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    SubjectRe: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
    Jann Horn <jannh@google.com> writes:

    > On Tue, Mar 10, 2020 at 10:33 PM Eric W. Biederman
    > <ebiederm@xmission.com> wrote:
    >> Jann Horn <jannh@google.com> writes:
    >> > On Sun, Mar 8, 2020 at 10:41 PM Eric W. Biederman <ebiederm@xmission.com> 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

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