linux-kernel - Re: proposal for meltdown-workaround with low overhead

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Date:   Thu, 4 Jan 2018 23:24:11 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Tony Luck <tony.luck@...il.com>
Cc:     Alexander Kleinsorge <aleks@...sik.tu-berlin.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: proposal for meltdown-workaround with low overhead

On Thu, 4 Jan 2018 15:09:28 -0800
Tony Luck <tony.luck@...il.com> wrote:

> On Thu, Jan 4, 2018 at 11:45 AM, Alexander Kleinsorge
> <aleks@...sik.tu-berlin.de> wrote:
> > As Meltdown-Issue depends on allowing to cause many exceptions (usually :
> > accessing an invalid address), we could restrict this misusage easy.  
> 
> The accesses to the invalid address are performed speculatively by the CPU in
> a code branch that is later found to be not taken. Hence there are no exceptions
> at all.

Actually for the 'spectre' attack you can sometimes see hints because
many of the obvious attack points end up causing a syscall to return an
errno value. One thing that might be interesting for the paranoid is
indeed to react to some of those (notably EINVAL, EFAULT) by stirring up
the mud before that process runs again.

Alan