Messages in this thread | | | Date | Mon, 31 Oct 2016 21:55:26 +0100 | From | Jann Horn <> | Subject | Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random |
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On Mon, Oct 31, 2016 at 09:45:59PM +0100, Florian Weimer wrote: > * 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.)
Well... not really with a 32-bit canary. 2^23 crashes to defeat a mitigation is not so much, even over the network. With a 64-bit canary, losing the 8 bits would be no problem at all IMO.
So I guess I should revise what I said: I think the nullbyte thing is fine for 64-bit canaries, but not for 32-bit ones.
> I was under the impression that the kernel performs far less > null-terminated string processing the average user space application, > especially on the stack.
Yes, that's true - the kernel allocates even small-ish temporary string buffers with kmalloc() to reduce stack usage. [unhandled content-type:application/pgp-signature] | |