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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 9 Mar 2017 18:43:59 -0800
Message-ID: <CALCETrXAARdzFP-gpzmF3_Se2BvoogHH-C1Q0jmwgy2+JUO9Pg@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/nmi: Fix and optimize the NMI stack check code
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Thu, Mar 9, 2017 at 2:42 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
>
> Andy Lutomirski reported an off by one in the NMI stack check
> for the nested NMI code, where if the stack pointer was one above
> the actual stack (stack start + STACK_SIZE) it would trigger a false
> positive. This is not that big of a deal because the stack pointer
> should never be that. Even if a stack was using the pages just
> above the NMI stack, it would require the stack about to overflow
> for this to trigger, which is a much bigger bug than this is fixing.
>
> Also, Linus Torvalds pointed out that doing two compares can be
> accomplish with a single compare. That is:
>
> ("reg" is top of stack we are comparing "stack" to)
>
>   cmpq reg, stack
>   jae label  // note, code had one off "ja" instead of "jae"
>   subq size, reg
>   cmpq reg, stack
>   jb label
>
> Is the same as:
>
>   subq $1, reg
>   subq stack, reg
>   cmpq size, reg
>   jae label
>
> The subq $1 was added into the leaq by doing:
>
>    leaq 5*8+7(%rsp), %rdx
>
> Added more comments as well.

Nice.