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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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