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, but still there might be a lock inversion of some kind here. At least as far as lockdep is concerned and we don't want anything like that. Eric
