hdlc_ppp: add range checks in ppp_cp_parse_cr()

There are a couple bugs here:
1) If opt[1] is zero then this results in a forever loop.  If the value
   is less than 2 then it is invalid.
2) It assumes that "len" is more than sizeof(valid_accm) or 6 which can
   result in memory corruption.

In the case of LCP_OPTION_ACCM, then  we should check "opt[1]" instead
of "len" because, if "opt[1]" is less than sizeof(valid_accm) then
"nak_len" gets out of sync and it can lead to memory corruption in the
next iterations through the loop.  In case of LCP_OPTION_MAGIC, the
only valid value for opt[1] is 6, but the code is trying to log invalid
data so we should only discard the data when "len" is less than 6
because that leads to a read overflow.

Reported-by: ChenNan Of Chaitin Security Research Lab  <whutchennan@gmail.com>
Fixes: e022c2f ("WAN: new synchronous PPP implementation for generic HDLC.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 66d42ed)
CVE-2020-25643
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>

drivers/net/wan/hdlc_ppp.c

@@ -383,18 +383,17 @@ static void ppp_cp_parse_cr(struct net_device *dev, u16 pid, u8 id,
 	}
 
 	for (opt = data; len; len -= opt[1], opt += opt[1]) {
-		if (len < 2 || len < opt[1]) {
-			dev->stats.rx_errors++;
-			kfree(out);
-			return; /* bad packet, drop silently */
-		}
+		if (len < 2 || opt[1] < 2 || len < opt[1])
+			goto err_out;
 
 		if (pid == PID_LCP)
 			switch (opt[0]) {
 			case LCP_OPTION_MRU:
 				continue; /* MRU always OK and > 1500 bytes? */
 
 			case LCP_OPTION_ACCM: /* async control character map */
+				if (opt[1] < sizeof(valid_accm))
+					goto err_out;
 				if (!memcmp(opt, valid_accm,
 					    sizeof(valid_accm)))
 					continue;
@@ -406,6 +405,8 @@ static void ppp_cp_parse_cr(struct net_device *dev, u16 pid, u8 id,
 				}
 				break;
 			case LCP_OPTION_MAGIC:
+				if (len < 6)
+					goto err_out;
 				if (opt[1] != 6 || (!opt[2] && !opt[3] &&
 						    !opt[4] && !opt[5]))
 					break; /* reject invalid magic number */
@@ -424,6 +425,11 @@ static void ppp_cp_parse_cr(struct net_device *dev, u16 pid, u8 id,
 		ppp_cp_event(dev, pid, RCR_GOOD, CP_CONF_ACK, id, req_len, data);
 
 	kfree(out);
+	return;
+
+err_out:
+	dev->stats.rx_errors++;
+	kfree(out);
 }
 
 static int ppp_rx(struct sk_buff *skb)