* 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 <j...@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 <j...@thejh.net> >> >> Acked-by: Kees Cook <keesc...@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.)