linux-kernel - Re: [PATCH] fork: make whole stack

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Message-ID: <CAGXu5j+Hz=AmmTAh3+QOv1wTG3HA60LPK0Dq6F8uybNQ5e+sHw@mail.gmail.com>
Date:   Mon, 31 Oct 2016 09:04:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jann@...jh.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Daniel Micay <danielmicay@...il.com>
Subject: Re: [PATCH] fork: make whole stack_canary random

On Mon, Oct 31, 2016 at 7:04 AM, Jann Horn <jann@...jh.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@...jh.net>

Acked-by: Kees Cook <keescook@...omium.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