Re: [PATCH 11/23] x86, kaiser: map entry stack variables

Andy Lutomirski
2017-11-23T03:32:15Z

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.