linux-kernel - Re: Linux 4.4.110

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Date:   Sat, 6 Jan 2018 14:16:40 +0100
From:   Willy Tarreau <w@....eu>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Greg KH <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        torvalds@...ux-foundation.org, stable@...r.kernel.org, lwn@....net,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: Linux 4.4.110

On Fri, Jan 05, 2018 at 09:24:31PM +0100, Willy Tarreau wrote:
> On Fri, Jan 05, 2018 at 07:58:04PM +0000, Alan Cox wrote:
> > and the techniques
> > to deal with rdtsc disabling are well known and used in other existing
> > attacks.
> 
> Yes i've tested one of them for the spectre poc, but it really did not
> work well, leading to about 1 among 10 bytes only to be valid. In fact
> either you run the counter thread on the other sibling of the same core
> and it significantly perturbates the local activity, or you run it on
> another core, and the time it takes to retrieve the time requires some
> L1+L2 traversal. I'm not saying it doesn't work at all, I'm saying that
> the accuracy is highly degraded and that can turn something 100%
> reproducible into something requiring a long time to run, making the
> attack more noticeable (and possibly letting observed data degrade
> during the period).

So I worked on an improved RDTSC emulation (attached) and it works
reasonably well on the spectre poc found online. Its accuracy is almost
as good as rdtsc on my i7-6700k on two threads running on the same core,
and 8-10 times worse on two distinct cores, but still leads to ~50%
success rate on the PoC. So my conclusion now is that it's indeed
pointless to invest time trying to make RDTSC less accessible/accurate.

Willy

View attachment "tsc.c" of type "text/plain" (3010 bytes)