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    Subject[PATCH v7 03/18] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE
    Date
    From: Andy Lutomirski <luto@kernel.org>

    This is temporary. It will allow the next few patches to be tested
    incrementally.

    Setting unsafe_fsgsbase is a root hole. Don't do it.

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
    Reviewed-by: Andi Kleen <ak@linux.intel.com>
    Reviewed-by: Andy Lutomirski <luto@kernel.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Randy Dunlap <rdunlap@infradead.org>
    ---
    Documentation/admin-guide/kernel-parameters.txt | 3 +++
    arch/x86/kernel/cpu/common.c | 24 ++++++++++++++++++++++++
    2 files changed, 27 insertions(+)

    diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    index fd03e2b..7fe1da0 100644
    --- a/Documentation/admin-guide/kernel-parameters.txt
    +++ b/Documentation/admin-guide/kernel-parameters.txt
    @@ -2827,6 +2827,9 @@
    no5lvl [X86-64] Disable 5-level paging mode. Forces
    kernel to use 4-level paging instead.

    + unsafe_fsgsbase [X86] Allow FSGSBASE instructions. This will be
    + replaced with a nofsgsbase flag.
    +
    no_console_suspend
    [HW] Never suspend the console
    Disable suspending of consoles during suspend and
    diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
    index 8739bdf..5743cb9 100644
    --- a/arch/x86/kernel/cpu/common.c
    +++ b/arch/x86/kernel/cpu/common.c
    @@ -366,6 +366,22 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
    }

    /*
    + * Temporary hack: FSGSBASE is unsafe until a few kernel code paths are
    + * updated. This allows us to get the kernel ready incrementally.
    + *
    + * Once all the pieces are in place, these will go away and be replaced with
    + * a nofsgsbase chicken flag.
    + */
    +static bool unsafe_fsgsbase;
    +
    +static __init int setup_unsafe_fsgsbase(char *arg)
    +{
    + unsafe_fsgsbase = true;
    + return 1;
    +}
    +__setup("unsafe_fsgsbase", setup_unsafe_fsgsbase);
    +
    +/*
    * Protection Keys are not available in 32-bit mode.
    */
    static bool pku_disabled;
    @@ -1344,6 +1360,14 @@ static void identify_cpu(struct cpuinfo_x86 *c)
    setup_smap(c);
    setup_umip(c);

    + /* Enable FSGSBASE instructions if available. */
    + if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
    + if (unsafe_fsgsbase)
    + cr4_set_bits(X86_CR4_FSGSBASE);
    + else
    + clear_cpu_cap(c, X86_FEATURE_FSGSBASE);
    + }
    +
    /*
    * The vendor-specific functions might have changed features.
    * Now we do "generic changes."
    --
    2.7.4
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