tipc: unclone unbundled buffers before forwarding

When extracting an individual message from a received "bundle" buffer,
we just create a clone of the base buffer, and adjust it to point into
the right position of the linearized data area of the latter. This works
well for regular message reception, but during periods of extremely high
load it may happen that an extracted buffer, e.g, a connection probe, is
reversed and forwarded through an external interface while the preceding
extracted message is still unhandled. When this happens, the header or
data area of the preceding message will be partially overwritten by a
MAC header, leading to unpredicatable consequences, such as a link
reset.

We now fix this by ensuring that the msg_reverse() function never
returns a cloned buffer, and that the returned buffer always contains
sufficient valid head and tail room to be forwarded.

Reported-by: Erik Hugne <[e-mail]>
Acked-by: Ying Xue <[e-mail]>
Signed-off-by: Jon Maloy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/msg.c

···
 #include "name_table.h"
 
 #define MAX_FORWARD_SIZE 1024
+#define BUF_HEADROOM (LL_MAX_HEADER + 48)
+#define BUF_TAILROOM 16
 
 static unsigned int align(unsigned int i)
 {
···
 		memcpy(hdr, &ohdr, BASIC_H_SIZE);
 		msg_set_hdr_sz(hdr, BASIC_H_SIZE);
 	}
+
+	if (skb_cloned(_skb) &&
+	    pskb_expand_head(_skb, BUF_HEADROOM, BUF_TAILROOM, GFP_KERNEL))
+		goto exit;
 
 	/* Now reverse the concerned fields */
 	msg_set_errcode(hdr, err);

net/tipc/msg.h

···
 
 #define TIPC_MEDIA_INFO_OFFSET	5
 
-/**
- * TIPC message buffer code
- *
- * TIPC message buffer headroom reserves space for the worst-case
- * link-level device header (in case the message is sent off-node).
- *
- * Note: Headroom should be a multiple of 4 to ensure the TIPC header fields
- *       are word aligned for quicker access
- */
-#define BUF_HEADROOM (LL_MAX_HEADER + 48)
-
 struct tipc_skb_cb {
 	void *handle;
 	struct sk_buff *tail;