linux-kernel - Re: [PATCH] [RFC] Deter exploit bruteforcing

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Message-ID: <54A87626.4070302@nod.at>
Date:	Sun, 04 Jan 2015 00:07:18 +0100
From:	Richard Weinberger <richard@....at>
To:	Pavel Machek <pavel@....cz>
CC:	Jiri Kosina <jkosina@...e.cz>, Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	Aaron Tomlin <atomlin@...hat.com>,
	DaeSeok Youn <daeseok.youn@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, vdavydov@...allels.com,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andy Lutomirski <luto@...capital.net>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH] [RFC] Deter exploit bruteforcing

Am 04.01.2015 um 00:01 schrieb Pavel Machek:
> On Sat 2015-01-03 23:44:18, Richard Weinberger wrote:
>> Am 03.01.2015 um 23:36 schrieb Pavel Machek:
>>>
>>>>>> No. This is not what this patch does.
>>>>>>
>>>>>>> But changing glibc to do sleep(30); abort(); instead of abort(); to
>>>>>>> slow down bruteforcing of canaries makes some kind of sense... and
>>>>>>> should be ok by default.
>>>>>>
>>>>>> As I saidn only focusing one the specific stack canary case is not enough.
>>>>>
>>>>> Ok, so I am now saying "adding random delays to the kernel, hoping
>>>>> they slow attacker down" is bad idea. Feel free to add my NAK to the
>>>>> patch.
>>>>
>>>> The patch does not add random delays nor is hope involved.
>>>>
>>>> It has a very clear purpose, it makes brute force attacks to forking
>>>> services unattractive.
>>>> Exploits often use the fact that after fork() the child has the same memory
>>>> as the parent and therefore an attacker can start fruitful brute force attacks
>>>> to brute stack canaries, offsets, etc. as the new child will always have mostly
>>>> the same memory layout as before.
>>>>
>>>> But I'll happily add your NAK to this series.
>>>
>>> Please do.
>>>
>>>>> If really neccessary, "kill_me_slowly()" syscall would be acceptable,
>>>>> but it seems just sleep(); abort(); combination is enough.
>>>>
>>>> The goal of the patch is not to protect only against brute forcing the stack canary.
>>>> It should protect against all kind of brute forcing using forking services.
>>>>
>>>>> glibc should cover 99% cases where this matters, please just fix glibc,
>>>>> others will follow.
>>>>
>>>> There are a lot of systems out there without glibc.
>>>
>>> Only "interesting" systems that are without glibc are androids, and
>>> they usually run very old kernels.
>>>
>>> If you implement sleep() in glibc, distros will enable it and you'll
>>> protect all the desktop users.
>>>
>>> If you implement it in kernel, it will not be compatible-enough to be
>>> enabled by default, and you'll be protecting special "high security"
>>> distros at most.
>>>
>>>> And many applications make system calls without going though any libc wrapper.
>>>> Hey, we want also protect esoteric distros like http://sta.li. :-)
>>>
>>> No, we don't. We want to maximize number of protected users. And
>>> patching glibc does that. (And then you can patch bionic. And then the
>>> small players will follow).
>>
>> And what about static linked programs or programs which do not use a libc wrapper
>> for system calls?
>> Say, any program written in go?
> 
> And what about my Atari 800XL?

If it runs Linux it can be protected.

> How many such programs are on common distributions? <1%
> 
> How many systems will your kernel hack leave unprotected? >70%
> 
> (Plus, reasonable languages like go should not really allow classical
> buffer overflows, and yes, you'll get protection if you statically
> link against glibc. And AFAICT this has nothing to do with syscalls,
> and everything to do with abort() implementation.).

Go does not use libc at all.

Anyway, you've stated your point.
I'm for a generic solution and not for one which works only for some systems.

Thanks,
//richard
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