[PATCH v2 4/4] x86/kernel/common.c: Disable SMEP by kernel option nosmep

["Fenghua Yu" <fenghua.yu@intel.com>]

From: Fenghua Yu <fenghua.yu@intel.com>

SMEP is enabled unconditionally on all CPUs that support it, the nosmep boot
option would turn it off shortly afterwards.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
---
 Documentation/kernel-parameters.txt |    4 ++++
 arch/x86/kernel/cpu/common.c        |   22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 0 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index cc85a92..76c67e5 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1664,6 +1664,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable non-executable mappings
 
+	nosmep		[X86]
+			Disable SMEP (Supervisor Mode Execution Protection)
+			even if it is supported by the processor.
+
 	noexec32	[X86-64]
 			This affects only 32-bit executables.
 			noexec32=on: enable non-executable mappings (default)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e2ced00..cd0762a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -254,6 +254,27 @@ static inline void squash_the_stupid_serial_number(struct cpuinfo_x86 *c)
 }
 #endif
 
+static int disable_smep __initdata;
+
+static __init int setup_nosmep(char *arg)
+{
+	disable_smep = 1;
+	return 1;
+}
+__setup("nosmep", setup_nosmep);
+
+/*
+ * If SMEP is supported by the processor, SMEP has been enabled in CR4 earlier.
+ * But if kernel option "nosmep" is given, we disable SMEP here.
+ */
+static __init void config_smep(struct cpuinfo_x86 *c)
+{
+	if (cpu_has(c, X86_FEATURE_SMEP) && unlikely(disable_smep)) {
+		setup_clear_cpu_cap(X86_FEATURE_SMEP);
+		clear_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
@@ -737,6 +758,7 @@ static void __cpuinit generic_identify(struct cpuinfo_x86 *c)
 	get_cpu_vendor(c);
 
 	get_cpu_cap(c);
+	config_smep(c);
 
 	if (c->cpuid_level >= 0x00000001) {
 		c->initial_apicid = (cpuid_ebx(1) >> 24) & 0xFF;
-- 
1.7.2