x86/bugs: Add a separate config for SSB

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 SSB CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240729164105.554296-10-leitao@debian.org

arch/x86/Kconfig

@@ -2741,6 +2741,16 @@ config MITIGATION_SRBDS
 	  using MDS techniques.
 	  See also
 	  <file:Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst>
+
+config MITIGATION_SSB
+	bool "Mitigate Speculative Store Bypass (SSB) hardware bug"
+	default y
+	help
+	  Enable mitigation for Speculative Store Bypass (SSB). SSB is a
+	  hardware security vulnerability and its exploitation takes advantage
+	  of speculative execution in a similar way to the Meltdown and Spectre
+	  security vulnerabilities.
+
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/kernel/cpu/bugs.c

@@ -2027,10 +2027,12 @@ static const struct {
 
 static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 {
-	enum ssb_mitigation_cmd cmd = SPEC_STORE_BYPASS_CMD_AUTO;
+	enum ssb_mitigation_cmd cmd;
 	char arg[20];
 	int ret, i;
 
+	cmd = IS_ENABLED(CONFIG_MITIGATION_SSB) ?
+		SPEC_STORE_BYPASS_CMD_AUTO : SPEC_STORE_BYPASS_CMD_NONE;
 	if (cmdline_find_option_bool(boot_command_line, "nospec_store_bypass_disable") ||
 	    cpu_mitigations_off()) {
 		return SPEC_STORE_BYPASS_CMD_NONE;
@@ -2038,7 +2040,7 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 		ret = cmdline_find_option(boot_command_line, "spec_store_bypass_disable",
 					  arg, sizeof(arg));
 		if (ret < 0)
-			return SPEC_STORE_BYPASS_CMD_AUTO;
+			return cmd;
 
 		for (i = 0; i < ARRAY_SIZE(ssb_mitigation_options); i++) {
 			if (!match_option(arg, ret, ssb_mitigation_options[i].option))
@@ -2049,8 +2051,8 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
 		}
 
 		if (i >= ARRAY_SIZE(ssb_mitigation_options)) {
-			pr_err("unknown option (%s). Switching to AUTO select\n", arg);
-			return SPEC_STORE_BYPASS_CMD_AUTO;
+			pr_err("unknown option (%s). Switching to default mode\n", arg);
+			return cmd;
 		}
 	}