x86/speculation: Add command line control for indirect branch speculation

BugLink: https://bugs.launchpad.net/bugs/1830176

commit fa1202ef upstream.

Add command line control for user space indirect branch speculation
mitigations. The new option is: spectre_v2_user=

The initial options are:

    -  on:   Unconditionally enabled
    - off:   Unconditionally disabled
    -auto:   Kernel selects mitigation (default off for now)

When the spectre_v2= command line argument is either 'on' or 'off' this
implies that the application to application control follows that state even
if a contradicting spectre_v2_user= argument is supplied.
Originally-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Casey Schaufler <casey.schaufler@intel.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Waiman Long <longman9394@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Dave Stewart <david.c.stewart@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20181125185005.082720373@linutronix.de
[bwh: Backported to 4.4:
 - Don't use __ro_after_init or cpu_smt_control
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[juergh:
 - Adjusted context.
 - Adjusted for Ubuntu-only IBRS/IBPB runtime controls.
 - Used __ro_after_init and cpu_smt_control which is available in Xenial.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Documentation/kernel-parameters.txt

@@ -3766,9 +3766,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	spectre_v2=	[X86] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.
+			The default operation protects the kernel from
+			user space attacks.
 
-			on   - unconditionally enable
-			off  - unconditionally disable
+			on   - unconditionally enable, implies
+			       spectre_v2_user=on
+			off  - unconditionally disable, implies
+			       spectre_v2_user=off
 			auto - kernel detects whether your CPU model is
 			       vulnerable
 
@@ -3778,6 +3782,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			CONFIG_RETPOLINE configuration option, and the
 			compiler with which the kernel was built.
 
+			Selecting 'on' will also enable the mitigation
+			against user space to user space task attacks.
+
+			Selecting 'off' will disable both the kernel and
+			the user space protections.
+
 			Specific mitigations can also be selected manually:
 
 			retpoline	  - replace indirect branches
@@ -3787,6 +3797,24 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Not specifying this option is equivalent to
 			spectre_v2=auto.
 
+	spectre_v2_user=
+			[X86] Control mitigation of Spectre variant 2
+		        (indirect branch speculation) vulnerability between
+		        user space tasks
+
+			on	- Unconditionally enable mitigations. Is
+				  enforced by spectre_v2=on
+
+			off     - Unconditionally disable mitigations. Is
+				  enforced by spectre_v2=off
+
+			auto    - Kernel selects the mitigation depending on
+				  the available CPU features and vulnerability.
+				  Default is off.
+
+			Not specifying this option is equivalent to
+			spectre_v2_user=auto.
+
 	spec_store_bypass_disable=
 			[HW] Control Speculative Store Bypass (SSB) Disable mitigation
 			(Speculative Store Bypass vulnerability)

arch/x86/include/asm/nospec-branch.h

@@ -204,6 +204,12 @@ enum spectre_v2_mitigation {
 	SPECTRE_V2_IBRS_ENHANCED,
 };
 
+/* The indirect branch speculation control variants */
+enum spectre_v2_user_mitigation {
+	SPECTRE_V2_USER_NONE,
+	SPECTRE_V2_USER_STRICT,
+};
+
 /* The Speculative Store Bypass disable variants */
 enum ssb_mitigation {
 	SPEC_STORE_BYPASS_NONE,
@@ -298,6 +304,8 @@ do {									\
 		native_wrmsrl(MSR_IA32_SPEC_CTRL, val);			\
  } while (0)
 
+DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
+
 DECLARE_STATIC_KEY_FALSE(mds_user_clear);
 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 

arch/x86/kernel/cpu/bugs.c

@@ -75,6 +75,9 @@ static u64 __ro_after_init x86_spec_ctrl_mask = SPEC_CTRL_IBRS;
 u64 __ro_after_init x86_amd_ls_cfg_base;
 u64 __ro_after_init x86_amd_ls_cfg_ssbd_mask;
 
+/* Control conditional STIPB in switch_to() */
+DEFINE_STATIC_KEY_FALSE(switch_to_cond_stibp);
+
 /* Control MDS CPU buffer clear before returning to user space */
 DEFINE_STATIC_KEY_FALSE(mds_user_clear);
 EXPORT_SYMBOL_GPL(mds_user_clear);
@@ -290,6 +293,9 @@ early_param("mds", mds_cmdline);
 static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
 	SPECTRE_V2_NONE;
 
+static enum spectre_v2_user_mitigation spectre_v2_user __ro_after_init =
+	SPECTRE_V2_USER_NONE;
+
 #ifdef RETPOLINE
 static bool spectre_v2_bad_module;
 
@@ -328,6 +334,108 @@ enum spectre_v2_mitigation_cmd {
 	SPECTRE_V2_CMD_RETPOLINE_AMD,
 };
 
+enum spectre_v2_user_cmd {
+	SPECTRE_V2_USER_CMD_NONE,
+	SPECTRE_V2_USER_CMD_AUTO,
+	SPECTRE_V2_USER_CMD_FORCE,
+};
+
+static const char * const spectre_v2_user_strings[] = {
+	[SPECTRE_V2_USER_NONE]		= "User space: Vulnerable",
+	[SPECTRE_V2_USER_STRICT]	= "User space: Mitigation: STIBP protection",
+};
+
+static const struct {
+	const char			*option;
+	enum spectre_v2_user_cmd	cmd;
+	bool				secure;
+} v2_user_options[] __initdata = {
+	{ "auto",	SPECTRE_V2_USER_CMD_AUTO,	false },
+	{ "off",	SPECTRE_V2_USER_CMD_NONE,	false },
+	{ "on",		SPECTRE_V2_USER_CMD_FORCE,	true  },
+};
+
+static void __init spec_v2_user_print_cond(const char *reason, bool secure)
+{
+	if (boot_cpu_has_bug(X86_BUG_SPECTRE_V2) != secure)
+		pr_info("spectre_v2_user=%s forced on command line.\n", reason);
+}
+
+static enum spectre_v2_user_cmd __init
+spectre_v2_parse_user_cmdline(enum spectre_v2_mitigation_cmd v2_cmd)
+{
+	char arg[20];
+	int ret, i;
+
+	switch (v2_cmd) {
+	case SPECTRE_V2_CMD_NONE:
+		return SPECTRE_V2_USER_CMD_NONE;
+	case SPECTRE_V2_CMD_FORCE:
+		return SPECTRE_V2_USER_CMD_FORCE;
+	default:
+		break;
+	}
+
+	ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
+				  arg, sizeof(arg));
+	if (ret < 0)
+		return SPECTRE_V2_USER_CMD_AUTO;
+
+	for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
+		if (match_option(arg, ret, v2_user_options[i].option)) {
+			spec_v2_user_print_cond(v2_user_options[i].option,
+						v2_user_options[i].secure);
+			return v2_user_options[i].cmd;
+		}
+	}
+
+	pr_err("Unknown user space protection option (%s). Switching to AUTO select\n", arg);
+	return SPECTRE_V2_USER_CMD_AUTO;
+}
+
+static void __init
+spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
+{
+	enum spectre_v2_user_mitigation mode = SPECTRE_V2_USER_NONE;
+	bool smt_possible = IS_ENABLED(CONFIG_SMP);
+
+	if (!boot_cpu_has(X86_FEATURE_IBPB) && !boot_cpu_has(X86_FEATURE_STIBP))
+		return;
+
+	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED ||
+	    cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
+		smt_possible = false;
+
+	switch (spectre_v2_parse_user_cmdline(v2_cmd)) {
+	case SPECTRE_V2_USER_CMD_AUTO:
+	case SPECTRE_V2_USER_CMD_NONE:
+		goto set_mode;
+	case SPECTRE_V2_USER_CMD_FORCE:
+		mode = SPECTRE_V2_USER_STRICT;
+		break;
+	}
+
+	/*
+	 * Initialize Indirect Branch Prediction Barrier if supported and not
+	 * disabled on the commandline
+	 */
+	if (boot_cpu_has(X86_FEATURE_IBPB)) {
+		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
+		if (!noibpb)
+			set_ibpb_enabled(1);   /* Enable IBPB */
+	}
+
+	/* If enhanced IBRS is enabled no STIPB required */
+	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+		return;
+
+set_mode:
+	spectre_v2_user = mode;
+	/* Only print the STIBP mode when SMT possible */
+	if (smt_possible)
+		pr_info("%s\n", spectre_v2_user_strings[mode]);
+}
+
 static const char * const spectre_v2_strings[] = {
 	[SPECTRE_V2_NONE]			= "Vulnerable",
 	[SPECTRE_V2_RETPOLINE_MINIMAL]		= "Vulnerable: Minimal generic ASM retpoline",
@@ -408,17 +516,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 
 static bool stibp_needed(void)
 {
-	if (spectre_v2_enabled == SPECTRE_V2_NONE)
-		return false;
-
 	/* Enhanced IBRS makes using STIBP unnecessary. */
 	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
 		return false;
 
-	if (!boot_cpu_has(X86_FEATURE_STIBP))
-		return false;
-
-	return true;
+	/* Check for strict user mitigation mode */
+	return spectre_v2_user == SPECTRE_V2_USER_STRICT;
 }
 
 static void update_stibp_msr(void *info)
@@ -504,16 +607,6 @@ static void __init spectre_v2_select_mitigation(void)
 	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
 
-	/*
-	 * Initialize Indirect Branch Prediction Barrier if supported and not
-	 * disabled on the commandline
-	 */
-	if (boot_cpu_has(X86_FEATURE_IBPB)) {
-		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
-		if (!noibpb)
-			set_ibpb_enabled(1);   /* Enable IBPB */
-	}
-
 	/*
 	 * Retpoline means the kernel is safe because it has no indirect
 	 * branches. Enhanced IBRS protects firmware too, so, enable restricted
@@ -534,6 +627,9 @@ static void __init spectre_v2_select_mitigation(void)
 		    mode != SPECTRE_V2_RETPOLINE_AMD)
 			set_ibrs_enabled(1);   /* Enable IBRS */
 	}
+
+	/* Set up IBPB and STIBP depending on the general spectre V2 command */
+	spectre_v2_user_select_mitigation(cmd);
 }
 
 #undef pr_fmt
@@ -1013,10 +1109,13 @@ static char *stibp_state(void)
 	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
 		return "";
 
-	if (x86_spec_ctrl_base & SPEC_CTRL_STIBP)
-		return ", STIBP";
-	else
-		return "";
+	switch (spectre_v2_user) {
+	case SPECTRE_V2_USER_NONE:
+		return ", STIBP: disabled";
+	case SPECTRE_V2_USER_STRICT:
+		return ", STIBP: forced";
+	}
+	return "";
 }
 
 static char *ibpb_state(void)