x86/speculation: Enable prctl mode for spectre_v2_user

Now that all prerequisites are in place:

- Add the prctl command line option

- Default the 'auto' mode to 'prctl'

- When SMT state changes, update the static key which controls the
conditional STIBP evaluation on context switch.

- At init update the static key which controls the conditional IBPB
evaluation on context switch.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Casey Schaufler <[e-mail]>
Cc: Asit Mallick <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Jon Masters <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Greg KH <[e-mail]>
Cc: Dave Stewart <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/[e-mail]

Documentation/admin-guide/kernel-parameters.txt

···
 			off     - Unconditionally disable mitigations. Is
 				  enforced by spectre_v2=off
 
+			prctl   - Indirect branch speculation is enabled,
+				  but mitigation can be enabled via prctl
+				  per thread.  The mitigation control state
+				  is inherited on fork.
+
 			auto    - Kernel selects the mitigation depending on
 				  the available CPU features and vulnerability.
-				  Default is off.
+				  Default is prctl.
 
 			Not specifying this option is equivalent to
 			spectre_v2_user=auto.

arch/x86/kernel/cpu/bugs.c

···
 	SPECTRE_V2_USER_CMD_NONE,
 	SPECTRE_V2_USER_CMD_AUTO,
 	SPECTRE_V2_USER_CMD_FORCE,
+	SPECTRE_V2_USER_CMD_PRCTL,
 };
 
 static const char * const spectre_v2_user_strings[] = {
 	[SPECTRE_V2_USER_NONE]		= "User space: Vulnerable",
 	[SPECTRE_V2_USER_STRICT]	= "User space: Mitigation: STIBP protection",
+	[SPECTRE_V2_USER_PRCTL]		= "User space: Mitigation: STIBP via prctl",
 };
 
 static const struct {
···
 	{ "auto",	SPECTRE_V2_USER_CMD_AUTO,	false },
 	{ "off",	SPECTRE_V2_USER_CMD_NONE,	false },
 	{ "on",		SPECTRE_V2_USER_CMD_FORCE,	true  },
+	{ "prctl",	SPECTRE_V2_USER_CMD_PRCTL,	false },
 };
 
 static void __init spec_v2_user_print_cond(const char *reason, bool secure)
···
 		smt_possible = false;
 
 	switch (spectre_v2_parse_user_cmdline(v2_cmd)) {
-	case SPECTRE_V2_USER_CMD_AUTO:
 	case SPECTRE_V2_USER_CMD_NONE:
 		goto set_mode;
 	case SPECTRE_V2_USER_CMD_FORCE:
 		mode = SPECTRE_V2_USER_STRICT;
+		break;
+	case SPECTRE_V2_USER_CMD_AUTO:
+	case SPECTRE_V2_USER_CMD_PRCTL:
+		mode = SPECTRE_V2_USER_PRCTL;
 		break;
 	}
 
···
 		switch (mode) {
 		case SPECTRE_V2_USER_STRICT:
 			static_branch_enable(&switch_mm_always_ibpb);
+			break;
+		case SPECTRE_V2_USER_PRCTL:
+			static_branch_enable(&switch_mm_cond_ibpb);
 			break;
 		default:
 			break;
···
 	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
 		return;
 
+	/*
+	 * If SMT is not possible or STIBP is not available clear the STIPB
+	 * mode.
+	 */
+	if (!smt_possible || !boot_cpu_has(X86_FEATURE_STIBP))
+		mode = SPECTRE_V2_USER_NONE;
 set_mode:
 	spectre_v2_user = mode;
 	/* Only print the STIBP mode when SMT possible */
···
 	on_each_cpu(update_stibp_msr, NULL, 1);
 }
 
+/* Update the static key controlling the evaluation of TIF_SPEC_IB */
+static void update_indir_branch_cond(void)
+{
+	if (sched_smt_active())
+		static_branch_enable(&switch_to_cond_stibp);
+	else
+		static_branch_disable(&switch_to_cond_stibp);
+}
+
 void arch_smt_update(void)
 {
 	/* Enhanced IBRS implies STIBP. No update required. */
···
 		update_stibp_strict();
 		break;
 	case SPECTRE_V2_USER_PRCTL:
+		update_indir_branch_cond();
 		break;
 	}
 
···
 	case SPECTRE_V2_USER_STRICT:
 		return ", STIBP: forced";
 	case SPECTRE_V2_USER_PRCTL:
-		return "";
+		if (static_key_enabled(&switch_to_cond_stibp))
+			return ", STIBP: conditional";
 	}
 	return "";
 }
···
 static char *ibpb_state(void)
 {
 	if (boot_cpu_has(X86_FEATURE_IBPB)) {
-		switch (spectre_v2_user) {
-		case SPECTRE_V2_USER_NONE:
-			return ", IBPB: disabled";
-		case SPECTRE_V2_USER_STRICT:
+		if (static_key_enabled(&switch_mm_always_ibpb))
 			return ", IBPB: always-on";
-		case SPECTRE_V2_USER_PRCTL:
-			return "";
-		}
+		if (static_key_enabled(&switch_mm_cond_ibpb))
+			return ", IBPB: conditional";
+		return ", IBPB: disabled";
 	}
 	return "";
 }