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Date:   Fri, 24 Nov 2017 11:12:12 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 12/43] x86/espfix/64: Stop assuming that pt_regs is on the
 entry stack

On Fri, Nov 24, 2017 at 10:25 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Nov 24, 2017 at 06:23:40PM +0100, Ingo Molnar wrote:
>> From: Andy Lutomirski <luto@...nel.org>
>>
>> When we start using an entry trampoline, a #GP from userspace will
>> be delivered on the entry stack, not on the task stack.  Fix the
>> espfix64 #DF fixup to set up #GP according to TSS.SP0, rather than
>> assuming that pt_regs + 1 == SP0.  This won't change anything
>> without an entry stack, but it will make the code continue to work
>> when an entry stack is added.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Borislav Petkov <bpetkov@...e.de>
>> Cc: Brian Gerst <brgerst@...il.com>
>> Cc: Dave Hansen <dave.hansen@...el.com>
>> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Link: https://lkml.kernel.org/r/b1ef4136616c6bd2a75d1fd2736d1d54437d65a8.1511497875.git.luto@kernel.org
>> Signed-off-by: Ingo Molnar <mingo@...nel.org>
>> ---
>>  arch/x86/kernel/traps.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index 2008dd0f8ccb..1bd43f044c62 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -359,7 +359,8 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>>               regs->cs == __KERNEL_CS &&
>>               regs->ip == (unsigned long)native_irq_return_iret)
>>       {
>> -             struct pt_regs *normal_regs = task_pt_regs(current);
>> +             struct pt_regs *normal_regs =
>> +                     (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
>
> Just let that line stick out. Also, you can shorten it by renaming
> normal_regs to something much shorter - it is a local variable and the
> comment already explains everything you you can just as well have:
>
>          struct pt_regs *r = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
>

Done, along with much better comments.

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_stack&id=6510485d026abdd144d170b1bc8508327acb5648

> --
> Regards/Gruss,
>     Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.

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