Re: [PATCH] exec: Avoid RLIMIT_STACK races with prlimit()

[Serge E. Hallyn]

Quoting Kees Cook (keescook@xxxxxxxxxxxx):
> While the defense-in-depth RLIMIT_STACK limit on setuid processes was
> protected against races from other threads calling setrlimit(), I missed
> protecting it against races from external processes calling prlimit().
> This adds locking around the change and makes sure that rlim_max is set
> too.
> 
> Reported-by: Ben Hutchings <ben.hutchings@xxxxxxxxxxxxxxx>
> Reported-by: Brad Spengler <spender@xxxxxxxxxxxxxx>
> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: Serge Hallyn <serge@xxxxxxxxxx>

Acked-by: Serge Hallyn <serge@xxxxxxxxxx>

The only thing i'm wondering is in do_prlimit():

. 1480         if (new_rlim) {
. 1481                 if (new_rlim->rlim_cur > new_rlim->rlim_max)
. 1482                         return -EINVAL;

that bit is done not under the lock.  Does that still allow a
race, if this check is done before the below block, and then the
rest proceeds after?

I *think* not, because later in do_prlimit() it will return -EPERM if

. 1500                 if (new_rlim->rlim_max > rlim->rlim_max &&
. 1501                                 !capable(CAP_SYS_RESOURCE))

Although rlim is gathered before the lock, but that is a struct *
so should be ok?

> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  fs/exec.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 1d6243d9f2b6..6be2aa0ab26f 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1340,10 +1340,15 @@ void setup_new_exec(struct linux_binprm * bprm)
>  		 * avoid bad behavior from the prior rlimits. This has to
>  		 * happen before arch_pick_mmap_layout(), which examines
>  		 * RLIMIT_STACK, but after the point of no return to avoid
> -		 * needing to clean up the change on failure.
> +		 * races from other threads changing the limits. This also
> +		 * must be protected from races with prlimit() calls.
>  		 */
> +		task_lock(current->group_leader);
>  		if (current->signal->rlim[RLIMIT_STACK].rlim_cur > _STK_LIM)
>  			current->signal->rlim[RLIMIT_STACK].rlim_cur = _STK_LIM;
> +		if (current->signal->rlim[RLIMIT_STACK].rlim_max > _STK_LIM)
> +			current->signal->rlim[RLIMIT_STACK].rlim_max = _STK_LIM;
> +		task_unlock(current->group_leader);
>  	}
>  
>  	arch_pick_mmap_layout(current->mm);
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security