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

From: Aleksa Sarai <asarai@suse.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Tycho Andersen <tycho@tycho.ws>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Akihiro Suda <suda.akihiro@lab.ntt.co.jp>,
	Oleg Nesterov <oleg@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF
Date: Thu, 13 Sep 2018 19:42:56 +1000	[thread overview]
Message-ID: <20180913094256.gpwrlu4gnjn6zyhn@mikami> (raw)
In-Reply-To: <CALCETrWZmN4FeCSwemfMeayupBmQ-NqpVWQuqSU34CLvzdx8gw@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 1712 bytes --]

On 2018-09-12, Andy Lutomirski <luto@amacapital.net> 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?

When we first discussed this I sent a (probably somewhat broken) patch
for "dup4" which would let you inject a file descriptor into a different
process -- I still think that having a method for injecting a file
descriptor without needing ptrace (and SCM_RIGHTS) shenanigans would be
generally useful. (With "dup4" you have a more obvious API for flags and
whether you allocate a new fd or use a specific one.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]