[Patch v5 11/16] x86/speculation: Add Spectre v2 app to app protection modes

[Tim Chen <tim.c.chen@linux.intel.com>]

Add new protection modes for Spectre v2 mitigations against
Spectre v2 attacks on user processes.  There are three modes:

	strict mode:
	In this mode, IBPB and STIBP are deployed full
	time to protect all processes.

	lite mode:
	In this mode, IBPB and STIBP are only deployed on
	processes marked with TIF_STIBP flag.

	none mode:
	In this mode, no mitigations are deployed.

The protection mode can be specified by the spectre_v2_app2app
boot parameter with the following semantics:

spectre_v2_app2app=
	off    - Turn off mitigation
	lite   - Protect processes which are marked non-dumpable
	strict - Protect all processes
	auto   - Kernel selects the mode

	Not specifying this option is equivalent to
	spectre_v2_app2app=auto.

	Setting spectre_v2=off will also turn off this mitigation.

	Setting spectre_v2=on implies unconditionally enabling
	this mitigation.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  20 ++++
 arch/x86/include/asm/nospec-branch.h            |   9 ++
 arch/x86/kernel/cpu/bugs.c                      | 153 +++++++++++++++++++++---
 arch/x86/mm/tlb.c                               |  23 +++-
 4 files changed, 185 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 81d1d5a..9c306e3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4215,6 +4215,26 @@
 			Not specifying this option is equivalent to
 			spectre_v2=auto.
 
+	spectre_v2_app2app=
+			[X86] Control mitigation of Spectre variant 2
+		        application to application (indirect branch speculation)
+			vulnerability.
+
+			off    - Unconditionally disable mitigations
+			lite   - Protect tasks which have requested restricted
+				 indirect branch speculation via the
+				 PR_SET_SPECULATION_CTRL prctl(). 
+			strict - Protect all processes
+			auto   - Kernel selects the mode
+
+			Not specifying this option is equivalent to
+			spectre_v2_app2app=auto.
+
+			Setting spectre_v2=off will also turn off this mitigation.
+
+			Setting spectre_v2=on implies unconditionally enabling
+			this mitigation.
+
 	spec_store_bypass_disable=
 			[HW] Control Speculative Store Bypass (SSB) Disable mitigation
 			(Speculative Store Bypass vulnerability)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 80dc144..587629d 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -3,6 +3,7 @@
 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
 #define _ASM_X86_NOSPEC_BRANCH_H_
 
+#include <linux/static_key.h>
 #include <asm/alternative.h>
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
@@ -226,6 +227,12 @@ enum spectre_v2_mitigation {
 	SPECTRE_V2_IBRS_ENHANCED,
 };
 
+enum spectre_v2_app2app_mitigation {
+	SPECTRE_V2_APP2APP_NONE,
+	SPECTRE_V2_APP2APP_LITE,
+	SPECTRE_V2_APP2APP_STRICT,
+};
+
 /* The Speculative Store Bypass disable variants */
 enum ssb_mitigation {
 	SPEC_STORE_BYPASS_NONE,
@@ -237,6 +244,8 @@ enum ssb_mitigation {
 extern char __indirect_thunk_start[];
 extern char __indirect_thunk_end[];
 
+DECLARE_STATIC_KEY_FALSE(spectre_v2_app_lite);
+
 /*
  * On VMEXIT we must ensure that no RSB predictions learned in the guest
  * can be followed in the host, by overwriting the RSB completely. Both
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6e1b910..21caade 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -133,6 +133,14 @@ enum spectre_v2_mitigation_cmd {
 	SPECTRE_V2_CMD_RETPOLINE_AMD,
 };
 
+enum spectre_v2_app2app_mitigation_cmd {
+	SPECTRE_V2_APP2APP_CMD_NONE,
+	SPECTRE_V2_APP2APP_CMD_AUTO,
+	SPECTRE_V2_APP2APP_CMD_FORCE,
+	SPECTRE_V2_APP2APP_CMD_LITE,
+	SPECTRE_V2_APP2APP_CMD_STRICT,
+};
+
 static const char *spectre_v2_strings[] = {
 	[SPECTRE_V2_NONE]			= "Vulnerable",
 	[SPECTRE_V2_RETPOLINE_MINIMAL]		= "Vulnerable: Minimal generic ASM retpoline",
@@ -142,12 +150,24 @@ static const char *spectre_v2_strings[] = {
 	[SPECTRE_V2_IBRS_ENHANCED]		= "Mitigation: Enhanced IBRS",
 };
 
+static const char *spectre_v2_app2app_strings[] = {
+	[SPECTRE_V2_APP2APP_NONE]   = "App-App Vulnerable",
+	[SPECTRE_V2_APP2APP_LITE]   = "App-App Mitigation: Protect branch speculation restricted tasks",
+	[SPECTRE_V2_APP2APP_STRICT] = "App-App Mitigation: Full app to app attack protection",
+};
+
+DEFINE_STATIC_KEY_FALSE(spectre_v2_app_lite);
+EXPORT_SYMBOL_GPL(spectre_v2_app_lite);
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
 static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
 	SPECTRE_V2_NONE;
 
+static enum spectre_v2_app2app_mitigation
+	spectre_v2_app2app_enabled __ro_after_init = SPECTRE_V2_APP2APP_NONE;
+
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 {
@@ -169,6 +189,9 @@ x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)
 		    static_cpu_has(X86_FEATURE_AMD_SSBD))
 			hostval |= ssbd_tif_to_spec_ctrl(ti->flags);
 
+		if (static_branch_unlikely(&spectre_v2_app_lite))
+			hostval |= stibp_tif_to_spec_ctrl(ti->flags);
+
 		if (hostval != guestval) {
 			msrval = setguest ? guestval : hostval;
 			wrmsrl(MSR_IA32_SPEC_CTRL, msrval);
@@ -275,6 +298,66 @@ static const struct {
 	{ "auto",              SPECTRE_V2_CMD_AUTO,              false },
 };
 
+static const struct {
+	const char *option;
+	enum spectre_v2_app2app_mitigation_cmd cmd;
+	bool secure;
+} app2app_options[] = {
+	{ "off",	SPECTRE_V2_APP2APP_CMD_NONE,   false },
+	{ "lite",	SPECTRE_V2_APP2APP_CMD_LITE,   false },
+	{ "strict",	SPECTRE_V2_APP2APP_CMD_STRICT, false },
+	{ "auto",	SPECTRE_V2_APP2APP_CMD_AUTO,   false },
+	/*
+	 * The "on" option is kept as last entry. It is implied by
+	 * spectre_v2=on boot parameter and it is not checked
+	 * in spectre_v2_app2app boot parameter.
+	 */
+	{ "on",		SPECTRE_V2_APP2APP_CMD_FORCE,  true  },
+};
+
+static enum spectre_v2_app2app_mitigation_cmd __init
+	    spectre_v2_parse_app2app_cmdline(enum spectre_v2_mitigation_cmd v2_cmd)
+{
+	enum spectre_v2_app2app_mitigation_cmd cmd;
+	char arg[20];
+	int ret, i;
+
+	if (v2_cmd == SPECTRE_V2_CMD_FORCE) {
+		cmd = SPECTRE_V2_APP2APP_CMD_FORCE;
+		goto show_cmd;
+	}
+
+	cmd = SPECTRE_V2_APP2APP_CMD_AUTO;
+	ret = cmdline_find_option(boot_command_line, "spectre_v2_app2app",
+				  arg, sizeof(arg));
+	if (ret < 0)
+		return SPECTRE_V2_APP2APP_CMD_AUTO;
+
+	/*
+	 * Don't check the last entry for forced mitigation. It is infered from
+	 * v2_cmd == SPECTRE_V2_CMD_FORCE
+	 */
+	for (i = 0; i < ARRAY_SIZE(app2app_options)-1; i++) {
+		if (!match_option(arg, ret, app2app_options[i].option))
+			continue;
+		cmd = app2app_options[i].cmd;
+		break;
+	}
+
+	if (i >= ARRAY_SIZE(app2app_options)) {
+		pr_err("unknown app to app protection option (%s). Switching to AUTO select\n", arg);
+		return SPECTRE_V2_APP2APP_CMD_AUTO;
+	}
+
+show_cmd:
+	if (app2app_options[i].secure)
+		spec2_print_if_secure(app2app_options[i].option);
+	else
+		spec2_print_if_insecure(app2app_options[i].option);
+
+	return cmd;
+}
+
 static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 {
 	char arg[20];
@@ -328,14 +411,19 @@ static bool stibp_needed(void)
 	/*
 	 * Determine if STIBP should be always on.
 	 * Using enhanced IBRS makes using STIBP unnecessary.
+	 * For lite option, STIBP is used only for task with
+	 * TIF_STIBP flag. STIBP is not always on for that case.
 	 */
 
-	if (spectre_v2_enabled == SPECTRE_V2_NONE)
+	if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
 		return false;
 
 	if (static_cpu_has(X86_FEATURE_USE_IBRS_ENHANCED))
 		return false;
 
+	if (static_branch_unlikely(&spectre_v2_app_lite))
+		return false;
+
 	if (!boot_cpu_has(X86_FEATURE_STIBP))
 		return false;
 
@@ -376,6 +464,8 @@ static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
 	enum spectre_v2_mitigation mode = SPECTRE_V2_NONE;
+	enum spectre_v2_app2app_mitigation_cmd app2app_cmd;
+	enum spectre_v2_app2app_mitigation app2app_mode;
 
 	/*
 	 * If the CPU is not affected and the command line mode is NONE or AUTO
@@ -452,12 +542,6 @@ 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 */
-	if (boot_cpu_has(X86_FEATURE_IBPB)) {
-		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
-		pr_info("Spectre v2 mitigation: Enabling Indirect Branch Prediction Barrier\n");
-	}
-
 	/*
 	 * Retpoline means the kernel is safe because it has no indirect
 	 * branches. Enhanced IBRS protects firmware too, so, enable restricted
@@ -474,6 +558,43 @@ static void __init spectre_v2_select_mitigation(void)
 		pr_info("Enabling Restricted Speculation for firmware calls\n");
 	}
 
+	app2app_mode = SPECTRE_V2_APP2APP_NONE;
+	if (!boot_cpu_has(X86_FEATURE_IBPB) ||
+	    !boot_cpu_has(X86_FEATURE_STIBP))
+		goto set_app2app_mode;
+
+	app2app_cmd = spectre_v2_parse_app2app_cmdline(cmd);
+
+	switch (app2app_cmd) {
+	case SPECTRE_V2_APP2APP_CMD_NONE:
+		break;
+
+	case SPECTRE_V2_APP2APP_CMD_LITE:
+	case SPECTRE_V2_APP2APP_CMD_AUTO:
+		app2app_mode = SPECTRE_V2_APP2APP_LITE;
+		break;
+
+	case SPECTRE_V2_APP2APP_CMD_STRICT:
+	case SPECTRE_V2_APP2APP_CMD_FORCE:
+		app2app_mode = SPECTRE_V2_APP2APP_STRICT;
+		break;
+	}
+
+	/*
+	 * Initialize Indirect Branch Prediction Barrier if supported
+	 * and not disabled explicitly
+	 */
+	if (app2app_mode != SPECTRE_V2_APP2APP_NONE) {
+		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
+		pr_info("Spectre v2 mitigation: Enabling Indirect Branch Prediction Barrier\n");
+	}
+
+set_app2app_mode:
+	spectre_v2_app2app_enabled = app2app_mode;
+	pr_info("%s\n", spectre_v2_app2app_strings[app2app_mode]);
+	if (app2app_mode == SPECTRE_V2_APP2APP_LITE)
+		static_branch_enable(&spectre_v2_app_lite);
+
 	/* Enable STIBP if appropriate */
 	arch_smt_update();
 }
@@ -873,21 +994,23 @@ static ssize_t l1tf_show_state(char *buf)
 
 static char *stibp_state(void)
 {
-	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED ||
+	    spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
 		return "";
-
-	if (x86_spec_ctrl_base & SPEC_CTRL_STIBP)
-		return ", STIBP";
+	else if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_LITE)
+		return ", STIBP-lite";
 	else
-		return "";
+		return ", STIBP-strict";
 }
 
 static char *ibpb_state(void)
 {
-	if (boot_cpu_has(X86_FEATURE_USE_IBPB))
-		return ", IBPB";
-	else
+	if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_NONE)
 		return "";
+	else if (spectre_v2_app2app_enabled == SPECTRE_V2_APP2APP_LITE)
+		return ", IBPB-lite";
+	else
+		return ", IBPB-strict";
 }
 
 static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index bddd6b3..9201470 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -184,14 +184,27 @@ static void sync_current_stack_to_mm(struct mm_struct *mm)
 static bool ibpb_needed(struct task_struct *tsk, u64 last_ctx_id)
 {
 	/*
-	 * Check if the current (previous) task has access to the memory
-	 * of the @tsk (next) task. If access is denied, make sure to
-	 * issue a IBPB to stop user->user Spectre-v2 attacks.
+	 * Don't issue IBPB when switching to kernel threads or staying in the
+	 * same mm context.
+	 */
+	if (!tsk || !tsk->mm || tsk->mm->context.ctx_id == last_ctx_id)
+		return false;
+
+	/*
+	 * If lite protection mode is enabled, check the STIBP thread flag.
+	 *
+	 * Otherwise check if the current (previous) task has access to the
+	 * the memory of the @tsk (next) task for strict app to app protection.
+	 * If access is denied, make sure to issue a IBPB to stop user->user
+	 * Spectre-v2 attacks.
 	 *
 	 * Note: __ptrace_may_access() returns 0 or -ERRNO.
 	 */
-	return (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
-		ptrace_may_access_sched(tsk, PTRACE_MODE_SPEC_IBPB));
+
+	if (static_branch_unlikely(&spectre_v2_app_lite))
+		return test_tsk_thread_flag(tsk, TIF_STIBP);
+	else
+		return ptrace_may_access_sched(tsk, PTRACE_MODE_SPEC_IBPB);
 }
 
 void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
-- 
2.9.4