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

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Date:	Fri, 19 Dec 2014 23:04:15 +0100
From:	Hector Marco <hecmargi@....es>
To:	Andy Lutomirski <luto@...capital.net>
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



El 12/12/14 a las 18:17, Andy Lutomirski escribió:
> 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

I don't think that mapping the VDSO in the mmap area breaks the
userspace. Actually, this is already happening with the current
implementation. You can see it by running:

setarch x86_64 -R cat /proc/self/maps


Do this break the userspace in some way ?


Regarding the solution to the offset2lib it seems that placing the
executable in a different memory region area could increase the
number of pages for the pages table (because it is more spread).
We should consider this before fixing the current implementation
(randomize_va_space=2).

I guess that the current implementation places the PIE executable in
the mmap base area jointly with the libraries in an attempt to reduce
the size of the page table.

Therefore, I can fix the current implementation (maintaining the
randomize_va_space=2) by moving the PIE executable from the mmap base
area to another one for x86*, ARM* and MIPS (as s390 and PowerPC do).
But we shall agree that this increment in the page table is not a
issue. Otherwise, the randomize_va_space=3 shall be considered.


Hector Marco.

>
>>
>>
>> Regards,
>> Hector Marco.
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