On 3/11/20 1:15 AM, Eric W. Biederman wrote: > 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. > Note that vm_access does also mmput under the exec_update_mutex. So I don't see a huge problem here. But maybe I missed something. Bernd.
