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From:   Jann Horn <jannh@google.com>
Date:   Wed, 11 Mar 2020 00:21:49 +0100
Message-ID: <CAG48ez1j2=pdj0nc1syHkh6X4d=aHuCH1srzA6hT7+32QD+6Gg@mail.gmail.com>
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
To:     "Eric W. Biederman" <ebiederm@xmission.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>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>
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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?