udp: preserve head state for IP_CMSG_PASSSEC

Paul Moore reported a SELinux/IP_PASSSEC regression
caused by missing skb->sp at recvmsg() time. We need to
preserve the skb head state to process the IP_CMSG_PASSSEC
cmsg.

With this commit we avoid releasing the skb head state in the
BH even if a secpath is attached to the current skb, and stores
the skb status (with/without head states) in the scratch area,
so that we can access it at skb deallocation time, without
incurring in cache-miss penalties.

This also avoids misusing the skb CB for ipv6 packets,
as introduced by the commit 0ddf3fb2c43d ("udp: preserve
skb->dst if required for IP options processing").

Clean a bit the scratch area helpers implementation, to
reduce the code differences between 32 and 64 bits build.

Reported-by: Paul Moore <paul@paul-moore.com>
Fixes: 0a463c78d25b ("udp: avoid a cache miss on dequeue")
Fixes: 0ddf3fb2c43d ("udp: preserve skb->dst if required for IP options processing")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 22 insertions, 11 deletions

diff --git a/include/net/udp.h b/include/net/udp.h
index 972ce4baab6b..56ce2d2a612d 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -305,33 +305,44 @@ struct sock *udp6_lib_lookup_skb(struct sk_buff *skb,
 /* UDP uses skb->dev_scratch to cache as much information as possible and avoid
  * possibly multiple cache miss on dequeue()
  */
-#if BITS_PER_LONG == 64
-
-/* truesize, len and the bit needed to compute skb_csum_unnecessary will be on
- * cold cache lines at recvmsg time.
- * skb->len can be stored on 16 bits since the udp header has been already
- * validated and pulled.
- */
 struct udp_dev_scratch {
-	u32 truesize;
+	/* skb->truesize and the stateless bit are embedded in a single field;
+	 * do not use a bitfield since the compiler emits better/smaller code
+	 * this way
+	 */
+	u32 _tsize_state;
+
+#if BITS_PER_LONG == 64
+	/* len and the bit needed to compute skb_csum_unnecessary
+	 * will be on cold cache lines at recvmsg time.
+	 * skb->len can be stored on 16 bits since the udp header has been
+	 * already validated and pulled.
+	 */
 	u16 len;
 	bool is_linear;
 	bool csum_unnecessary;
+#endif
 };
 
+static inline struct udp_dev_scratch *udp_skb_scratch(struct sk_buff *skb)
+{
+	return (struct udp_dev_scratch *)&skb->dev_scratch;
+}
+
+#if BITS_PER_LONG == 64
 static inline unsigned int udp_skb_len(struct sk_buff *skb)
 {
-	return ((struct udp_dev_scratch *)&skb->dev_scratch)->len;
+	return udp_skb_scratch(skb)->len;
 }
 
 static inline bool udp_skb_csum_unnecessary(struct sk_buff *skb)
 {
-	return ((struct udp_dev_scratch *)&skb->dev_scratch)->csum_unnecessary;
+	return udp_skb_scratch(skb)->csum_unnecessary;
 }
 
 static inline bool udp_skb_is_linear(struct sk_buff *skb)
 {
-	return ((struct udp_dev_scratch *)&skb->dev_scratch)->is_linear;
+	return udp_skb_scratch(skb)->is_linear;
 }
 
 #else