Re: [PATCH] [RFC] Deter exploit bruteforcing

Richard Weinberger
2015-01-02T11:00:19Z

Am 02.01.2015 um 06:11 schrieb Pavel Machek:
> On Tue 2014-12-30 10:40:15, Kees Cook wrote:
>> On Wed, Dec 24, 2014 at 1:39 PM, Richard Weinberger <richard@nod.at> wrote:
>>> While exploring the offset2lib attack I remembered that
>>> grsecurity has an interesting feature to make such attacks
>>> much harder. Exploits can brute stack canaries often very easily
>>> if the target is a forking server like sshd or Apache httpd.
>>> The problem is that after fork() the child has by definition
>>> exactly the same memory as the parent and therefore also the same
>>> stack canaries.
>>> The attacker can guess the stack canaries byte by byte.
>>> After 256 times 7 forks() a good exploit can find the correct
>>> canary value.
>>>
>>> The basic idea behind this patch is to delay fork() if a child died
>>> due to a fatal error.
>>> Currently it delays fork() by 30 seconds if the parent tries to fork()
>>> within 60 seconds after a child died due to a fatal error.
>>>
>>> I'm sure you'll hate this patch but I want to find out how much you hate it
>>> and whether there is a little chance to get it mainline in a modified form.
>>> Later I'd make it depend on a new Kconfig option and off by default
>>> and the timing constants changeable via sysctl.
>
> Does this break trinity, crashme, and similar programs?

If they fork() without execve() and a child dies very fast the next fork()
will be throttled.
This is why I'd like to make this feature disabled by default.

> Can you detect it died due to the stack canary? Then, the patch might
> be actually acceptable.

I don't think so as this is glibc specific.

Thanks,
//richard