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SubjectRe: [PATCH] exec: Avoid RLIMIT_STACK races with prlimit()
Quoting Kees Cook (keescook@chromium.org):
> 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@codethink.co.uk>
> Reported-by: Brad Spengler <spender@grsecurity.net>
> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
> Cc: stable@vger.kernel.org
> Cc: James Morris <james.l.morris@oracle.com>
> Cc: Serge Hallyn <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

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@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Jiri Slaby <jslaby@suse.cz>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> 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

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