x86/bhi: Add BHI mitigation knob

Branch history clearing software sequences and hardware control
BHI_DIS_S were defined to mitigate Branch History Injection (BHI).

Add cmdline spectre_bhi={on|off|auto} to control BHI mitigation:

 auto - Deploy the hardware mitigation BHI_DIS_S, if available.
 on   - Deploy the hardware mitigation BHI_DIS_S, if available,
        otherwise deploy the software sequence at syscall entry and
	VMexit.
 off  - Turn off BHI mitigation.

The default is auto mode which does not deploy the software sequence
mitigation.  This is because of the hardening done in the syscall
dispatch path, which is the likely target of BHI.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>

Documentation/admin-guide/hw-vuln/spectre.rst

@@ -138,11 +138,10 @@ associated with the source address of the indirect branch. Specifically,
 the BHB might be shared across privilege levels even in the presence of
 Enhanced IBRS.
 
-Currently the only known real-world BHB attack vector is via
-unprivileged eBPF. Therefore, it's highly recommended to not enable
-unprivileged eBPF, especially when eIBRS is used (without retpolines).
-For a full mitigation against BHB attacks, it's recommended to use
-retpolines (or eIBRS combined with retpolines).
+Previously the only known real-world BHB attack vector was via unprivileged
+eBPF. Further research has found attacks that don't require unprivileged eBPF.
+For a full mitigation against BHB attacks it is recommended to set BHI_DIS_S or
+use the BHB clearing sequence.
 
 Attack scenarios
 ----------------
@@ -430,6 +429,21 @@ The possible values in this file are:
   'PBRSB-eIBRS: Not affected'  CPU is not affected by PBRSB
   ===========================  =======================================================
 
+  - Branch History Injection (BHI) protection status:
+
+.. list-table::
+
+ * - BHI: Not affected
+   - System is not affected
+ * - BHI: Retpoline
+   - System is protected by retpoline
+ * - BHI: BHI_DIS_S
+   - System is protected by BHI_DIS_S
+ * - BHI: SW loop
+   - System is protected by software clearing sequence
+ * - BHI: Syscall hardening
+   - Syscalls are hardened against BHI
+
 Full mitigation might require a microcode update from the CPU
 vendor. When the necessary microcode is not available, the kernel will
 report vulnerability.
@@ -484,7 +498,11 @@ Spectre variant 2
 
    Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
    boot, by setting the IBRS bit, and they're automatically protected against
-   Spectre v2 variant attacks.
+   some Spectre v2 variant attacks. The BHB can still influence the choice of
+   indirect branch predictor entry, and although branch predictor entries are
+   isolated between modes when eIBRS is enabled, the BHB itself is not isolated
+   between modes. Systems which support BHI_DIS_S will set it to protect against
+   BHI attacks.
 
    On Intel's enhanced IBRS systems, this includes cross-thread branch target
    injections on SMT systems (STIBP). In other words, Intel eIBRS enables
@@ -638,6 +656,21 @@ kernel command line.
 		spectre_v2=off. Spectre variant 1 mitigations
 		cannot be disabled.
 
+	spectre_bhi=
+
+		[X86] Control mitigation of Branch History Injection
+		(BHI) vulnerability. Syscalls are hardened against BHI
+		regardless of this setting. This setting affects the deployment
+		of the HW BHI control and the SW BHB clearing sequence.
+
+		on
+			unconditionally enable.
+		off
+			unconditionally disable.
+		auto
+			enable if hardware mitigation
+			control(BHI_DIS_S) is available.
+
 For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
 
 Mitigation selection guide

Documentation/admin-guide/kernel-parameters.txt

@@ -6063,6 +6063,17 @@
 	sonypi.*=	[HW] Sony Programmable I/O Control Device driver
 			See Documentation/admin-guide/laptops/sonypi.rst
 
+	spectre_bhi=	[X86] Control mitigation of Branch History Injection
+			(BHI) vulnerability. Syscalls are hardened against BHI
+			reglardless of this setting. This setting affects the
+			deployment of the HW BHI control and the SW BHB
+			clearing sequence.
+
+			on   - unconditionally enable.
+			off  - unconditionally disable.
+			auto - (default) enable only if hardware mitigation
+			       control(BHI_DIS_S) is available.
+
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.
 			The default operation protects the kernel from

arch/x86/Kconfig

@@ -2633,6 +2633,31 @@ config MITIGATION_RFDS
 	  stored in floating point, vector and integer registers.
 	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
 
+choice
+	prompt "Clear branch history"
+	depends on CPU_SUP_INTEL
+	default SPECTRE_BHI_AUTO
+	help
+	  Enable BHI mitigations. BHI attacks are a form of Spectre V2 attacks
+	  where the branch history buffer is poisoned to speculatively steer
+	  indirect branches.
+	  See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config SPECTRE_BHI_ON
+	bool "on"
+	help
+	  Equivalent to setting spectre_bhi=on command line parameter.
+config SPECTRE_BHI_OFF
+	bool "off"
+	help
+	  Equivalent to setting spectre_bhi=off command line parameter.
+config SPECTRE_BHI_AUTO
+	bool "auto"
+	help
+	  Equivalent to setting spectre_bhi=auto command line parameter.
+
+endchoice
+
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/include/asm/cpufeatures.h

@@ -468,6 +468,7 @@
 #define X86_FEATURE_AMD_LBR_PMC_FREEZE	(21*32+ 0) /* AMD LBR and PMC Freeze */
 #define X86_FEATURE_CLEAR_BHB_LOOP	(21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */
 #define X86_FEATURE_BHI_CTRL		(21*32+ 2) /* "" BHI_DIS_S HW control available */
+#define X86_FEATURE_CLEAR_BHB_HW	(21*32+ 3) /* "" BHI_DIS_S HW control enabled */
 
 /*
  * BUG word(s)

arch/x86/kernel/cpu/bugs.c

@@ -1607,6 +1607,74 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
 	dump_stack();
 }
 
+/*
+ * Set BHI_DIS_S to prevent indirect branches in kernel to be influenced by
+ * branch history in userspace. Not needed if BHI_NO is set.
+ */
+static bool __init spec_ctrl_bhi_dis(void)
+{
+	if (!boot_cpu_has(X86_FEATURE_BHI_CTRL))
+		return false;
+
+	x86_spec_ctrl_base |= SPEC_CTRL_BHI_DIS_S;
+	update_spec_ctrl(x86_spec_ctrl_base);
+	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_HW);
+
+	return true;
+}
+
+enum bhi_mitigations {
+	BHI_MITIGATION_OFF,
+	BHI_MITIGATION_ON,
+	BHI_MITIGATION_AUTO,
+};
+
+static enum bhi_mitigations bhi_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_SPECTRE_BHI_ON)  ? BHI_MITIGATION_ON  :
+	IS_ENABLED(CONFIG_SPECTRE_BHI_OFF) ? BHI_MITIGATION_OFF :
+					     BHI_MITIGATION_AUTO;
+
+static int __init spectre_bhi_parse_cmdline(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strcmp(str, "off"))
+		bhi_mitigation = BHI_MITIGATION_OFF;
+	else if (!strcmp(str, "on"))
+		bhi_mitigation = BHI_MITIGATION_ON;
+	else if (!strcmp(str, "auto"))
+		bhi_mitigation = BHI_MITIGATION_AUTO;
+	else
+		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
+
+	return 0;
+}
+early_param("spectre_bhi", spectre_bhi_parse_cmdline);
+
+static void __init bhi_select_mitigation(void)
+{
+	if (bhi_mitigation == BHI_MITIGATION_OFF)
+		return;
+
+	/* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
+	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
+	    !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
+		return;
+
+	if (spec_ctrl_bhi_dis())
+		return;
+
+	if (!IS_ENABLED(CONFIG_X86_64))
+		return;
+
+	if (bhi_mitigation == BHI_MITIGATION_AUTO)
+		return;
+
+	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
+	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -1718,6 +1786,9 @@ static void __init spectre_v2_select_mitigation(void)
 	    mode == SPECTRE_V2_RETPOLINE)
 		spec_ctrl_disable_kernel_rrsba();
 
+	if (boot_cpu_has(X86_BUG_BHI))
+		bhi_select_mitigation();
+
 	spectre_v2_enabled = mode;
 	pr_info("%s\n", spectre_v2_strings[mode]);
 
@@ -2733,6 +2804,21 @@ static char *pbrsb_eibrs_state(void)
 	}
 }
 
+static const char * const spectre_bhi_state(void)
+{
+	if (!boot_cpu_has_bug(X86_BUG_BHI))
+		return "; BHI: Not affected";
+	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_HW))
+		return "; BHI: BHI_DIS_S";
+	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
+		return "; BHI: SW loop";
+	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
+		 !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
+		return "; BHI: Retpoline";
+
+	return "; BHI: Vulnerable (Syscall hardening enabled)";
+}
+
 static ssize_t spectre_v2_show_state(char *buf)
 {
 	if (spectre_v2_enabled == SPECTRE_V2_LFENCE)
@@ -2745,13 +2831,15 @@ static ssize_t spectre_v2_show_state(char *buf)
 	    spectre_v2_enabled == SPECTRE_V2_EIBRS_LFENCE)
 		return sysfs_emit(buf, "Vulnerable: eIBRS+LFENCE with unprivileged eBPF and SMT\n");
 
-	return sysfs_emit(buf, "%s%s%s%s%s%s%s\n",
+	return sysfs_emit(buf, "%s%s%s%s%s%s%s%s\n",
 			  spectre_v2_strings[spectre_v2_enabled],
 			  ibpb_state(),
 			  boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? "; IBRS_FW" : "",
 			  stibp_state(),
 			  boot_cpu_has(X86_FEATURE_RSB_CTXSW) ? "; RSB filling" : "",
 			  pbrsb_eibrs_state(),
+			  spectre_bhi_state(),
+			  /* this should always be at the end */
 			  spectre_v2_module_string());
 }