[NET]: Add skb->truesize assertion checking.

Add some sanity checking.  truesize should be at least sizeof(struct
sk_buff) plus the current packet length.  If not, then truesize is
seriously mangled and deserves a kernel log message.

Currently we'll do the check for release of stream socket buffers.

But we can add checks to more spots over time.

Incorporating ideas from Herbert Xu.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -344,6 +344,13 @@ extern void	      skb_over_panic(struct sk_buff *skb, int len,
 				     void *here);
 extern void	      skb_under_panic(struct sk_buff *skb, int len,
 				      void *here);
+extern void	      skb_truesize_bug(struct sk_buff *skb);
+
+static inline void skb_truesize_check(struct sk_buff *skb)
+{
+	if (unlikely((int)skb->truesize < sizeof(struct sk_buff) + skb->len))
+		skb_truesize_bug(skb);
+}
 
 extern int skb_append_datato_frags(struct sock *sk, struct sk_buff *skb,
 			int getfrag(void *from, char *to, int offset,

include/net/sock.h

@@ -454,6 +454,7 @@ static inline void sk_stream_set_owner_r(struct sk_buff *skb, struct sock *sk)
 
 static inline void sk_stream_free_skb(struct sock *sk, struct sk_buff *skb)
 {
+	skb_truesize_check(skb);
 	sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
 	sk->sk_wmem_queued   -= skb->truesize;
 	sk->sk_forward_alloc += skb->truesize;

net/core/skbuff.c

@@ -112,6 +112,14 @@ void skb_under_panic(struct sk_buff *skb, int sz, void *here)
 	BUG();
 }
 
+void skb_truesize_bug(struct sk_buff *skb)
+{
+	printk(KERN_ERR "SKB BUG: Invalid truesize (%u) "
+	       "len=%u, sizeof(sk_buff)=%Zd\n",
+	       skb->truesize, skb->len, sizeof(struct sk_buff));
+}
+EXPORT_SYMBOL(skb_truesize_bug);
+
 /* 	Allocate a new skbuff. We do this ourselves so we can fill in a few
  *	'private' fields and also do memory statistics to find all the
  *	[BEEP] leaks.

net/core/stream.c

@@ -176,6 +176,7 @@ void sk_stream_rfree(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
 
+	skb_truesize_check(skb);
 	atomic_sub(skb->truesize, &sk->sk_rmem_alloc);
 	sk->sk_forward_alloc += skb->truesize;
 }