospectre_v2", handle_nospectre_v2);
 void setup_spectre_v2(void)
 {
-	if (no_spectrev2)
+	if (no_spectrev2 || cpu_mitigations_off())
 		do_btb_flush_fixups();
 	else
 		btb_flush_enabled = true;
···
 
 	stf_enabled_flush_types = type;
 
-	if (!no_stf_barrier)
+	if (!no_stf_barrier && !cpu_mitigations_off())
 		stf_barrier_enable(enable);
 }
 

arch/powerpc/kernel/setup_64.c

···
 
 	enabled_flush_types = types;
 
-	if (!no_rfi_flush)
+	if (!no_rfi_flush && !cpu_mitigations_off())
 		rfi_flush_enable(enable);
 }
 

arch/s390/kernel/nospec-branch.c

···
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/module.h>
 #include <linux/device.h>
+#include <linux/cpu.h>
 #include <asm/nospec-branch.h>
 
 static int __init nobp_setup_early(char *str)
···
 
 void __init nospec_auto_detect(void)
 {
-	if (test_facility(156)) {
+	if (test_facility(156) || cpu_mitigations_off()) {
 		/*
 		 * The machine supports etokens.
 		 * Disable expolines and disable nobp.

arch/x86/kernel/cpu/bugs.c

···
 	char arg[20];
 	int ret, i;
 
-	if (cmdline_find_option_bool(boot_command_line, "nospectre_v2"))
+	if (cmdline_find_option_bool(boot_command_line, "nospectre_v2") ||
+	    cpu_mitigations_off())
 		return SPECTRE_V2_CMD_NONE;
 
 	ret = cmdline_find_option(boot_command_line, "spectre_v2", arg, sizeof(arg));
···
 	char arg[20];
 	int ret, i;
 
-	if (cmdline_find_option_bool(boot_command_line, "nospec_store_bypass_disable")) {
+	if (cmdline_find_option_bool(boot_command_line, "nospec_store_bypass_disable") ||
+	    cpu_mitigations_off()) {
 		return SPEC_STORE_BYPASS_CMD_NONE;
 	} else {
 		ret = cmdline_find_option(boot_command_line, "spec_store_bypass_disable",
···
 
 	if (!boot_cpu_has_bug(X86_BUG_L1TF))
 		return;
+
+	if (cpu_mitigations_off())
+		l1tf_mitigation = L1TF_MITIGATION_OFF;
+	else if (cpu_mitigations_auto_nosmt())
+		l1tf_mitigation = L1TF_MITIGATION_FLUSH_NOSMT;
 
 	override_cache_bits(&boot_cpu_data);
 

arch/x86/mm/pti.c

···
 #include <linux/spinlock.h>
 #include <linux/mm.h>
 #include <linux/uaccess.h>
+#include <linux/cpu.h>
 
 #include <asm/cpufeature.h>
 #include <asm/hypervisor.h>
···
 		}
 	}
 
-	if (cmdline_find_option_bool(boot_command_line, "nopti")) {
+	if (cmdline_find_option_bool(boot_command_line, "nopti") ||
+	    cpu_mitigations_off()) {
 		pti_mode = PTI_FORCE_OFF;
 		pti_print_if_insecure("disabled on command line.");
 		return;

include/linux/cpu.h

···
 static inline void cpu_smt_check_topology(void) { }
 #endif
 
+/*
+ * These are used for a global "mitigations=" cmdline option for toggling
+ * optional CPU mitigations.
+ */
+enum cpu_mitigations {
+	CPU_MITIGATIONS_OFF,
+	CPU_MITIGATIONS_AUTO,
+	CPU_MITIGATIONS_AUTO_NOSMT,
+};
+
+extern enum cpu_mitigations cpu_mitigations;
+
+/* mitigations=off */
+static inline bool cpu_mitigations_off(void)
+{
+	return cpu_mitigations == CPU_MITIGATIONS_OFF;
+}
+
+/* mitigations=auto,nosmt */
+static inline bool cpu_mitigations_auto_nosmt(void)
+{
+	return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
+}
+
 #endif /* _LINUX_CPU_H_ */

kernel/cpu.c

···
 #endif
 	this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
 }
+
+enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
+
+static int __init mitigations_parse_cmdline(char *arg)
+{
+	if (!strcmp(arg, "off"))
+		cpu_mitigations = CPU_MITIGATIONS_OFF;
+	else if (!strcmp(arg, "auto"))
+		cpu_mitigations = CPU_MITIGATIONS_AUTO;
+	else if (!strcmp(arg, "auto,nosmt"))
+		cpu_mitigations = CPU_MITIGATIONS_AUTO_NOSMT;
+
+	return 0;
+}
+early_param("mitigations", mitigations_parse_cmdline);