[TCP]: Fix sk_forward_alloc underflow in tcp_sendmsg

I've finally found a potential cause of the sk_forward_alloc underflows
that people have been reporting sporadically.

When tcp_sendmsg tacks on extra bits to an existing TCP_PAGE we don't
check sk_forward_alloc even though a large amount of time may have
elapsed since we allocated the page.  In the mean time someone could've
come along and liberated packets and reclaimed sk_forward_alloc memory.

This patch makes tcp_sendmsg check sk_forward_alloc every time as we
do in do_tcp_sendpages.
 
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 11 insertions, 8 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index e51e626e9af1..cf628261da52 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1232,9 +1232,8 @@ static inline struct page *sk_stream_alloc_page(struct sock *sk)
 {
         struct page *page = NULL;
 
-        if (sk_stream_wmem_schedule(sk, PAGE_SIZE))
-                page = alloc_pages(sk->sk_allocation, 0);
-        else {
+        page = alloc_pages(sk->sk_allocation, 0);
+        if (!page) {
                 sk->sk_prot->enter_memory_pressure();
                 sk_stream_moderate_sndbuf(sk);
         }
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 854f6d0c4bb3..cbcc9fc47783 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -769,19 +769,23 @@ new_segment:
                                         if (off == PAGE_SIZE) {
                                                 put_page(page);
                                                 TCP_PAGE(sk) = page = NULL;
+                                                TCP_OFF(sk) = off = 0;
                                         }
-                                }
+                                } else
+                                        BUG_ON(off);
+
+                                if (copy > PAGE_SIZE - off)
+                                        copy = PAGE_SIZE - off;
+
+                                if (!sk_stream_wmem_schedule(sk, copy))
+                                        goto wait_for_memory;
 
                                 if (!page) {
                                         /* Allocate new cache page. */
                                         if (!(page = sk_stream_alloc_page(sk)))
                                                 goto wait_for_memory;
-                                        off = 0;
                                 }
 
-                                if (copy > PAGE_SIZE - off)
-                                        copy = PAGE_SIZE - off;
-
                                 /* Time to copy data. We are close to
                                  * the end! */
                                 err = skb_copy_to_page(sk, from, skb, page,