[NET]: Fix module reference counts for loadable protocol modules

I have been experimenting with loadable protocol modules, and ran into
several issues with module reference counting.

The first issue was that __module_get failed at the BUG_ON check at
the top of the routine (checking that my module reference count was
not zero) when I created the first socket. When sk_alloc() is called,
my module reference count was still 0. When I looked at why sctp
didn't have this problem, I discovered that sctp creates a control
socket during module init (when the module ref count is not 0), which
keeps the reference count non-zero. This section has been updated to
address the point Stephen raised about checking the return value of
try_module_get().

The next problem arose when my socket init routine returned an error.
This resulted in my module reference count being decremented below 0.
My socket ops->release routine was also being called. The issue here
is that sock_release() calls the ops->release routine and decrements
the ref count if sock->ops is not NULL. Since the socket probably
didn't get correctly initialized, this should not be done, so we will
set sock->ops to NULL because we will not call try_module_get().

While searching for another bug, I also noticed that sys_accept() has
a possibility of doing a module_put() when it did not do an
__module_get so I re-ordered the call to security_socket_accept().

Signed-off-by: Frank Filz <ffilzlnx@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 8 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index ac63b56e23b2..928d2a1d6d8e 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -660,16 +660,20 @@ struct sock *sk_alloc(int family, unsigned int __nocast priority,
                         sock_lock_init(sk);
                 }
                 
-                if (security_sk_alloc(sk, family, priority)) {
-                        if (slab != NULL)
-                                kmem_cache_free(slab, sk);
-                        else
-                                kfree(sk);
-                        sk = NULL;
-                } else
-                        __module_get(prot->owner);
+                if (security_sk_alloc(sk, family, priority))
+                        goto out_free;
+
+                if (!try_module_get(prot->owner))
+                        goto out_free;
         }
         return sk;
+
+out_free:
+        if (slab != NULL)
+                kmem_cache_free(slab, sk);
+        else
+                kfree(sk);
+        return NULL;
 }
 
 void sk_free(struct sock *sk)