[PATCH v4 08/11] x86/bugs: Add a separate config for Spectre V2

[Breno Leitao]

Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
where some mitigations have entries in Kconfig, and they could be
modified, while others mitigations do not have Kconfig entries, and
could not be controlled at build time.

Create an entry for the Spectre V2 CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 12 ++++++++++++
 arch/x86/kernel/cpu/bugs.c |  9 +++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 22d324581a60..33e125a28f79 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2716,6 +2716,18 @@ config MITIGATION_SPECTRE_V1
 	  memory access bounds check.
 	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
 
+config MITIGATION_SPECTRE_V2
+	bool "Mitigate SPECTRE V2 hardware bug"
+	default y
+	help
+	  Enable mitigation for Spectre V2 (Branch Target Injection). Spectre
+	  V2 is a class of side channel attacks that takes advantage of
+	  indirect branch predictors inside the processor. In Spectre variant 2
+	  attacks, the attacker can steer speculative indirect branches in the
+	  victim to gadget code by poisoning the branch target buffer of a CPU
+	  used for predicting indirect branch addresses.
+	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
 config MITIGATION_SRBDS
 	bool "Mitigate Special Register Buffer Data Sampling (SRBDS) hardware bug"
 	depends on CPU_SUP_INTEL
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 8292a96d376c..45cbc6f994ca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1452,17 +1452,18 @@ static void __init spec_v2_print_cond(const char *reason, bool secure)
 
 static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 {
-	enum spectre_v2_mitigation_cmd cmd = SPECTRE_V2_CMD_AUTO;
+	enum spectre_v2_mitigation_cmd cmd;
 	char arg[20];
 	int ret, i;
 
+	cmd = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?  SPECTRE_V2_CMD_AUTO : SPECTRE_V2_CMD_NONE;
 	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));
 	if (ret < 0)
-		return SPECTRE_V2_CMD_AUTO;
+		return cmd;
 
 	for (i = 0; i < ARRAY_SIZE(mitigation_options); i++) {
 		if (!match_option(arg, ret, mitigation_options[i].option))
@@ -1472,8 +1473,8 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	}
 
 	if (i >= ARRAY_SIZE(mitigation_options)) {
-		pr_err("unknown option (%s). Switching to AUTO select\n", arg);
-		return SPECTRE_V2_CMD_AUTO;
+		pr_err("unknown option (%s). Switching to default mode\n", arg);
+		return cmd;
 	}
 
 	if ((cmd == SPECTRE_V2_CMD_RETPOLINE ||
-- 
2.43.0