[PATCH 4.9 059/101] tcp: fix over estimation in sk_forced_mem_schedule()

[Greg Kroah-Hartman]

From: Eric Dumazet <edumazet@xxxxxxxxxx>

commit c4ee118561a0f74442439b7b5b486db1ac1ddfeb upstream.

sk_forced_mem_schedule() has a bug similar to ones fixed
in commit 7c80b038d23e ("net: fix sk_wmem_schedule() and
sk_rmem_schedule() errors")

While this bug has little chance to trigger in old kernels,
we need to fix it before the following patch.

Fixes: d83769a580f1 ("tcp: fix possible deadlock in tcp_send_fin()")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reviewed-by: Wei Wang <weiwan@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_output.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2986,11 +2986,12 @@ begin_fwd:
  */
 void sk_forced_mem_schedule(struct sock *sk, int size)
 {
-	int amt;
+	int delta, amt;
 
-	if (size <= sk->sk_forward_alloc)
+	delta = size - sk->sk_forward_alloc;
+	if (delta <= 0)
 		return;
-	amt = sk_mem_pages(size);
+	amt = sk_mem_pages(delta);
 	sk->sk_forward_alloc += amt * SK_MEM_QUANTUM;
 	sk_memory_allocated_add(sk, amt);