bgpd: Check against extended community unit size for link bandwidth

If we receive a malformed packets, this could lead ptr_get_be64() reading
the packets more than needed (heap overflow).

```
Using host libthread_db library "/lib/aarch64-linux-gnu/libthread_db.so.1".
    0 0xaaaaaadf86ec in __asan_memcpy (/home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/.libs/bgpd+0x3586ec) (BuildId: 78123cd26ada92b8b59fc0d74d292ba70c9d2e01)
    1 0xaaaaaaeb60fc in ptr_get_be64 /home/ubuntu/frr-public/frr_public_private-libfuzzer/./lib/stream.h:377:2
    2 0xaaaaaaeb5b90 in ecommunity_linkbw_present /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_ecommunity.c:1895:10
    3 0xaaaaaae50f30 in bgp_attr_ext_communities /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:2639:8
    4 0xaaaaaae49d58 in bgp_attr_parse /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_attr.c:3776:10
    5 0xaaaaab063260 in bgp_update_receive /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:2371:20
    6 0xaaaaab05df00 in bgp_process_packet /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_packet.c:4063:11
    7 0xaaaaaae36110 in LLVMFuzzerTestOneInput /home/ubuntu/frr-public/frr_public_private-libfuzzer/bgpd/bgp_main.c:582:3
```

This is triggered when receiving such a packet (malformed):

```
(gdb) bt
0  ecommunity_linkbw_present (ecom=0x555556287990, bw=bw@entry=0x7fffffffda68)
    at bgpd/bgp_ecommunity.c:1802
1  0x000055555564fcac in bgp_attr_ext_communities (args=0x7fffffffd840) at bgpd/bgp_attr.c:2619
2  bgp_attr_parse (peer=peer@entry=0x55555628cdf0, attr=attr@entry=0x7fffffffd960, size=size@entry=20,
    mp_update=mp_update@entry=0x7fffffffd940, mp_withdraw=mp_withdraw@entry=0x7fffffffd950)
    at bgpd/bgp_attr.c:3755
3  0x00005555556aa655 in bgp_update_receive (connection=connection@entry=0x5555562aa030,
    peer=peer@entry=0x55555628cdf0, size=size@entry=41) at bgpd/bgp_packet.c:2324
4  0x00005555556afab7 in bgp_process_packet (thread=<optimized out>) at bgpd/bgp_packet.c:3897
5  0x00007ffff7ac2f73 in event_call (thread=thread@entry=0x7fffffffdc70) at lib/event.c:2011
6  0x00007ffff7a6fb90 in frr_run (master=0x555555bc7c90) at lib/libfrr.c:1212
7  0x00005555556457e1 in main (argc=<optimized out>, argv=<optimized out>) at bgpd/bgp_main.c:543
(gdb) p *ecom
$1 = {refcnt = 1, unit_size = 8 '\b', disable_ieee_floating = false, size = 2, val = 0x555556282150 "",
  str = 0x5555562a9c30 "UNK:0, 255 UNK:2, 6"}
```

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

bgpd/bgp_ecommunity.c

@@ -1856,7 +1856,7 @@ ecommunity_add_origin_validation_state(enum rpki_states rpki_state,
  */
 const uint8_t *ecommunity_linkbw_present(struct ecommunity *ecom, uint64_t *bw)
 {
-	const uint8_t *eval;
+	const uint8_t *data;
 	uint32_t i;
 
 	if (bw)
@@ -1869,10 +1869,19 @@ const uint8_t *ecommunity_linkbw_present(struct ecommunity *ecom, uint64_t *bw)
 		const uint8_t *pnt;
 		uint8_t type, sub_type;
 
-		eval = pnt = (ecom->val + (i * ecom->unit_size));
+		data = pnt = (ecom->val + (i * ecom->unit_size));
 		type = *pnt++;
 		sub_type = *pnt++;
 
+		const uint8_t *end = data + ecom->unit_size;
+		size_t len = end - data;
+
+		/* Sanity check for extended communities lenght, to avoid
+		 * overrun when dealing with bits, e.g. ptr_get_be64().
+		 */
+		if (len < ecom->unit_size)
+			return NULL;
+
 		if ((type == ECOMMUNITY_ENCODE_AS ||
 		     type == ECOMMUNITY_ENCODE_AS_NON_TRANS) &&
 		    sub_type == ECOMMUNITY_LINK_BANDWIDTH) {
@@ -1886,19 +1895,22 @@ const uint8_t *ecommunity_linkbw_present(struct ecommunity *ecom, uint64_t *bw)
 							 ? bwval
 							 : ieee_float_uint32_to_uint32(
 								   bwval));
-			return eval;
+			return data;
 		} else if (type == ECOMMUNITY_ENCODE_AS4 &&
 			   sub_type == ECOMMUNITY_EXTENDED_LINK_BANDWIDTH) {
 			uint64_t bwval;
 
+			if (len < IPV6_ECOMMUNITY_SIZE)
+				return NULL;
+
 			pnt += 2; /* Reserved */
 			pnt = ptr_get_be64(pnt, &bwval);
 			(void)pnt;
 
 			if (bw)
 				*bw = bwval;
 
-			return eval;
+			return data;
 		}
 	}