From: Ingo Molnar <mingo@kernel.org>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Fri, 20 Oct 2017 10:13:20 +0200
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Cc: "H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>,
	Thomas Garnier <thgarnie@google.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	"David S . Miller" <davem@davemloft.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Matthias Kaehlcke <mka@chromium.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
	Brian Gerst <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Tejun Heo <tj@kernel.org>,
	Christo
To: Pavel Machek <pavel@ucw.cz>
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* Pavel Machek <pavel@ucw.cz> wrote:

> On Mon 2017-09-25 09:33:42, Ingo Molnar wrote:
> > 
> > * Pavel Machek <pavel@ucw.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