x86, smap: Turn on Supervisor Mode Access Prevention

If Supervisor Mode Access Prevention is available and not disabled by
the user, turn it on.  Also fix the expansion of SMEP (Supervisor Mode
Execution Prevention.)
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-10-git-send-email-hpa@linux.intel.com

Documentation/kernel-parameters.txt

@@ -1812,8 +1812,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable non-executable mappings
 
+	nosmap		[X86]
+			Disable SMAP (Supervisor Mode Access Prevention)
+			even if it is supported by processor.
+
 	nosmep		[X86]
-			Disable SMEP (Supervisor Mode Execution Protection)
+			Disable SMEP (Supervisor Mode Execution Prevention)
 			even if it is supported by processor.
 
 	noexec32	[X86-64]

arch/x86/kernel/cpu/common.c

@@ -278,6 +278,31 @@ static __cpuinit void setup_smep(struct cpuinfo_x86 *c)
 	}
 }
 
+static int disable_smap __cpuinitdata;
+static __init int setup_disable_smap(char *arg)
+{
+	disable_smap = 1;
+	return 1;
+}
+__setup("nosmap", setup_disable_smap);
+
+static __cpuinit void setup_smap(struct cpuinfo_x86 *c)
+{
+	if (cpu_has(c, X86_FEATURE_SMAP)) {
+		if (unlikely(disable_smap)) {
+			setup_clear_cpu_cap(X86_FEATURE_SMAP);
+			clear_in_cr4(X86_CR4_SMAP);
+		} else {
+			set_in_cr4(X86_CR4_SMAP);
+			/*
+			 * Don't use clac() here since alternatives
+			 * haven't run yet...
+			 */
+			asm volatile(__stringify(__ASM_CLAC) ::: "memory");
+		}
+	}
+}
+
 /*
  * Some CPU features depend on higher CPUID levels, which may not always
  * be available due to CPUID level capping or broken virtualization
@@ -713,6 +738,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 	filter_cpuid_features(c, false);
 
 	setup_smep(c);
+	setup_smap(c);
 
 	if (this_cpu->c_bsp_init)
 		this_cpu->c_bsp_init(c);