xsk: fix to reject invalid flags in xsk_bind

Passing a non-existing flag in the sxdp_flags member of struct
sockaddr_xdp was, incorrectly, silently ignored. This patch addresses
that behavior, and rejects any non-existing flags.

We have examined existing user space code, and to our best knowledge,
no one is relying on the current incorrect behavior. AF_XDP is still
in its infancy, so from our perspective, the risk of breakage is very
low, and addressing this problem now is important.

Fixes: 965a9909 ("xsk: add support for bind for Rx")
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xsk.c

@@ -407,6 +407,10 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
 	if (sxdp->sxdp_family != AF_XDP)
 		return -EINVAL;
 
+	flags = sxdp->sxdp_flags;
+	if (flags & ~(XDP_SHARED_UMEM | XDP_COPY | XDP_ZEROCOPY))
+		return -EINVAL;
+
 	mutex_lock(&xs->mutex);
 	if (xs->dev) {
 		err = -EBUSY;
@@ -425,7 +429,6 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
 	}
 
 	qid = sxdp->sxdp_queue_id;
-	flags = sxdp->sxdp_flags;
 
 	if (flags & XDP_SHARED_UMEM) {
 		struct xdp_sock *umem_xs;