linux-kernel - Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys

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Date:   Tue, 6 Feb 2018 12:58:27 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Luis Henriques <lhenriques@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Greg KH <gregkh@...uxfoundation.org>, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read
 under speculation

On Tue, Feb 6, 2018 at 12:49 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> Can you use @cc to make an asm statement that outputs both the masked
> array index and the "if" condition?  I can never remember the syntax,
> but something like:

Yes. Although I'd actually suggest just using an "asm goto" if we
really want to optimize this. Give the "index_mask_nospec()" a third
argument that is the label to jump to for overflow.

Then you can just decide how to implement it best for any particular
architecture (and compiler limitation).

              Linus