conf: Bind inbound ports with CAP_NET_BIND_SERVICE before isolate_user()

Even if CAP_NET_BIND_SERVICE is granted, we'll lose the capability in
the target user namespace as we isolate the process, which means
we're unable to bind to low ports at that point.

Bind inbound ports, and only those, before isolate_user(). Keep the
handling of outbound ports (for pasta mode only) after the setup of
the namespace, because that's where we'll bind them.

To this end, initialise the netlink socket for the init namespace
before isolate_user() as well, as we actually need to know the
addresses of the upstream interface before binding ports, in case
they're not explicitly passed by the user.

As we now call nl_sock_init() twice, checking its return code from
conf() twice looks a bit heavy: make it exit(), instead, as we
can't do much if we don't have netlink sockets.

While at it:

- move the v4_only && v6_only options check just after the first
  option processing loop, as this is more strictly related to
  option parsing proper

- update the man page, explaining that CAP_NET_BIND_SERVICE is
  *not* the preferred way to bind ports, because passt and pasta
  can be abused to allow other processes to make effective usage
  of it. Add a note about the recommended sysctl instead

- simplify nl_sock_init_do() now that it's called once for each
  case

Reported-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 40 insertions, 7 deletions

diff --git a/passt.1 b/passt.1
index 7d113f2..4848087 100644
--- a/passt.1
+++ b/passt.1
@@ -771,14 +771,47 @@ possible to bind sockets to foreign addresses.
 
 .SS Binding to low numbered ports (well-known or system ports, up to 1023)
 
-If the port forwarding configuration requires binding to port numbers lower than
-1024, \fBpasst\fR and \fBpasta\fR will try to bind to them, but will fail if not
-running as root, or without the \fICAP_NET_BIND_SERVICE\fR Linux capability, see
-\fBservices\fR(5) and \fBcapabilities\fR(7). To grant the
-\fICAP_NET_BIND_SERVICE\fR capability to passt, you can issue, as root:
+If the port forwarding configuration requires binding to ports with numbers
+lower than 1024, \fBpasst\fR and \fBpasta\fR will try to bind to them, but will
+fail, unless, either:
+
+.IP \(bu 2
+the \fIsys.net.ipv4.ip_unprivileged_port_start\fR sysctl is set to the number
+of the lowest port \fBpasst\fR and \fBpasta\fR need. For example, as root:
+
+.nf
+	sysctl -w net.ipv4.ip_unprivileged_port_start=443
+.fi
+
+\fBNote\fR: this is the recommended way of enabling \fBpasst\fR and \fBpasta\fR
+to bind to ports with numbers below 1024.
+
+.IP \(bu
+or the \fICAP_NET_BIND_SERVICE\fR Linux capability is granted, see
+\fBservices\fR(5) and \fBcapabilities\fR(7).
+
+This is, in general, \fBnot the recommended way\fR, because \fBpasst\fR and
+\fBpasta\fR might be used as vector to effectively use this capability from
+another process.
+
+However, if your environment is sufficiently controlled by an LSM (Linux
+Security Module) such as \fIAppArmor\fR, \fISELinux\fR, \fISmack\fR or
+\fITOMOYO\fR, and no other processes can interact in such a way in virtue of
+this, granting this capability to \fBpasst\fR and \fBpasta\fR only can
+effectively prevent other processes from utilising it.
+
+Note that this will not work for automatic detection and forwarding of ports
+with \fBpasta\fR, because \fBpasta\fR will relinquish this capability at
+runtime.
+
+To grant this capability, you can issue, as root:
+
+.nf
+	for p in $(which passt passt.avx2); do
+		setcap 'cap_net_bind_service=+ep' "${p}"
+	done
+.fi
 
-.RS
-setcap 'cap_net_bind_service=+ep' $(which passt)
 .RE
 
 .SS ICMP/ICMPv6 Echo sockets