Date: Wed, 15 Oct 2014 15:35:18 +0200
From: David Madore <david+ml@madore.org>
To: Linux Kernel mailing-list <linux-kernel@vger.kernel.org>
Subject: feature suggestion: implement SO_PEERCRED on local AF_INET/AF_INET6
 sockets (allow uid-based identification on localhost)
Message-ID: <20141015133518.GA7179@achernar.madore.org>
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Given an AF_UNIX socket, the getsockopt(, SOL_SOCKET, SO_PEERCRED,,)
call allows one endpoint to authenticate the other endpoint's pid, uid
and gid.

The call is valid on AF_INET and AF_INET6 sockets but returns no data
(pid=0, uid=-1, gid=-1).  Obviously it is meaningless to try to get
such credentials from a INET/INET6 socket in general, but there is one
case where it would make sense: namely, when the endpoint is local
(i.e., when the socket is a connection to the same machine, e.g., when
connecting to 127.0.0.0/8 or ::1/32).

Being able to authenticate local INET/INET6 sockets would be immensely
valuable for a number of programs, to provide some kind of access
control to local sockets.  For example, ssh allows port forwarding
using the -L and -D options: by default or by option (cf. the
GatewayPorts option of ssh), these port-forwarding sockets can be
restricted to localhost, but of course they cannot be restricted to
the user running ssh, which makes them a huge security problem.  Many
programs suffer from the same problem (they restrict some kind of
connection to localhost, but they of course cannot make a restriction
on which user will be able to connect).

One cannot simply retort "these programs should be using Unix-domain
sockets instead": I don't think many browsers support using a SOCKS
proxy or an HTTP proxy over a Unix-domain socket, and in the latter
case I'm not even sure it would make sense (protocol-wise).

If I believe <URL:
http://www.lehman.cuny.edu/cgi-bin/man-cgi?getpeerucred+3
 > ("The system currently supports both sides of connection end-points
for local AF_UNIX, AF_INET, and AF_INET6 sockets"), Solaris, or at
least some version thereof, support authentication of local AF_INET
and AF_INET6 sockets.

I think it would be wonderful if Linux had this.  I'm willing to work
on the implementation if it is considered *a priori* acceptable for
inclusion.

The data seems to be available, since it is exposed in /proc/net/tcp
and /proc/net/tcp6 and whatnots (implementation details left aside, it
is merely a question of matching a line with opposite endpoints to the
current socket and returning it).

[In principle, a userland program can parse /proc/net/tcp so it does
not need the feature I am suggesting, but in practice parsing a text
file to communicate with the kernel is yucky at best, and probably not
very robust (e.g., /proc might not be mounted), and it would be very
difficult to convince, say, the OpenSSH authors to include code that
parses the Linux /proc/net/tcp format (or even link with a library
which does this) in order to add access-control on ssh port-forwards:
having this under a more standard getsockpot() interface is cleaner
and opens at least some kind of hope that programs would agree to use
it.]

Question number 1: If this feature were implemented, would it be
considered acceptable for inclusion in the kernel?  (If there is some
reason why it can't be accepted, I'd like to know in advance, to avoid
working in vain.)

Question number 2: A priori, how difficult would it be to implement
this?  (As mentioned above, it seems trivial in principle to merely go
through the local endpoints to find a matching connection, but maybe
there are locking issues that I don't understand that make it much
more difficult than it would seem.)  Any guidelines on implementation?
(I imagine one should try to fill sk->sk_peer_cred at connect time,
but I don't really know how difficult this might turn out.)

Any comments on the matter are welcome.

Happy hacking,

-- 
     David A. Madore
   ( http://www.madore.org/~david/ )
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