net: Fix sock_wfree() race

Commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
(net: No more expensive sock_hold()/sock_put() on each tx)
opens a window in sock_wfree() where another cpu
might free the socket we are working on.

A fix is to call sk->sk_write_space(sk) while still
holding a reference on sk.

Reported-by: Jike Song <albcamus@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/sock.c b/net/core/sock.c
index 77fbfed..7626b6a 100644 (file)
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1228,17 +1228,22 @@ void __init sk_init(void)
 void sock_wfree(struct sk_buff *skb)
 {
        struct sock *sk = skb->sk;
-       int res;
+       unsigned int len = skb->truesize;
 
-       /* In case it might be waiting for more memory. */
-       res = atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc);
-       if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE))
+       if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE)) {
+               /*
+                * Keep a reference on sk_wmem_alloc, this will be released
+                * after sk_write_space() call
+                */
+               atomic_sub(len - 1, &sk->sk_wmem_alloc);
                sk->sk_write_space(sk);
+               len = 1;
+       }
        /*
-        * if sk_wmem_alloc reached 0, we are last user and should
-        * free this sock, as sk_free() call could not do it.
+        * if sk_wmem_alloc reaches 0, we must finish what sk_free()
+        * could not do because of in-flight packets
         */
-       if (res == 0)
+       if (atomic_sub_and_test(len, &sk->sk_wmem_alloc))
                __sk_free(sk);
 }
 EXPORT_SYMBOL(sock_wfree);