Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random

Florian Weimer
2016-10-31T20:46:18Z

* Jann Horn:

> On Mon, Oct 31, 2016 at 09:04:02AM -0700, Kees Cook wrote:
>> On Mon, Oct 31, 2016 at 7:04 AM, Jann Horn <jann@thejh.net> wrote:
>> > On machines with sizeof(unsigned long)==8, this ensures that the more
>> > significant 32 bits of stack_canary are random, too.
>> > stack_canary is defined as unsigned long, all the architectures with stack
>> > protector support already pick the stack_canary of init as a random
>> > unsigned long, and get_random_long() should be as fast as get_random_int(),
>> > so there seems to be no good reason against this.
>> >
>> > This should help if someone tries to guess a stack canary with brute force.
>> >
>> > (This change has been made in PaX already, with a different RNG.)
>> >
>> > Signed-off-by: Jann Horn <jann@thejh.net>
>>
>> Acked-by: Kees Cook <keescook@chromium.org>
>>
>> (A separate change might be to make sure that the leading byte is
>> zeroed. Entropy of the value, I think, is less important than blocking
>> canary exposures from unbounded str* functions. Brute forcing kernel
>> stack canaries isn't like it bruting them in userspace...)
>
> Yeah, makes sense. Especially on 64bit, 56 bits of entropy ought to be
> enough anyway.

So you two approve of the way glibc does this currently? (See the
other thread.)

I was under the impression that the kernel performs far less
null-terminated string processing the average user space application,
especially on the stack. (A lot of userspace code assumes large
stacks and puts essentially arbitrarily long strings into VLAs.)