x86/bugs: Enable STIBP for JMP2RET

For untrained return thunks to be fully effective, STIBP must be enabled
or SMT disabled.
Co-developed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -5201,11 +5201,17 @@
 			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.
+			off          - no mitigation
+			auto         - automatically select a migitation
+			auto,nosmt   - automatically select a mitigation,
+				       disabling SMT if necessary for
+				       the full mitigation (only on Zen1
+				       and older without STIBP).
+			unret        - force enable untrained return thunks,
+				       only effective on AMD f15h-f17h
+				       based systems.
+			unret,nosmt  - like unret, will disable SMT when STIBP
+			               is not available.
 
 			Selecting 'auto' will choose a mitigation method at run
 			time according to the CPU.

arch/x86/kernel/cpu/bugs.c

@@ -783,19 +783,34 @@ static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
 static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
 	RETBLEED_CMD_AUTO;
 
+static int __ro_after_init retbleed_nosmt = false;
+
 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);
+	while (str) {
+		char *next = strchr(str, ',');
+		if (next) {
+			*next = 0;
+			next++;
+		}
+
+		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 if (!strcmp(str, "nosmt")) {
+			retbleed_nosmt = true;
+		} else {
+			pr_err("Ignoring unknown retbleed option (%s).", str);
+		}
+
+		str = next;
+	}
 
 	return 0;
 }
@@ -841,6 +856,10 @@ static void __init retbleed_select_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
 		setup_force_cpu_cap(X86_FEATURE_UNRET);
 
+		if (!boot_cpu_has(X86_FEATURE_STIBP) &&
+		    (retbleed_nosmt || cpu_mitigations_auto_nosmt()))
+			cpu_smt_disable(false);
+
 		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
 		    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
 			pr_err(RETBLEED_UNTRAIN_MSG);
@@ -1087,6 +1106,13 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
 		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
 
+	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+		if (mode != SPECTRE_V2_USER_STRICT &&
+		    mode != SPECTRE_V2_USER_STRICT_PREFERRED)
+			pr_info("Selecting STIBP always-on mode to complement retbleed mitigation'\n");
+		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
+	}
+
 	spectre_v2_user_stibp = mode;
 
 set_mode:
@@ -2097,10 +2123,18 @@ static ssize_t srbds_show_state(char *buf)
 
 static ssize_t retbleed_show_state(char *buf)
 {
-	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");
+	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
+	    if (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; SMT %s\n",
+			   retbleed_strings[retbleed_mitigation],
+			   !sched_smt_active() ? "disabled" :
+			   spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT ||
+			   spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT_PREFERRED ?
+			   "enabled with STIBP protection" : "vulnerable");
+	}
 
 	return sprintf(buf, "%s\n", retbleed_strings[retbleed_mitigation]);
 }