Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Kees Cook
2020-03-10T20:56:00Z

On Mon, Mar 09, 2020 at 02:02:37PM -0500, Eric W. Biederman wrote:
> exec: Add exec_update_mutex to replace cred_guard_mutex
>
> The cred_guard_mutex is problematic as it is held over possibly
> indefinite waits for userspace. The possilbe indefinite waits for
> userspace that I have identified are: The cred_guard_mutex is held in
> PTRACE_EVENT_EXIT waiting for the tracer. The cred_guard_mutex is
> held over "put_user(0, tsk->clear_child_tid)" in exit_mm(). The
> cred_guard_mutex is held over "get_user(futex_offset, ...") in
> exit_robust_list. The cred_guard_mutex held over copy_strings.

I suspect you're not trying to make a comprehensive list here, but do
you want to mention seccomp too (since it's yet another weird case).

> [...]
> Holding a mutex over any of those possibly indefinite waits for
> userspace does not appear necessary. Add exec_update_mutex that will
> just cover updating the process during exec where the permissions and
> the objects pointed to by the task struct may be out of sync.

Should the specific resources be pointed out here? creds, mm, ... ?

But otherwise, yup, looks sane:

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook