x86/kaiser: Check boottime cmdline params

AMD (and possibly other vendors) are not affected by the leak
KAISER is protecting against.

Keep the "nopti" for traditional reasons and add pti=<on|off|auto>
like upstream.
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

CVE-2017-5754
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Documentation/kernel-parameters.txt

@@ -3062,6 +3062,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	pt.		[PARIDE]
 			See Documentation/blockdev/paride.txt.
 
+	pti=		[X86_64]
+			Control KAISER 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/kaiser.c

@@ -16,6 +16,7 @@
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
 #include <asm/desc.h>
+#include <asm/cmdline.h>
 
 int kaiser_enabled __read_mostly = 1;
 EXPORT_SYMBOL(kaiser_enabled);	/* for inlined TLB flush functions */
@@ -264,6 +265,43 @@ static void __init kaiser_init_all_pgds(void)
 	WARN_ON(__ret);							\
 } while (0)
 
+void __init kaiser_check_boottime_disable(void)
+{
+	bool enable = true;
+	char arg[5];
+	int ret;
+
+	ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
+	if (ret > 0) {
+		if (!strncmp(arg, "on", 2))
+			goto enable;
+
+		if (!strncmp(arg, "off", 3))
+			goto disable;
+
+		if (!strncmp(arg, "auto", 4))
+			goto skip;
+	}
+
+	if (cmdline_find_option_bool(boot_command_line, "nopti"))
+		goto disable;
+
+skip:
+	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
+		goto disable;
+
+enable:
+	if (enable)
+		setup_force_cpu_cap(X86_FEATURE_KAISER);
+
+	return;
+
+disable:
+	pr_info("Kernel/User page tables isolation: disabled\n");
+	kaiser_enabled = 0;
+	setup_clear_cpu_cap(X86_FEATURE_KAISER);
+}
+
 /*
  * If anything in here fails, we will likely die on one of the
  * first kernel->user transitions and init will die.  But, we
@@ -275,12 +313,10 @@ void __init kaiser_init(void)
 {
 	int cpu;
 
-	if (!kaiser_enabled) {
-		setup_clear_cpu_cap(X86_FEATURE_KAISER);
-		return;
-	}
+	kaiser_check_boottime_disable();
 
-	setup_force_cpu_cap(X86_FEATURE_KAISER);
+	if (!kaiser_enabled)
+		return;
 
 	kaiser_init_all_pgds();
 
@@ -424,16 +460,3 @@ void kaiser_flush_tlb_on_return_to_user(void)
 			X86_CR3_PCID_USER_FLUSH | KAISER_SHADOW_PGD_OFFSET);
 }
 EXPORT_SYMBOL(kaiser_flush_tlb_on_return_to_user);
-
-static int __init x86_nokaiser_setup(char *s)
-{
-	/* nopti doesn't accept parameters */
-	if (s)
-		return -EINVAL;
-
-	kaiser_enabled = 0;
-	pr_info("Kernel/User page tables isolation: disabled\n");
-
-	return 0;
-}
-early_param("nopti", x86_nokaiser_setup);