Re: [PATCH] fork: make whole stack_canary random

Kees Cook
2016-10-31T16:04:07Z

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...)

-Kees

> ---
> kernel/fork.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 623259fc794d..d577e2c5d14f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -518,7 +518,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
> set_task_stack_end_magic(tsk);
>
> #ifdef CONFIG_CC_STACKPROTECTOR
> - tsk->stack_canary = get_random_int();
> + tsk->stack_canary = get_random_long();
> #endif
>
> /*
> --
> 2.1.4
>

--
Kees Cook
Nexus Security