gtp: allow -1 to be specified as file description from userspace

Existing user space applications maintained by the Osmocom project are
breaking since a recent fix that addresses incorrect error checking.

Restore operation for user space programs that specify -1 as file
descriptor to skip GTPv0 or GTPv1 only sockets.

Fixes: defd8b3 ("gtp: fix a potential NULL pointer dereference")
Reported-by: Pau Espin Pedrol <pespin@sysmocom.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Oliver Smith <osmith@sysmocom.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241022144825.66740-1-pablo@netfilter.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: ummakynes

drivers/net/gtp.c

@@ -1702,20 +1702,24 @@ static int gtp_encap_enable(struct gtp_dev *gtp, struct nlattr *data[])
 		return -EINVAL;
 
 	if (data[IFLA_GTP_FD0]) {
-		u32 fd0 = nla_get_u32(data[IFLA_GTP_FD0]);
+		int fd0 = nla_get_u32(data[IFLA_GTP_FD0]);
 
-		sk0 = gtp_encap_enable_socket(fd0, UDP_ENCAP_GTP0, gtp);
-		if (IS_ERR(sk0))
-			return PTR_ERR(sk0);
+		if (fd0 >= 0) {
+			sk0 = gtp_encap_enable_socket(fd0, UDP_ENCAP_GTP0, gtp);
+			if (IS_ERR(sk0))
+				return PTR_ERR(sk0);
+		}
 	}
 
 	if (data[IFLA_GTP_FD1]) {
-		u32 fd1 = nla_get_u32(data[IFLA_GTP_FD1]);
+		int fd1 = nla_get_u32(data[IFLA_GTP_FD1]);
 
-		sk1u = gtp_encap_enable_socket(fd1, UDP_ENCAP_GTP1U, gtp);
-		if (IS_ERR(sk1u)) {
-			gtp_encap_disable_sock(sk0);
-			return PTR_ERR(sk1u);
+		if (fd1 >= 0) {
+			sk1u = gtp_encap_enable_socket(fd1, UDP_ENCAP_GTP1U, gtp);
+			if (IS_ERR(sk1u)) {
+				gtp_encap_disable_sock(sk0);
+				return PTR_ERR(sk1u);
+			}
 		}
 	}