Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF

Andy Lutomirski
2018-09-19T19:58:20Z

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> wrote:
>> >>> The idea here is that the userspace handler should be able to pass an fd
>> >>> back to the trapped task, for example so it can be returned from socket().
>> >>>
>> >>> I've proposed one API here, but I'm open to other options. In particular,
>> >>> this only lets you return an fd from a syscall, which may not be enough in
>> >>> all cases. For example, if an fd is written to an output parameter instead
>> >>> 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 netlink
>> >>> 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 new
>> >> 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 *number* right away, but what if another thread uses the number before you actually 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-“kill” signals to interrupt the whole process? It might be the case that we don’t really need to clean up from signals if there’s 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