net: adjust skb->truesize in pskb_expand_head()

Slava Shwartsman reported a warning in skb_try_coalesce(), when we
detect skb->truesize is completely wrong.

In his case, issue came from IPv6 reassembly coping with malicious
datagrams, that forced various pskb_may_pull() to reallocate a bigger
skb->head than the one allocated by NIC driver before entering GRO
layer.

Current code does not change skb->truesize, leaving this burden to
callers if they care enough.

Blindly changing skb->truesize in pskb_expand_head() is not
easy, as some producers might track skb->truesize, for example
in xmit path for back pressure feedback (sk->sk_wmem_alloc)

We can detect the cases where it should be safe to change
skb->truesize :

1) skb is not attached to a socket.
2) If it is attached to a socket, destructor is sock_edemux()

My audit gave only two callers doing their own skb->truesize
manipulation.

I had to remove skb parameter in sock_edemux macro when
CONFIG_INET is not set to avoid a compile error.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Slava Shwartsman <slavash@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1534,7 +1534,7 @@ void sock_efree(struct sk_buff *skb);
 #ifdef CONFIG_INET
 void sock_edemux(struct sk_buff *skb);
 #else
-#define sock_edemux(skb) sock_efree(skb)
+#define sock_edemux sock_efree
 #endif
 
 int sock_setsockopt(struct socket *sock, int level, int op,

net/core/skbuff.c

@@ -1192,10 +1192,10 @@ EXPORT_SYMBOL(__pskb_copy_fclone);
 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 		     gfp_t gfp_mask)
 {
-	int i;
-	u8 *data;
-	int size = nhead + skb_end_offset(skb) + ntail;
+	int i, osize = skb_end_offset(skb);
+	int size = osize + nhead + ntail;
 	long off;
+	u8 *data;
 
 	BUG_ON(nhead < 0);
 
@@ -1257,6 +1257,14 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	skb->hdr_len  = 0;
 	skb->nohdr    = 0;
 	atomic_set(&skb_shinfo(skb)->dataref, 1);
+
+	/* It is not generally safe to change skb->truesize.
+	 * For the moment, we really care of rx path, or
+	 * when skb is orphaned (not attached to a socket).
+	 */
+	if (!skb->sk || skb->destructor == sock_edemux)
+		skb->truesize += size - osize;
+
 	return 0;
 
 nofrags:

net/netlink/af_netlink.c

@@ -1210,11 +1210,9 @@ static struct sk_buff *netlink_trim(struct sk_buff *skb, gfp_t allocation)
 		skb = nskb;
 	}
 
-	if (!pskb_expand_head(skb, 0, -delta,
+	pskb_expand_head(skb, 0, -delta,
 			 (allocation & ~__GFP_DIRECT_RECLAIM) |
-			      __GFP_NOWARN | __GFP_NORETRY))
-		skb->truesize -= delta;
-
+			 __GFP_NOWARN | __GFP_NORETRY);
 	return skb;
 }
 

net/wireless/util.c

@@ -618,8 +618,6 @@ int ieee80211_data_from_8023(struct sk_buff *skb, const u8 *addr,
 
 		if (pskb_expand_head(skb, head_need, 0, GFP_ATOMIC))
 			return -ENOMEM;
-
-		skb->truesize += head_need;
 	}
 
 	if (encaps_data) {