conn: ensure control message size is respected in StdNetBind

This commit re-slices received control messages in StdNetBind to the
value the OS reports on a successful read. Previously, the len of this
slice would always be srcControlSize, which could result in control
message values leaking through a sync.Pool round trip. This is
unlikely with the IP_PKTINFO socket option set successfully, but
should be guarded against.

Signed-off-by: James Tucker <[email protected]>
Signed-off-by: Jordan Whited <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>

conn/bind_std.go

@@ -229,7 +229,7 @@ func (s *StdNetBind) makeReceiveIPv4(pc *ipv4.PacketConn, conn *net.UDPConn) Rec
 			sizes[i] = msg.N
 			addrPort := msg.Addr.(*net.UDPAddr).AddrPort()
 			ep := asEndpoint(addrPort)
-			getSrcFromControl(msg.OOB, ep)
+			getSrcFromControl(msg.OOB[:msg.NN], ep)
 			eps[i] = ep
 		}
 		return numMsgs, nil
@@ -262,7 +262,7 @@ func (s *StdNetBind) makeReceiveIPv6(pc *ipv6.PacketConn, conn *net.UDPConn) Rec
 			sizes[i] = msg.N
 			addrPort := msg.Addr.(*net.UDPAddr).AddrPort()
 			ep := asEndpoint(addrPort)
-			getSrcFromControl(msg.OOB, ep)
+			getSrcFromControl(msg.OOB[:msg.NN], ep)
 			eps[i] = ep
 		}
 		return numMsgs, nil