[PATCH iproute] libnetlink: ignore netlink messages not from kernel

[Stephen Hemminger <stephen@networkplumber.org>]

A process with CAP_NET_ADMIN in its own netns can send forged
netlink messages to a monitor, by multicast to an all-nsid listener
in another netns or by unicast to the monitor's port.

Filter on nl_pid in __rtnl_recvmsg() and use it in rtnl_listen().
Dequeue the datagram when peeking, and restore the value-result
msg fields before retrying.

Reported-by: Anthropic OSS scanner
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/libnetlink.c | 51 ++++++++++++++++++++++++++----------------------
 1 file changed, 28 insertions(+), 23 deletions(-)

diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index edf3a8ba..a07cbeef 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -825,13 +825,21 @@ static int rtnl_dump_error(const struct rtnl_handle *rth,
 
 static int __rtnl_recvmsg(int fd, struct msghdr *msg, int flags)
 {
+	const struct sockaddr_nl *nladdr = msg->msg_name;
+	socklen_t namelen = msg->msg_namelen;
+	size_t controllen = msg->msg_controllen;
 	int len;
 
-	do {
-		len = recvmsg(fd, msg, flags);
-	} while (len < 0 && (errno == EINTR || errno == EAGAIN));
+retry:
+	/* value-result fields, restore if discarding a message */
+	msg->msg_namelen = namelen;
+	msg->msg_controllen = controllen;
 
+	len = recvmsg(fd, msg, flags);
 	if (len < 0) {
+		if (errno == EINTR || errno == EAGAIN)
+			goto retry;
+
 		fprintf(stderr, "netlink receive error %s (%d)\n",
 			strerror(errno), errno);
 		return -errno;
@@ -842,6 +850,19 @@ static int __rtnl_recvmsg(int fd, struct msghdr *msg, int flags)
 		return -ENODATA;
 	}
 
+	if (msg->msg_namelen != sizeof(*nladdr)) {
+		fprintf(stderr, "Sender address length == %u\n",
+			msg->msg_namelen);
+		return -EINVAL;
+	}
+
+	/* Only accept messages from kernel, discard anything else */
+	if (nladdr->nl_pid != 0) {
+		if (flags & MSG_PEEK)
+			recv(fd, NULL, 0, 0);
+		goto retry;
+	}
+
 	return len;
 }
 
@@ -1248,27 +1269,11 @@ int rtnl_listen(struct rtnl_handle *rtnl,
 		}
 
 		iov.iov_len = sizeof(buf);
-		status = recvmsg(rtnl->fd, &msg, 0);
-
-		if (status < 0) {
-			if (errno == EINTR || errno == EAGAIN)
-				continue;
-			fprintf(stderr, "netlink receive error %s (%d)\n",
-				strerror(errno), errno);
-			if (errno == ENOBUFS)
-				continue;
-			return -1;
-		}
-		if (status == 0) {
-			fprintf(stderr, "EOF on netlink\n");
+		status = __rtnl_recvmsg(rtnl->fd, &msg, 0);
+		if (status == -ENOBUFS)
+			continue;
+		if (status < 0)
 			return -1;
-		}
-		if (msg.msg_namelen != sizeof(nladdr)) {
-			fprintf(stderr,
-				"Sender address length == %d\n",
-				msg.msg_namelen);
-			exit(1);
-		}
 
 		if (rtnl->flags & RTNL_HANDLE_F_LISTEN_ALL_NSID) {
 			memset(&ctrl, 0, sizeof(ctrl));
-- 
2.53.0