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Date:   Wed, 22 Nov 2017 19:31:52 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Andrew Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 11/23] x86, kaiser: map entry stack variables

On Wed, Nov 22, 2017 at 4:34 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> From: Dave Hansen <dave.hansen@...ux.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@...ux.intel.com>
> Cc: Moritz Lipp <moritz.lipp@...k.tugraz.at>
> Cc: Daniel Gruss <daniel.gruss@...k.tugraz.at>
> Cc: Michael Schwarz <michael.schwarz@...k.tugraz.at>
> Cc: Richard Fellner <richard.fellner@...dent.tugraz.at>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Kees Cook <keescook@...gle.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: x86@...nel.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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