[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, 8 insertions, 5 deletions

diff --git a/net/socket.c b/net/socket.c
index dbd1a6851edd..3145103cdf54 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1145,8 +1145,11 @@ static int __sock_create(int family, int type, int protocol, struct socket **res
         if (!try_module_get(net_families[family]->owner))
                 goto out_release;
 
-        if ((err = net_families[family]->create(sock, protocol)) < 0)
+        if ((err = net_families[family]->create(sock, protocol)) < 0) {
+                sock->ops = NULL;
                 goto out_module_put;
+        }
+
         /*
          * Now to bump the refcnt of the [loadable] module that owns this
          * socket at sock_release time we decrement its refcnt.
@@ -1360,16 +1363,16 @@ asmlinkage long sys_accept(int fd, struct sockaddr __user *upeer_sockaddr, int _
         newsock->type = sock->type;
         newsock->ops = sock->ops;
 
-        err = security_socket_accept(sock, newsock);
-        if (err)
-                goto out_release;
-
         /*
          * We don't need try_module_get here, as the listening socket (sock)
          * has the protocol module (sock->ops->owner) held.
          */
         __module_get(newsock->ops->owner);
 
+        err = security_socket_accept(sock, newsock);
+        if (err)
+                goto out_release;
+
         err = sock->ops->accept(sock, newsock, sock->file->f_flags);
         if (err < 0)
                 goto out_release;