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Date:   Thu, 23 Nov 2017 14:16:33 +0100
From:   Denys Vlasenko <vda.linux@...glemail.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 09/18] x86/asm: Move SYSENTER_stack to the beginning of
 struct tss_struct

On Wed, Nov 22, 2017 at 5:44 AM, Andy Lutomirski <luto@...nel.org> wrote:
> I want SYSENTER_stack to have reliable overflow detection, which
> means that it needs to be at the bottom of a page, not the top.
> Move it to the beginning of struct tss_struct and page-align it.
>
> Also add an assertion to make sure that the fixed hardware TSS
> doesn't cross a page boundary.
>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/processor.h | 21 ++++++++++++---------
>  arch/x86/kernel/cpu/common.c     | 22 ++++++++++++++++++++++
>  2 files changed, 34 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index d32a3c88a968..8f5dac9dfbdc 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -327,7 +327,16 @@ struct x86_hw_tss {
>
>  struct tss_struct {
>         /*
> -        * The hardware state:
> +        * Space for the temporary SYSENTER stack, used for SYSENTER
> +        * and the entry trampoline as well.
> +        */
> +       unsigned long           SYSENTER_stack_canary;
> +       unsigned long           SYSENTER_stack[64];
> +
> +       /*
> +        * The fixed hardware portion.  This must not cross a page boundary
> +        * at risk of violating the SDM's advice and potentially triggering
> +        * errata.
>          */
>         struct x86_hw_tss       x86_tss;
>
> @@ -338,15 +347,9 @@ struct tss_struct {
>          * be within the limit.
>          */
>         unsigned long           io_bitmap[IO_BITMAP_LONGS + 1];
> +} __attribute__((__aligned__(PAGE_SIZE)));
>
> -       /*
> -        * Space for the temporary SYSENTER stack.
> -        */
> -       unsigned long           SYSENTER_stack_canary;
> -       unsigned long           SYSENTER_stack[64];
> -} ____cacheline_aligned;

You may also move this initializer in process.c:

.SYSENTER_stack_canary  = STACK_END_MAGIC,

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