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?
