net: Reset secmark when scrubbing packet

skb_scrub_packet() is called when a packet switches between a context
such as between underlay and overlay, between namespaces, or between
L3 subnets.

While we already scrub the packet mark, connection tracking entry,
and cached destination, the security mark/context is left intact.

It seems wrong to inherit the security context of a packet when going
from overlay to underlay or across forwarding paths.
Signed-off-by: NThomas Graf <tgraf@suug.ch>
Acked-by: NFlavio Leitner <fbl@sysclose.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -4148,6 +4148,7 @@ void skb_scrub_packet(struct sk_buff *skb, bool xnet)
 	skb->ignore_df = 0;
 	skb_dst_drop(skb);
 	skb->mark = 0;
+	skb_init_secmark(skb);
 	secpath_reset(skb);
 	nf_reset(skb);
 	nf_reset_trace(skb);