Quoting Kees Cook ([email protected]):
> 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 <[email protected]>
> Reported-by: Brad Spengler <[email protected]>
> Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
> Cc: [email protected]
> Cc: James Morris <[email protected]>
> Cc: Serge Hallyn <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
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 <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: Jiri Slaby <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> 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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