x86/retbleed: Add call depth tracking mitigation

The fully secure mitigation for RSB underflow on Intel SKL CPUs is IBRS,
which inflicts up to 30% penalty for pathological syscall heavy work loads.

Software based call depth tracking and RSB refill is not perfect, but
reduces the attack surface massively. The penalty for the pathological case
is about 8% which is still annoying but definitely more palatable than IBRS.

Add a retbleed=stuff command line option to enable the call depth tracking
and software refill of the RSB.

This gives admins a choice. IBeeRS are safe and cause headaches, call depth
tracking is considered to be s(t)ufficiently safe.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111149.029587352@infradead.org

arch/x86/kernel/cpu/bugs.c

@@ -787,6 +787,7 @@ enum retbleed_mitigation {
 	RETBLEED_MITIGATION_IBPB,
 	RETBLEED_MITIGATION_IBRS,
 	RETBLEED_MITIGATION_EIBRS,
+	RETBLEED_MITIGATION_STUFF,
 };
 
 enum retbleed_mitigation_cmd {
@@ -794,6 +795,7 @@ enum retbleed_mitigation_cmd {
 	RETBLEED_CMD_AUTO,
 	RETBLEED_CMD_UNRET,
 	RETBLEED_CMD_IBPB,
+	RETBLEED_CMD_STUFF,
 };
 
 static const char * const retbleed_strings[] = {
@@ -802,6 +804,7 @@ static const char * const retbleed_strings[] = {
 	[RETBLEED_MITIGATION_IBPB]	= "Mitigation: IBPB",
 	[RETBLEED_MITIGATION_IBRS]	= "Mitigation: IBRS",
 	[RETBLEED_MITIGATION_EIBRS]	= "Mitigation: Enhanced IBRS",
+	[RETBLEED_MITIGATION_STUFF]	= "Mitigation: Stuffing",
 };
 
 static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
@@ -831,6 +834,8 @@ static int __init retbleed_parse_cmdline(char *str)
 			retbleed_cmd = RETBLEED_CMD_UNRET;
 		} else if (!strcmp(str, "ibpb")) {
 			retbleed_cmd = RETBLEED_CMD_IBPB;
+		} else if (!strcmp(str, "stuff")) {
+			retbleed_cmd = RETBLEED_CMD_STUFF;
 		} else if (!strcmp(str, "nosmt")) {
 			retbleed_nosmt = true;
 		} else {
@@ -879,6 +884,21 @@ static void __init retbleed_select_mitigation(void)
 		}
 		break;
 
+	case RETBLEED_CMD_STUFF:
+		if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING) &&
+		    spectre_v2_enabled == SPECTRE_V2_RETPOLINE) {
+			retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
+
+		} else {
+			if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING))
+				pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
+			else
+				pr_err("WARNING: kernel not compiled with CALL_DEPTH_TRACKING.\n");
+
+			goto do_cmd_auto;
+		}
+		break;
+
 do_cmd_auto:
 	case RETBLEED_CMD_AUTO:
 	default:
@@ -916,6 +936,12 @@ static void __init retbleed_select_mitigation(void)
 		mitigate_smt = true;
 		break;
 
+	case RETBLEED_MITIGATION_STUFF:
+		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+		setup_force_cpu_cap(X86_FEATURE_CALL_DEPTH);
+		x86_set_skl_return_thunk();
+		break;
+
 	default:
 		break;
 	}
@@ -926,7 +952,7 @@ static void __init retbleed_select_mitigation(void)
 
 	/*
 	 * Let IBRS trump all on Intel without affecting the effects of the
-	 * retbleed= cmdline option.
+	 * retbleed= cmdline option except for call depth based stuffing
 	 */
 	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
 		switch (spectre_v2_enabled) {
@@ -939,6 +965,7 @@ static void __init retbleed_select_mitigation(void)
 			retbleed_mitigation = RETBLEED_MITIGATION_EIBRS;
 			break;
 		default:
+			if (retbleed_mitigation != RETBLEED_MITIGATION_STUFF)
 				pr_err(RETBLEED_INTEL_MSG);
 		}
 	}
@@ -1413,6 +1440,7 @@ static void __init spectre_v2_select_mitigation(void)
 		if (IS_ENABLED(CONFIG_CPU_IBRS_ENTRY) &&
 		    boot_cpu_has_bug(X86_BUG_RETBLEED) &&
 		    retbleed_cmd != RETBLEED_CMD_OFF &&
+		    retbleed_cmd != RETBLEED_CMD_STUFF &&
 		    boot_cpu_has(X86_FEATURE_IBRS) &&
 		    boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
 			mode = SPECTRE_V2_IBRS;