net: sctp: keep owned chunk in destructor_arg instead of skb->cb

It's just silly to hold the skb destructor argument around inside
skb->cb[] as we currently do in SCTP.

Nowadays, we're sort of cheating on data accounting in the sense
that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
sndbuf again when transmitting packet"), we orphan the skb already
in the SCTP output path, i.e. giving back charged data memory, and
use a different destructor only to make sure the sk doesn't vanish
on skb destruction time. Thus, cb[] is still valid here as we
operate within the SCTP layer. (It's generally actually a big
candidate for future rework, imho.)

However, storing the destructor in the cb[] can easily cause issues
should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
layer, since cb[] may get overwritten by lower layers and thus can
corrupt the chunk pointer. There are no such issues at present,
but lets keep the chunk in destructor_arg, as this is the actual
purpose for it.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -162,7 +162,7 @@ static inline void sctp_set_owner_w(struct sctp_chunk *chunk)
 
 	chunk->skb->destructor = sctp_wfree;
 	/* Save the chunk pointer in skb for sctp_wfree to use later.  */
-	*((struct sctp_chunk **)(chunk->skb->cb)) = chunk;
+	skb_shinfo(chunk->skb)->destructor_arg = chunk;
 
 	asoc->sndbuf_used += SCTP_DATA_SNDSIZE(chunk) +
 				sizeof(struct sk_buff) +
@@ -6870,14 +6870,10 @@ static void sctp_wake_up_waiters(struct sock *sk,
  */
 static void sctp_wfree(struct sk_buff *skb)
 {
-	struct sctp_association *asoc;
-	struct sctp_chunk *chunk;
-	struct sock *sk;
+	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
+	struct sctp_association *asoc = chunk->asoc;
+	struct sock *sk = asoc->base.sk;
 
-	/* Get the saved chunk pointer.  */
-	chunk = *((struct sctp_chunk **)(skb->cb));
-	asoc = chunk->asoc;
-	sk = asoc->base.sk;
 	asoc->sndbuf_used -= SCTP_DATA_SNDSIZE(chunk) +
 				sizeof(struct sk_buff) +
 				sizeof(struct sctp_chunk);