linux-kernel - Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard

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Date:   Wed, 11 Mar 2020 11:29:08 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc\@vger.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>,
        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Bernd Edlinger <bernd.edlinger@...mail.de> writes:

> On 3/11/20 1:15 AM, Eric W. Biederman wrote:
>> Jann Horn <jannh@...gle.com> writes:
>> 
>>> On Tue, Mar 10, 2020 at 10:33 PM Eric W. Biederman
>>> <ebiederm@...ssion.com> wrote:
>>>> Jann Horn <jannh@...gle.com> writes:
>>>>> On Sun, Mar 8, 2020 at 10:41 PM Eric W. Biederman <ebiederm@...ssion.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.

The issue is that to prevent deadlock locks must always be taken
in the same order.

Taking mmap_sem then exec_update_mutex at the start of the function,
then taking exec_update_mutex then mmap_sem in mmput, takes the
two locks in two different orders.   Which means that in the right
set or circumstances:

thread1:                                thread2:
   obtain mmap_sem                      optain exec_update_mutex
      wait for exec_update_mutex        wait for mmap_sem

Which guarantees that neither thread will make progress.

The fix is easy I just need to take exec_update_mutex a few lines
earlier.

Eric