tap: add missing verification for short frame

The cited commit missed to check against the validity of the frame length
in the tap_get_user_xdp() path, which could cause a corrupted skb to be
sent downstack. Even before the skb is transmitted, the
tap_get_user_xdp()-->skb_set_network_header() may assume the size is more
than ETH_HLEN. Once transmitted, this could either cause out-of-bound
access beyond the actual length, or confuse the underlayer with incorrect
or inconsistent header length in the skb metadata.

In the alternative path, tap_get_user() already prohibits short frame which
has the length less than Ethernet header size from being transmitted.

This is to drop any frame shorter than the Ethernet header size just like
how tap_get_user() does.

CVE: CVE-2024-41090
Link: https://lore.kernel.org/netdev/[email protected]/
Fixes: 0efac27 ("tap: accept an array of XDP buffs through sendmsg()")
Cc: [email protected]
Signed-off-by: Si-Wei Liu <[email protected]>
Signed-off-by: Dongli Zhang <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>
Reviewed-by: Paolo Abeni <[email protected]>
Reviewed-by: Jason Wang <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: siwliu-kernel

drivers/net/tap.c

@@ -1177,6 +1177,11 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
 	struct sk_buff *skb;
 	int err, depth;
 
+	if (unlikely(xdp->data_end - xdp->data < ETH_HLEN)) {
+		err = -EINVAL;
+		goto err;
+	}
+
 	if (q->flags & IFF_VNET_HDR)
 		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);