x86, cpu: Enable/disable Supervisor Mode Execution Protection

Enable/disable newly documented SMEP (Supervisor Mode Execution Protection) CPU
feature in kernel. CR4.SMEP (bit 20) is 0 at power-on. If the feature is
supported by CPU (X86_FEATURE_SMEP), enable SMEP by setting CR4.SMEP. New kernel
option nosmep disables the feature even if the feature is supported by CPU.

[ hpa: moved the call to setup_smep() until after the vendor-specific
  initialization; that ensures that CPUID features are unmasked.  We
  will still run it before we have userspace (never mind uncontrolled
  userspace). ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
LKML-Reference: <1305157865-31727-1-git-send-email-fenghua.yu@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 173f3a3fa1a6..cbc70a27430c 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -254,6 +254,25 @@ static inline void squash_the_stupid_serial_number(struct cpuinfo_x86 *c)
 }
 #endif
 
+static int disable_smep __initdata;
+static __init int setup_disable_smep(char *arg)
+{
+        disable_smep = 1;
+        return 1;
+}
+__setup("nosmep", setup_disable_smep);
+
+static __init void setup_smep(struct cpuinfo_x86 *c)
+{
+        if (cpu_has(c, X86_FEATURE_SMEP)) {
+                if (unlikely(disable_smep)) {
+                        setup_clear_cpu_cap(X86_FEATURE_SMEP);
+                        clear_in_cr4(X86_CR4_SMEP);
+                } else
+                        set_in_cr4(X86_CR4_SMEP);
+        }
+}
+
 /*
  * Some CPU features depend on higher CPUID levels, which may not always
  * be available due to CPUID level capping or broken virtualization
@@ -667,6 +686,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
         c->cpu_index = 0;
 #endif
         filter_cpuid_features(c, false);
+
+        setup_smep(c);
 }
 
 void __init early_cpu_init(void)
@@ -752,6 +773,8 @@ static void __cpuinit generic_identify(struct cpuinfo_x86 *c)
 #endif
         }
 
+        setup_smep(c);
+
         get_model_name(c); /* Default name */
 
         detect_nopl(c);