net: cls_bpf: fix auto generation of per list handles

commit 3f2ab135 upstream.

When creating a bpf classifier in tc with priority collisions and
invoking automatic unique handle assignment, cls_bpf_grab_new_handle()
will return a wrong handle id which in fact is non-unique. Usually
altering of specific filters is being addressed over major id, but
in case of collisions we result in a filter chain, where handle ids
address individual cls_bpf_progs inside the classifier.

Issue is, in cls_bpf_grab_new_handle() we probe for head->hgen handle
in cls_bpf_get() and in case we found a free handle, we're supposed
to use exactly head->hgen. In case of insufficient numbers of handles,
we bail out later as handle id 0 is not allowed.

Fixes: 7d1d65cb ("net: sched: cls_bpf: add BPF-based classifier")
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/sched/cls_bpf.c

@@ -233,15 +233,21 @@ static u32 cls_bpf_grab_new_handle(struct tcf_proto *tp,
 				   struct cls_bpf_head *head)
 {
 	unsigned int i = 0x80000000;
+	u32 handle;
 
 	do {
 		if (++head->hgen == 0x7FFFFFFF)
 			head->hgen = 1;
 	} while (--i > 0 && cls_bpf_get(tp, head->hgen));
-	if (i == 0)
+
+	if (unlikely(i == 0)) {
 		pr_err("Insufficient number of handles\n");
+		handle = 0;
+	} else {
+		handle = head->hgen;
+	}
 
-	return i;
+	return handle;
 }
 
 static int cls_bpf_change(struct net *net, struct sk_buff *in_skb,