Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Bernd Edlinger
2020-03-11T06:33:41Z

On 3/11/20 1:15 AM, Eric W. Biederman wrote:
> 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.
>

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.