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

[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@xxxxxxxxx> 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@xxxxxxxxx>
> 
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> (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.

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