[PATCH net 1/3] net: hsr: privatize interlink-bound skbs before address mutation

[Xin Xie <xiexinet@gmail.com>]

On an HSR RedBox, frames forwarded to the interlink go through
hsr_get_untagged_frame() and arrive in hsr_xmit() as
skb_clone(frame->skb_std). Two reachable alias cases make the writes
there unsafe:

(a) A tagged multicast or broadcast frame received on a slave is
    also delivered to the master through another clone of the same
    buffer: hsr_deliver_master() writes the node MAC and hsr_xmit()
    writes hsr->macaddress_redbox into the same six source bytes, so
    the local stack receives the RedBox MAC instead of the
    originating node's - deterministic without backpressure.

(b) A master-originated frame forwarded to the interlink aliases the
    original TX skb, which packet taps or the TX path may still hold.
    The master-origin substitutions (hsr_addr_subst_dest() and the
    outgoing-slave source write) and the RedBox rewrite would all
    modify that shared data.

Privatize the interlink-bound skb with skb_cow() before any address
mutation whenever it can alias a live consumer: for all
master-originated frames, and for ring frames the master also
consumes (is_local_dest && !is_local_exclusive - the exact inverse
of the port loop's master skip rules, so the condition does not
depend on port order). Every interlink skb reaching hsr_xmit() is a
clone from get_untagged_frame(), so the selected cases always pay
one private copy. On the hardware tag-insertion path, the selected
master-originated and locally consumed ring frames are now isolated
before the interlink address write: the interlink write can no
longer change an already queued clone of the same frame. Ring frames
the master does not consume (is_local_dest == false, e.g. unicast
ring-to-SAN) keep the zero-copy path; aliases that already exist
there - for example between the interlink clone and a peer slave's
hardware-tag-insertion clone - are pre-existing and unchanged by
this patch. On allocation failure the frame is dropped with the same
accounting as a tagged-frame creation failure.

Fixes: 5055cccfc2d1 ("net: hsr: Provide RedBox support (HSR-SAN)")
Signed-off-by: Xin Xie <xiexinet@gmail.com>
---
 net/hsr/hsr_forward.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/net/hsr/hsr_forward.c b/net/hsr/hsr_forward.c
index 0774981a65..67aaf5a862 100644
--- a/net/hsr/hsr_forward.c
+++ b/net/hsr/hsr_forward.c
@@ -420,6 +420,22 @@ static void hsr_deliver_master(struct sk_buff *skb, struct net_device *dev,
 static int hsr_xmit(struct sk_buff *skb, struct hsr_port *port,
 		    struct hsr_frame_info *frame)
 {
+	/* An interlink-bound skb from get_untagged_frame() can still alias
+	 * another live consumer: for master-originated frames the clone
+	 * shares the original TX skb (which taps or the TX path may still
+	 * hold); for ring frames the master also consumes them when they
+	 * are destined to the local node without being exclusive to it.
+	 * Privatize before any address mutation.
+	 */
+	if (port->type == HSR_PT_INTERLINK &&
+	    (frame->port_rcv->type == HSR_PT_MASTER ||
+	     (frame->is_local_dest && !frame->is_local_exclusive)) &&
+	    skb_cow(skb, 0)) {
+		frame->port_rcv->dev->stats.rx_dropped++;
+		kfree_skb(skb);
+		return NET_XMIT_DROP;
+	}
+
 	if (frame->port_rcv->type == HSR_PT_MASTER) {
 		hsr_addr_subst_dest(frame->node_src, skb, port);
 
-- 
2.43.0