linux-kernel - Re: [PATCH 16/18] net: mpls: prevent bounds-check bypass via speculative execution

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Message-ID: <CA+55aFxkBkyDOApZF9zHzLY4nDkX+mxVOZQqAxGxNP6eUikBgw@mail.gmail.com>
Date:   Mon, 8 Jan 2018 20:21:09 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Netdev <netdev@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 16/18] net: mpls: prevent bounds-check bypass via
 speculative execution

On Mon, Jan 8, 2018 at 8:13 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>     # carry will be clear if idx >= max
>     cmpq %idx,%max

Bah. Other way around.

        cmpq %max,%idx

I'm a moron.

>     # mask will be clear if carry was clear, ~0 otherwise
>     sbbq %mask,%mask
>
> to generate mask directly. I might have screwed that up. Worth perhaps trying?

More importantly, worth _testing_ and fixing my hand-waving "asm like
this" crap.

But I do think that simple two-instruction cmpq/sbbq sequence could
get it right in just two trivial ALU instructions.

          Linus