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Date: Fri, 20 Oct 2017 10:13:20 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Pavel Machek <pavel@....cz>
Cc: "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>,
Thomas Garnier <thgarnie@...gle.com>,
Herbert Xu <herbert@...dor.apana.org.au>,
"David S . Miller" <davem@...emloft.net>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>,
Matthias Kaehlcke <mka@...omium.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
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"H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>,
Rob Landley <rob@...dley.net>, Baoquan He <bhe@...hat.com>,
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the arch/x86 maintainers <x86@...nel.org>,
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Linus Torvalds <torvalds@...ux-foundation.org>,
Borislav Petkov <bp@...en8.de>
Subject: Re: x86: PIE support and option to extend KASLR randomization
* Pavel Machek <pavel@....cz> wrote:
> On Mon 2017-09-25 09:33:42, Ingo Molnar wrote:
> >
> > * Pavel Machek <pavel@....cz> wrote:
> >
> > > > For example, there would be collision with regular user-space mappings, right?
> > > > Can local unprivileged users use mmap(MAP_FIXED) probing to figure out where
> > > > the kernel lives?
> > >
> > > Local unpriviledged users can probably get your secret bits using cache probing
> > > and jump prediction buffers.
> > >
> > > Yes, you don't want to leak the information using mmap(MAP_FIXED), but CPU will
> > > leak it for you, anyway.
> >
> > Depends on the CPU I think, and CPU vendors are busy trying to mitigate this
> > angle.
>
> I believe any x86 CPU running Linux will leak it. And with CPU vendors
> putting "artifical inteligence" into branch prediction, no, I don't
> think it is going to get better.
>
> That does not mean we shoudl not prevent mmap() info leak, but...
That might or might not be so, but there's a world of a difference between
running a relatively long statistical attack figuring out the kernel's
location, versus being able to programmatically probe the kernel's location
by using large MAP_FIXED user-space mmap()s, within a few dozen microseconds
or so and a 100% guaranteed, non-statistical result.
Thanks,
Ingo
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