[PATCH 2/2] net: reset pending ARP state when request is done

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

net_new() creates a struct net_connection and calls arp_request()
to resolve unicast addresses. On success, arp_request() will
populate a buffer within the net_connection with the destination MAC
address.

If arp_request() aborts due to an error, it will leave the global
pending_arp.ether pointing at the buffer, which is promptly freed
leading to a dangling pointer and a use-after-free if we happen to get
an ARP response just after the error occurred.

Fix this memory safety issue by always clearing all of  pending_arp once
we are done with it, including error cases.

Reported-by: Sohaib Mohamed <sohaib.amhmd@gmail.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 net/net.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/net/net.c b/net/net.c
index d00d942b514c..5e0ddd2d335d 100644
--- a/net/net.c
+++ b/net/net.c
@@ -220,24 +220,28 @@ static int arp_request(struct eth_device *edev, IPaddr_t dest, unsigned char *et
 
 	ret = eth_send(edev, arp_packet, ETHER_HDR_SIZE + ARP_HDR_SIZE);
 	if (ret)
-		return ret;
+		goto out;
 	arp_start = get_time_ns();
 
 	while (arp_wait_ip) {
-		if (ctrlc())
-			return -EINTR;
+		if (ctrlc()) {
+			ret = -EINTR;
+			goto out;
+		}
 
 		if (is_timeout(arp_start, 3 * SECOND)) {
 			printf("T ");
 			arp_start = get_time_ns();
 			ret = eth_send(edev, arp_packet, ETHER_HDR_SIZE + ARP_HDR_SIZE);
 			if (ret)
-				return ret;
+				goto out;
 			retries++;
 		}
 
-		if (retries > PKT_NUM_RETRIES)
-			return -ETIMEDOUT;
+		if (retries > PKT_NUM_RETRIES) {
+			ret = -ETIMEDOUT;
+			goto out;
+		}
 
 		net_poll();
 	}
@@ -245,7 +249,11 @@ static int arp_request(struct eth_device *edev, IPaddr_t dest, unsigned char *et
 	pr_debug("Got ARP REPLY for %pI4: %02x:%02x:%02x:%02x:%02x:%02x\n",
 		 &dest, ether[0], ether[1], ether[2], ether[3], ether[4],
 		 ether[5]);
-	return 0;
+
+out:
+	pending_arp.ip = 0;
+	pending_arp.ether = NULL;
+	return ret;
 }
 
 void net_poll(void)
-- 
2.47.3