[PATCH] sctp: load transport header after sk_filter

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

Do not cache pointers into the skb linear segment across sk_filter.
The function call can trigger pskb_expand_head.

Signed-off-by: Willem de Bruijn <willemb@google.com>

---

No Fixes tag: existed at the start of git log.

It is probably not necessary to store chunk->sctp_hdr at all, as
chunk->skb exists. That is harder to verify, so not suitable for net.

SCTP sk_filter trimming does not work as expected for most packets.
sctp_inq_pop marks most truncated linear packets as discarded with
pdiscard = 1. The following crude change would make trimming behave as
expected on linear data chunks but is probably not safe as is.

-        if (unlikely(skb_is_nonlinear(chunk->skb))) {
+        if (1) {
---
 net/sctp/input.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index a701527..47cf460 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -112,7 +112,6 @@ int sctp_rcv(struct sk_buff *skb)
 	struct sctp_ep_common *rcvr;
 	struct sctp_transport *transport = NULL;
 	struct sctp_chunk *chunk;
-	struct sctphdr *sh;
 	union sctp_addr src;
 	union sctp_addr dest;
 	int family;
@@ -127,8 +126,6 @@ int sctp_rcv(struct sk_buff *skb)
 	if (skb_linearize(skb))
 		goto discard_it;
 
-	sh = sctp_hdr(skb);
-
 	/* Pull up the IP and SCTP headers. */
 	__skb_pull(skb, skb_transport_offset(skb));
 	if (skb->len < sizeof(struct sctphdr))
@@ -230,7 +227,7 @@ int sctp_rcv(struct sk_buff *skb)
 	chunk->rcvr = rcvr;
 
 	/* Remember the SCTP header. */
-	chunk->sctp_hdr = sh;
+	chunk->sctp_hdr = sctp_hdr(skb);
 
 	/* Set the source and destination addresses of the incoming chunk.  */
 	sctp_init_addrs(chunk, &src, &dest);
-- 
2.8.0.rc3.226.g39d4020