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Date:   Thu, 23 Nov 2017 07:02:58 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Denys Vlasenko <vda.linux@...glemail.com>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 Thu, Nov 23, 2017 at 5:16 AM, Denys Vlasenko
<vda.linux@...glemail.com> wrote:
> 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,

Hmm.  I think I'll leave it for now to avoid churn and then I'll just
delete it entirely once the dust settles.  The canary is rather
pointless given that we'll have an actual guard page.

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