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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Jann Horn <jannh@google.com>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        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>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
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        <CAG48ez13XXWNRLrPFRHRsvPKSwSK1-6k+1F7QujWOJtVuk0QHg@mail.gmail.com>
Date:   Tue, 10 Mar 2020 16:30:51 -0500
In-Reply-To: <CAG48ez13XXWNRLrPFRHRsvPKSwSK1-6k+1F7QujWOJtVuk0QHg@mail.gmail.com>
        (Jann Horn's message of "Tue, 10 Mar 2020 22:21:28 +0100")
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Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
Sender: linux-kernel-owner@vger.kernel.org
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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, but still there might be a
lock inversion of some kind here.  At least as far as lockdep is
concerned and we don't want anything like that.

Eric