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

[Jann Horn]

On Sun, Mar 8, 2020 at 10:41 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> The cred_guard_mutex is problematic.  The cred_guard_mutex is held
> over the userspace accesses as the arguments from userspace are read.
> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
> threads are killed.  The cred_guard_mutex is held over
> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>
> Any of those can result in deadlock, as the cred_guard_mutex is held
> over a possible indefinite userspace waits for userspace.
>
> Add exec_update_mutex that is only held over exec updating process
> with the new contents of exec, so that code that needs not to be
> confused by exec changing the mm and the cred in ways that can not
> happen during ordinary execution of a process.
>
> The plan is to switch the users of cred_guard_mutex to
> exec_udpate_mutex one by one.  This lets us move forward while still
> being careful and not introducing any regressions.
[...]
> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
>                         return -EINTR;
>                 }
>         }
> +
> +       ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
> +       if (ret)
> +               return ret;

We're already holding the old mmap_sem, and now nest the
exec_update_mutex inside it; but then while still holding the
exec_update_mutex, we do mmput(), which can e.g. end up in ksm_exit(),
which can do down_write(&mm->mmap_sem) from __ksm_exit(). So I think
at least lockdep will be unhappy, and I'm not sure whether it's an
actual problem or not.