slip: Check if rstate is initialized before uncompressing

On receiving a packet the state index points to the rstate which must be
used to fill up IP and TCP headers. But if the state index points to a
rstate which is unitialized, i.e. filled with zeros, it gets stuck in an
infinite loop inside ip_fast_csum trying to compute the ip checsum of a
header with zero length.

89.666953: <2> [<ffffff9dd3e94d38>] slhc_uncompress+0x464/0x468
89.666965: <2> [<ffffff9dd3e87d88>] ppp_receive_nonmp_frame+0x3b4/0x65c
89.666978: <2> [<ffffff9dd3e89dd4>] ppp_receive_frame+0x64/0x7e0
89.666991: <2> [<ffffff9dd3e8a708>] ppp_input+0x104/0x198
89.667005: <2> [<ffffff9dd3e93868>] pppopns_recv_core+0x238/0x370
89.667027: <2> [<ffffff9dd4428fc8>] __sk_receive_skb+0xdc/0x250
89.667040: <2> [<ffffff9dd3e939e4>] pppopns_recv+0x44/0x60
89.667053: <2> [<ffffff9dd4426848>] __sock_queue_rcv_skb+0x16c/0x24c
89.667065: <2> [<ffffff9dd4426954>] sock_queue_rcv_skb+0x2c/0x38
89.667085: <2> [<ffffff9dd44f7358>] raw_rcv+0x124/0x154
89.667098: <2> [<ffffff9dd44f7568>] raw_local_deliver+0x1e0/0x22c
89.667117: <2> [<ffffff9dd44c8ba0>] ip_local_deliver_finish+0x70/0x24c
89.667131: <2> [<ffffff9dd44c92f4>] ip_local_deliver+0x100/0x10c

./scripts/faddr2line vmlinux slhc_uncompress+0x464/0x468 output:
ip_fast_csum at arch/arm64/include/asm/checksum.h:40
(inlined by) slhc_uncompress at drivers/net/slip/slhc.c:615

Adding a variable to indicate if the current rstate is initialized. If
such a packet arrives, move to toss state.

Signed-off-by: Tejaswi Tanikella <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/slip/slhc.c

···
 		if(x < 0 || x > comp->rslot_limit)
 			goto bad;
 
+		/* Check if the cstate is initialized */
+		if (!comp->rstate[x].initialized)
+			goto bad;
+
 		comp->flags &=~ SLF_TOSS;
 		comp->recv_current = x;
 	} else {
···
 	if (cs->cs_tcp.doff > 5)
 	  memcpy(cs->cs_tcpopt, icp + ihl*4 + sizeof(struct tcphdr), (cs->cs_tcp.doff - 5) * 4);
 	cs->cs_hsize = ihl*2 + cs->cs_tcp.doff*2;
+	cs->initialized = true;
 	/* Put headers back on packet
 	 * Neither header checksum is recalculated
 	 */

include/net/slhc_vj.h

···
  */
 struct cstate {
 	byte_t	cs_this;	/* connection id number (xmit) */
+	bool	initialized;	/* true if initialized */
 	struct cstate *next;	/* next in ring (xmit) */
 	struct iphdr cs_ip;	/* ip/tcp hdr from most recent packet */
 	struct tcphdr cs_tcp;