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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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