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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 19 Sep 2018 12:58:20 -0700
Message-ID: <CALCETrVuP0nQ5B01-iWrcQ87hisz7=jA08-Uisqud_r+zc74NQ@mail.gmail.com>
Subject: Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF
To:     Tycho Andersen <tycho@tycho.ws>
Cc:     Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux API <linux-api@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Tyler Hicks <tyhicks@canonical.com>,
        Akihiro Suda <suda.akihiro@lab.ntt.co.jp>,
        Jann Horn <jannh@google.com>
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On Wed, Sep 19, 2018 at 7:38 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> On Wed, Sep 19, 2018 at 07:19:56AM -0700, Andy Lutomirski wrote:
>>
>>
>> > On Sep 19, 2018, at 2:55 AM, Tycho Andersen <tycho@tycho.ws> wrote:
>> >
>> >> On Wed, Sep 12, 2018 at 04:52:38PM -0700, Andy Lutomirski wrote:
>> >>> On Thu, Sep 6, 2018 at 8:28 AM, Tycho Andersen <tycho@tycho.ws> wrot=
e:
>> >>> The idea here is that the userspace handler should be able to pass a=
n fd
>> >>> back to the trapped task, for example so it can be returned from soc=
ket().
>> >>>
>> >>> I've proposed one API here, but I'm open to other options. In partic=
ular,
>> >>> this only lets you return an fd from a syscall, which may not be eno=
ugh in
>> >>> all cases. For example, if an fd is written to an output parameter i=
nstead
>> >>> of returned, the current API can't handle this. Another case is that
>> >>> netlink takes as input fds sometimes (IFLA_NET_NS_FD, e.g.). If netl=
ink
>> >>> ever decides to install an fd and output it, we wouldn't be able to =
handle
>> >>> this either.
>> >>
>> >> An alternative could be to have an API (an ioctl on the listener,
>> >> perhaps) that just copies an fd into the tracee.  There would be the
>> >> obvious set of options: do we replace an existing fd or allocate a ne=
w
>> >> one, and is it CLOEXEC.  Then the tracer could add an fd and then
>> >> return it just like it's a regular number.
>> >>
>> >> I feel like this would be more flexible and conceptually simpler, but
>> >> maybe a little slower for the common cases.  What do you think?
>> >
>> > I'm just implementing this now, and there's one question: when do we
>> > actually do the fd install? Should we do it when the user calls
>> > SECCOMP_NOTIF_PUT_FD, or when the actual response is sent? It feels
>> > like we should do it when the response is sent, instead of doing it
>> > right when SECCOMP_NOTIF_PUT_FD is called, since if there's a
>> > subsequent signal and the tracer decides to discard the response,
>> > we'll have to implement some delete mechanism to delete the fd, but it
>> > would have already been visible to the process, etc. So I'll go
>> > forward with this unless there are strong objections, but I thought
>> > I'd point it out just to avoid another round trip.
>> >
>> >
>>
>> Can you do that non-racily?  That is, you need to commit to an fd *numbe=
r* right away, but what if another thread uses the number before you actual=
ly install the fd?
>
> I was thinking we could just do an __alloc_fd() and then do the
> fd_install() when the response is sent or clean up the case that the
> listener or task dies. I haven't actually tried to run the code yet,
> so it's possible the locking won't work :)

I would be very surprised if the locking works.  How can you run a
thread in a process when another thread has allocated but not
installed an fd and is blocked for an arbitrarily long time?

>
>> Do we really allow non-=E2=80=9Ckill=E2=80=9D signals to interrupt the w=
hole process?  It might be the case that we don=E2=80=99t really need to cl=
ean up from signals if there=E2=80=99s a guarantee that the thread dies.
>
> Yes, we do, because of this: https://lkml.org/lkml/2018/3/15/1122
>

I'm still not sure I see the problem.  Suppose I'm implementing a user
notifier for a nasty syscall like recvmsg().  If I'm the tracer, by
the time I decide to install an fd, I've committed to returning
something other than -EINTR, even if a non-fatal signal is sent before
I finish.  No rollback should be necessary.

In the (unlikely?) event that some tracer needs to be able to rollback
an fd installation to return -EINTR, a SECCOMP_NOTIF_CLOSE_FD
operation should be good enough, I think.  Or maybe PUT_FD can put -1
to delete an fd.

--Andy