x86/bugs: Add AMD retbleed= boot parameter

Add the "retbleed=<value>" boot parameter to select a mitigation for
RETBleed. Possible values are "off", "auto" and "unret"
(JMP2RET mitigation). The default value is "auto".

Currently, "retbleed=auto" will select the unret mitigation on
AMD and Hygon and no mitigation on Intel (JMP2RET is not effective on
Intel).

  [peterz: rebase; add hygon]
  [jpoimboe: cleanups]

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -5197,6 +5197,21 @@
 
 	retain_initrd	[RAM] Keep initrd memory after extraction
 
+	retbleed=	[X86] Control mitigation of RETBleed (Arbitrary
+			Speculative Code Execution with Return Instructions)
+			vulnerability.
+
+			off         - unconditionally disable
+			auto        - automatically select a migitation
+			unret       - force enable untrained return thunks,
+				      only effective on AMD Zen {1,2}
+				      based systems.
+
+			Selecting 'auto' will choose a mitigation method at run
+			time according to the CPU.
+
+			Not specifying this option is equivalent to retbleed=auto.
+
 	rfkill.default_state=
 		0	"airplane mode".  All wifi, bluetooth, wimax, gps, fm,
 			etc. communication is blocked by default.

arch/x86/Kconfig

@@ -475,6 +475,9 @@ config RETPOLINE
 config CC_HAS_SLS
 	def_bool $(cc-option,-mharden-sls=all)
 
+config CC_HAS_RETURN_THUNK
+	def_bool $(cc-option,-mfunction-return=thunk-extern)
+
 config SLS
 	bool "Mitigate Straight-Line-Speculation"
 	depends on CC_HAS_SLS && X86_64

arch/x86/kernel/cpu/bugs.c

@@ -37,6 +37,7 @@
 #include "cpu.h"
 
 static void __init spectre_v1_select_mitigation(void);
+static void __init retbleed_select_mitigation(void);
 static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
@@ -120,6 +121,12 @@ void __init check_bugs(void)
 
 	/* Select the proper CPU mitigations before patching alternatives: */
 	spectre_v1_select_mitigation();
+	retbleed_select_mitigation();
+	/*
+	 * spectre_v2_select_mitigation() relies on the state set by
+	 * retbleed_select_mitigation(); specifically the STIBP selection is
+	 * forced for UNRET.
+	 */
 	spectre_v2_select_mitigation();
 	ssb_select_mitigation();
 	l1tf_select_mitigation();
@@ -752,6 +759,100 @@ static int __init nospectre_v1_cmdline(char *str)
 }
 early_param("nospectre_v1", nospectre_v1_cmdline);
 
+#undef pr_fmt
+#define pr_fmt(fmt)     "RETBleed: " fmt
+
+enum retbleed_mitigation {
+	RETBLEED_MITIGATION_NONE,
+	RETBLEED_MITIGATION_UNRET,
+};
+
+enum retbleed_mitigation_cmd {
+	RETBLEED_CMD_OFF,
+	RETBLEED_CMD_AUTO,
+	RETBLEED_CMD_UNRET,
+};
+
+const char * const retbleed_strings[] = {
+	[RETBLEED_MITIGATION_NONE]	= "Vulnerable",
+	[RETBLEED_MITIGATION_UNRET]	= "Mitigation: untrained return thunk",
+};
+
+static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
+	RETBLEED_MITIGATION_NONE;
+static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
+	RETBLEED_CMD_AUTO;
+
+static int __init retbleed_parse_cmdline(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!strcmp(str, "off"))
+		retbleed_cmd = RETBLEED_CMD_OFF;
+	else if (!strcmp(str, "auto"))
+		retbleed_cmd = RETBLEED_CMD_AUTO;
+	else if (!strcmp(str, "unret"))
+		retbleed_cmd = RETBLEED_CMD_UNRET;
+	else
+		pr_err("Unknown retbleed option (%s). Defaulting to 'auto'\n", str);
+
+	return 0;
+}
+early_param("retbleed", retbleed_parse_cmdline);
+
+#define RETBLEED_UNTRAIN_MSG "WARNING: BTB untrained return thunk mitigation is only effective on AMD/Hygon!\n"
+#define RETBLEED_COMPILER_MSG "WARNING: kernel not compiled with RETPOLINE or -mfunction-return capable compiler!\n"
+
+static void __init retbleed_select_mitigation(void)
+{
+	if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off())
+		return;
+
+	switch (retbleed_cmd) {
+	case RETBLEED_CMD_OFF:
+		return;
+
+	case RETBLEED_CMD_UNRET:
+		retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		break;
+
+	case RETBLEED_CMD_AUTO:
+	default:
+		if (!boot_cpu_has_bug(X86_BUG_RETBLEED))
+			break;
+
+		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
+		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
+			retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+		break;
+	}
+
+	switch (retbleed_mitigation) {
+	case RETBLEED_MITIGATION_UNRET:
+
+		if (!IS_ENABLED(CONFIG_RETPOLINE) ||
+		    !IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK)) {
+			pr_err(RETBLEED_COMPILER_MSG);
+			retbleed_mitigation = RETBLEED_MITIGATION_NONE;
+			break;
+		}
+
+		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+		setup_force_cpu_cap(X86_FEATURE_UNRET);
+
+		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
+		    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
+			pr_err(RETBLEED_UNTRAIN_MSG);
+		break;
+
+	default:
+		break;
+	}
+
+	pr_info("%s\n", retbleed_strings[retbleed_mitigation]);
+}
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
@@ -1996,7 +2097,12 @@ static ssize_t srbds_show_state(char *buf)
 
 static ssize_t retbleed_show_state(char *buf)
 {
-	return sprintf(buf, "Vulnerable\n");
+	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET &&
+	    (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
+	     boot_cpu_data.x86_vendor != X86_VENDOR_HYGON))
+		return sprintf(buf, "Vulnerable: untrained return thunk on non-Zen uarch\n");
+
+	return sprintf(buf, "%s\n", retbleed_strings[retbleed_mitigation]);
 }
 
 static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,