tcp: fix crashes in do_tcp_sendpages()

Recent network changes allowed high order pages being used
for skb fragments.

This uncovered a bug in do_tcp_sendpages() which was assuming its caller
provided an array of order-0 page pointers.

We only have to deal with a single page in this function, and its order
is irrelevant.
Reported-by: NWilly Tarreau <w@1wt.eu>
Tested-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -830,8 +830,8 @@ static int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
 	return mss_now;
 }
 
-static ssize_t do_tcp_sendpages(struct sock *sk, struct page **pages, int poffset,
-			 size_t psize, int flags)
+static ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
+				size_t size, int flags)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	int mss_now, size_goal;
@@ -858,12 +858,9 @@ static ssize_t do_tcp_sendpages(struct sock *sk, struct page **pages, int poffse
 	if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
 		goto out_err;
 
-	while (psize > 0) {
+	while (size > 0) {
 		struct sk_buff *skb = tcp_write_queue_tail(sk);
-		struct page *page = pages[poffset / PAGE_SIZE];
 		int copy, i;
-		int offset = poffset % PAGE_SIZE;
-		int size = min_t(size_t, psize, PAGE_SIZE - offset);
 		bool can_coalesce;
 
 		if (!tcp_send_head(sk) || (copy = size_goal - skb->len) <= 0) {
@@ -912,8 +909,8 @@ static ssize_t do_tcp_sendpages(struct sock *sk, struct page **pages, int poffse
 			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;
 
 		copied += copy;
-		poffset += copy;
-		if (!(psize -= copy))
+		offset += copy;
+		if (!(size -= copy))
 			goto out;
 
 		if (skb->len < size_goal || (flags & MSG_OOB))
@@ -960,7 +957,7 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset,
 					flags);
 
 	lock_sock(sk);
-	res = do_tcp_sendpages(sk, &page, offset, size, flags);
+	res = do_tcp_sendpages(sk, page, offset, size, flags);
 	release_sock(sk);
 	return res;
 }