[TCP]: Compute in_sacked properly when we split up a TSO frame.

The problem is that the SACK fragmenting code may incorrectly call
tcp_fragment() with a length larger than the skb->len.  This happens
when the skb on the transmit queue completely falls to the LHS of the
SACK.

And add a BUG() check to tcp_fragment() so we can spot this kind of
error more quickly in the future.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 11 insertions, 7 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 29222b964951..a7537c7bbd06 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -979,14 +979,19 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
                         if (!before(TCP_SKB_CB(skb)->seq, end_seq))
                                 break;
 
+                        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
+                                !before(end_seq, TCP_SKB_CB(skb)->end_seq);
+
                         pcount = tcp_skb_pcount(skb);
 
-                        if (pcount > 1 &&
-                            (after(start_seq, TCP_SKB_CB(skb)->seq) ||
-                             before(end_seq, TCP_SKB_CB(skb)->end_seq))) {
+                        if (pcount > 1 && !in_sack &&
+                            after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
                                 unsigned int pkt_len;
 
-                                if (after(start_seq, TCP_SKB_CB(skb)->seq))
+                                in_sack = !after(start_seq,
+                                                 TCP_SKB_CB(skb)->seq);
+
+                                if (!in_sack)
                                         pkt_len = (start_seq -
                                                    TCP_SKB_CB(skb)->seq);
                                 else
@@ -999,9 +1004,6 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
 
                         fack_count += pcount;
 
-                        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
-                                !before(end_seq, TCP_SKB_CB(skb)->end_seq);
-
                         sacked = TCP_SKB_CB(skb)->sacked;
 
                         /* Account D-SACK for retransmitted packet. */
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c10e4435e3b1..b018e31b6530 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -435,6 +435,8 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
         int nsize, old_factor;
         u16 flags;
 
+        BUG_ON(len >= skb->len);
+
         nsize = skb_headlen(skb) - len;
         if (nsize < 0)
                 nsize = 0;