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Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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From:   Bernd Edlinger <bernd.edlinger@hotmail.de>
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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.