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Subject[PATCH v3 4/4] x86/kernel/cpu/common.c: Disable SMEP by kernel option nosmep
Date
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..9dcec58 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) && 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


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