x86/pti: Add the pti= cmdline option and documentation

Keep the "nopti" optional for traditional reasons.

[ tglx: Don't allow force on when running on XEN PV and made 'on'
	printout conditional ]

Requested-by: Linus Torvalds <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirsky <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Laight <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Eduardo Valentin <[email protected]>
Cc: Greg KH <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: suryasaimadhu

Documentation/admin-guide/kernel-parameters.txt

@@ -3255,6 +3255,12 @@
 	pt.		[PARIDE]
 			See Documentation/blockdev/paride.txt.
 
+	pti=		[X86_64]
+			Control user/kernel address space isolation:
+			on - enable
+			off - disable
+			auto - default setting
+
 	pty.legacy_count=
 			[KNL] Number of legacy pty's. Overwrites compiled-in
 			default number.

arch/x86/mm/pti.c

@@ -54,21 +54,45 @@ static void __init pti_print_if_insecure(const char *reason)
 		pr_info("%s\n", reason);
 }
 
+static void __init pti_print_if_secure(const char *reason)
+{
+	if (!boot_cpu_has_bug(X86_BUG_CPU_INSECURE))
+		pr_info("%s\n", reason);
+}
+
 void __init pti_check_boottime_disable(void)
 {
+	char arg[5];
+	int ret;
+
 	if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
 		pti_print_if_insecure("disabled on XEN PV.");
 		return;
 	}
 
+	ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
+	if (ret > 0)  {
+		if (ret == 3 && !strncmp(arg, "off", 3)) {
+			pti_print_if_insecure("disabled on command line.");
+			return;
+		}
+		if (ret == 2 && !strncmp(arg, "on", 2)) {
+			pti_print_if_secure("force enabled on command line.");
+			goto enable;
+		}
+		if (ret == 4 && !strncmp(arg, "auto", 4))
+			goto autosel;
+	}
+
 	if (cmdline_find_option_bool(boot_command_line, "nopti")) {
 		pti_print_if_insecure("disabled on command line.");
 		return;
 	}
 
+autosel:
 	if (!boot_cpu_has_bug(X86_BUG_CPU_INSECURE))
 		return;
-
+enable:
 	setup_force_cpu_cap(X86_FEATURE_PTI);
 }