linux-kernel - Re: [PATCH 14/18] x86/boot/64: Stop initializing TSS.sp0 at boot

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Message-ID: <CALCETrXtX5DL7aV8eEWQhQgWBHaS2CYay6DFR82XULSqXDYrxQ@mail.gmail.com>
Date:   Sat, 28 Oct 2017 01:10:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.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>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 14/18] x86/boot/64: Stop initializing TSS.sp0 at boot

On Fri, Oct 27, 2017 at 11:21 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 10/26/2017 01:26 AM, Andy Lutomirski wrote:
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -48,7 +48,8 @@
>>   */
>>  __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
>>       .x86_tss = {
>> -             .sp0 = TOP_OF_INIT_STACK,
>> +             /* Initialize sp0 to a value that is definitely invalid. */
>> +             .sp0 = (1UL << (BITS_PER_LONG-1)) + 1,
>
> This confused me at first: How does this not poison the init task's stack?
>
> Should the comment maybe say something like:
>
>         The hardware only uses .sp0 (or sp1 or sp2 for that matter) when
>         doing ring transitions.  Since the init task never runs anything
>         other than ring 0, it has no need for a valid value here.
>         Poison it.
>
> to clarify what's going on?

Done.