linux-kernel - Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec

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Date:   Thu, 4 Jan 2018 21:56:54 +0100
From:   Pavel Machek <pavel@....cz>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "alan@...ux.intel.com" <alan@...ux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Hi!


> > It falls there so that the cpu only issues reads with known good 'index' values.
> >
> >> I suspect it would be better to have those barriers in the tun/tap
> >> interfaces where userspace can inject packets and thus time them.  Then
> >> the code could still speculate and go fast for remote packets.
> >>
> >> Or does the speculation stomping have to be immediately at the place
> >> where we use data from userspace to perform a table lookup?
> >
> > The speculation stomping barrier has to be between where we validate
> > the input and when we may speculate on invalid input.
> 
> So a serializing instruction at the kernel/user boundary (like say
> loading cr3) is not enough?  That would seem to break any chance of a
> controlled timing.

Timing attack is not as straightforward as this.

You can assume cache snooping from second CPU _while_ kernel is
executing. Yes, that will mean timing, but....

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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