Jann Horn <jannh@xxxxxxxxxx> writes: > 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? Clearly I didn't look far enough. I will adjust this so that exec_update_mutex is taken before mmap_sem. Anything else is just asking for trouble. Eric
