net: introduce SO_PEERGROUPS getsockopt

This adds the new getsockopt(2) option SO_PEERGROUPS on SOL_SOCKET to
retrieve the auxiliary groups of the remote peer. It is designed to
naturally extend SO_PEERCRED. That is, the underlying data is from the
same credentials. Regarding its syntax, it is based on SO_PEERSEC. That
is, if the provided buffer is too small, ERANGE is returned and @optlen
is updated. Otherwise, the information is copied, @optlen is set to the
actual size, and 0 is returned.

While SO_PEERCRED (and thus `struct ucred') already returns the primary
group, it lacks the auxiliary group vector. However, nearly all access
controls (including kernel side VFS and SYSVIPC, but also user-space
polkit, DBus, ...) consider the entire set of groups, rather than just
the primary group. But this is currently not possible with pure
SO_PEERCRED. Instead, user-space has to work around this and query the
system database for the auxiliary groups of a UID retrieved via
SO_PEERCRED.

Unfortunately, there is no race-free way to query the auxiliary groups
of the PID/UID retrieved via SO_PEERCRED. Hence, the current user-space
solution is to use getgrouplist(3p), which itself falls back to NSS and
whatever is configured in nsswitch.conf(3). This effectively checks
which groups we *would* assign to the user if it logged in *now*. On
normal systems it is as easy as reading /etc/group, but with NSS it can
resort to quering network databases (eg., LDAP), using IPC or network
communication.

Long story short: Whenever we want to use auxiliary groups for access
checks on IPC, we need further IPC to talk to the user/group databases,
rather than just relying on SO_PEERCRED and the incoming socket. This
is unfortunate, and might even result in dead-locks if the database
query uses the same IPC as the original request.

So far, those recursions / dead-locks have been avoided by using
primitive IPC for all crucial NSS modules. However, we want to avoid
re-inventing the wheel for each NSS module that might be involved in
user/group queries. Hence, we would preferably make DBus (and other IPC
that supports access-management based on groups) work without resorting
to the user/group database. This new SO_PEERGROUPS ioctl would allow us
to make dbus-daemon work without ever calling into NSS.

Cc: Michal Sekletar <msekleta@redhat.com>
Cc: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1078,6 +1078,18 @@ static void cred_to_ucred(struct pid *pid, const struct cred *cred,
 	}
 }
 
+static int groups_to_user(gid_t __user *dst, const struct group_info *src)
+{
+	struct user_namespace *user_ns = current_user_ns();
+	int i;
+
+	for (i = 0; i < src->ngroups; i++)
+		if (put_user(from_kgid_munged(user_ns, src->gid[i]), dst + i))
+			return -EFAULT;
+
+	return 0;
+}
+
 int sock_getsockopt(struct socket *sock, int level, int optname,
 		    char __user *optval, int __user *optlen)
 {
@@ -1231,6 +1243,27 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
 		goto lenout;
 	}
 
+	case SO_PEERGROUPS:
+	{
+		int ret, n;
+
+		if (!sk->sk_peer_cred)
+			return -ENODATA;
+
+		n = sk->sk_peer_cred->group_info->ngroups;
+		if (len < n * sizeof(gid_t)) {
+			len = n * sizeof(gid_t);
+			return put_user(len, optlen) ? -EFAULT : -ERANGE;
+		}
+		len = n * sizeof(gid_t);
+
+		ret = groups_to_user((gid_t __user *)optval,
+				     sk->sk_peer_cred->group_info);
+		if (ret)
+			return ret;
+		goto lenout;
+	}
+
 	case SO_PEERNAME:
 	{
 		char address[128];