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

[Jann Horn]

On Tue, Mar 10, 2020 at 10:33 PM Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Jann Horn <jannh@xxxxxxxxxx> writes:
> > 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.
>
> Good point.  I should double check the lock ordering here with mmap_sem.
> It doesn't look like mmput takes mmap_sem

You sure about that? mmput() -> __mmput() -> ksm_exit() ->
__ksm_exit() -> down_write(&mm->mmap_sem)

Or also: mmput() -> __mmput() -> khugepaged_exit() ->
__khugepaged_exit() -> down_write(&mm->mmap_sem)

Or is there a reason why those paths can't happen?