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    Subject[PATCH 15/30] x86, kaiser: map entry stack variables
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    From: Dave Hansen <dave.hansen@linux.intel.com>

    There are times where the kernel is entered but there is not a
    safe stack, like at SYSCALL entry. To obtain a safe stack, the
    per-cpu variables 'rsp_scratch' and 'cpu_current_top_of_stack'
    are used to save the old %rsp value and to find where the kernel
    stack should start.

    You can not directly manipulate the CR3 register. You can only
    'MOV' to it from another register, which means a register must be
    clobbered in order to do any CR3 manipulation. User-mapping
    these variables allows us to obtain a safe stack and use it for
    temporary storage *before* CR3 is switched.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
    Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
    Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
    Cc: Richard Fellner <richard.fellner@student.tugraz.at>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Kees Cook <keescook@google.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: x86@kernel.org
    ---

    b/arch/x86/kernel/cpu/common.c | 2 +-
    b/arch/x86/kernel/process_64.c | 2 +-
    2 files changed, 2 insertions(+), 2 deletions(-)

    diff -puN arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/cpu/common.c
    --- a/arch/x86/kernel/cpu/common.c~kaiser-user-map-stack-helper-vars 2017-11-10 11:22:13.203244939 -0800
    +++ b/arch/x86/kernel/cpu/common.c 2017-11-10 11:22:13.209244939 -0800
    @@ -1447,7 +1447,7 @@ DEFINE_PER_CPU_ALIGNED(struct stack_cana
    * trampoline, not the thread stack. Use an extra percpu variable to track
    * the top of the kernel stack directly.
    */
    -DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
    +DEFINE_PER_CPU_USER_MAPPED(unsigned long, cpu_current_top_of_stack) =
    (unsigned long)&init_thread_union + THREAD_SIZE;
    EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);

    diff -puN arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars arch/x86/kernel/process_64.c
    --- a/arch/x86/kernel/process_64.c~kaiser-user-map-stack-helper-vars 2017-11-10 11:22:13.205244939 -0800
    +++ b/arch/x86/kernel/process_64.c 2017-11-10 11:22:13.209244939 -0800
    @@ -59,7 +59,7 @@
    #include <asm/unistd_32_ia32.h>
    #endif

    -__visible DEFINE_PER_CPU(unsigned long, rsp_scratch);
    +__visible DEFINE_PER_CPU_USER_MAPPED(unsigned long, rsp_scratch);

    /* Prints also some state that isn't saved in the pt_regs */
    void __show_regs(struct pt_regs *regs, int all)
    _
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      Last update: 2017-11-10 20:37    [W:2.081 / U:0.076 seconds]
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