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>

1 files changed, 12 insertions, 7 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 77fbfed332e..7626b6aacd6 100644
--- 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);