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    SubjectRe: [PATCH 11/23] x86, kaiser: map entry stack variables
    On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen
    <dave.hansen@linux.intel.com> wrote:
    >
    > 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 2017-11-22 15:45:50.128619736 -0800
    > +++ b/arch/x86/kernel/cpu/common.c 2017-11-22 15:45:50.134619736 -0800
    > @@ -1524,7 +1524,7 @@ EXPORT_PER_CPU_SYMBOL(__preempt_count);
    > * the top of the kernel 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;

    This is in an x86_32-only section and should be dropped, I think.

    > 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 2017-11-22 15:45:50.130619736 -0800
    > +++ b/arch/x86/kernel/process_64.c 2017-11-22 15:45:50.134619736 -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);
    >

    This shouldn't be needed any more either.

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