linux-kernel - Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

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Date:	Fri, 12 Dec 2014 09:17:05 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Hector Marco <hecmargi@....es>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Anton Blanchard <anton@...ba.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Daney <ddaney.cavm@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arun Chandran <achandran@...sta.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ismael Ripoll <iripoll@...ca.upv.es>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hanno Böck <hanno@...eck.de>,
	Will Deacon <will.deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Kees Cook <keescook@...omium.org>,
	Reno Robert <renorobert@...il.com>
Subject: Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack

On Dec 12, 2014 8:33 AM, "Hector Marco" <hecmargi@....es> wrote:
>
> Hello,
>
> I agree. I don't think a new randomization mode will be needed, just fix
> the current randomize_va_space=2. Said other way: fixing the offset2lib
> will not break any current program and so, no need to add additional
> configuration options. May be we shall wait for some inputs
> from the list (may be we are missing something).
>
>
> Regarding to VDSO, definitively, is not randomized enough in 64bits.
> Brute force attacks would be pretty fast even from the network.
> I have identified the bug and seems quite easy to fix it.
>
> On 32bit systems, this is not a issue because it is mapped in the
> mmap area. In order to fix the VDSO on 64bit, the following
> considerations shall
> be discussed:
>
>
> Performance:
>     It seems (reading the kernel comments) that the random allocation
>     algorithm tries to place the VDSO in the same PTE than the stack.

The comment is wrong.  It means PTE table.

>     But since the permissions of the stack and the VDSO are different
>     it seems that are getting right the opposite.

Permissions have page granularity, so this isn't a problem.

>
>     Effectively VDSO shall be correctly randomized because it contains
>     enough useful exploitable stuff.
>
>     I think that the possible solution is follow the x86_32 approach
>     which consist on map the VDSO in the mmap area.
>
>     It would be better fix VDSO in a different patch ? I can send a
>     patch which fixes the VDSO on 64 bit.
>

What are the considerations for 64-bit memory layout?  I haven't
touched it because I don't want to break userspace, but I don't know
what to be careful about.

--Andy

>
>
> Regards,
> Hector Marco.
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