[PATCH v2 net-next 3/7] socket: Restore sock_create_kern().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

Let's restore sock_create_kern() that holds a netns reference.

Now, it's the same as the version before commit 26abe14379f8 ("net:
Modify sk_alloc to not reference count the netns of kernel sockets.").

Back then, after creating a socket in init_net, we used sk_change_net()
to drop the netns ref and switch to another netns, but now we can
simply use __sock_create_kern() instead.

  $ git blame -L:sk_change_net include/net/sock.h 26abe14379f8~

DEBUG_NET_WARN_ON_ONCE() is to catch a path calling sock_create_kern()
from __net_init functions, since doing so would leak the netns as
__net_exit functions cannot run until the socket is removed.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
v2: s/ret/err/ in sock_create_kern() for clarity
---
 include/linux/net.h |  2 ++
 net/socket.c        | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index 12180e00f882..b60e3afab344 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -254,6 +254,8 @@ bool sock_is_registered(int family);
 int sock_create(int family, int type, int proto, struct socket **res);
 int __sock_create_kern(struct net *net, int family, int type, int proto,
 		       struct socket **res);
+int sock_create_kern(struct net *net, int family, int type, int proto,
+		     struct socket **res);
 int sock_create_lite(int family, int type, int proto, struct socket **res);
 struct socket *sock_alloc(void);
 void sock_release(struct socket *sock);
diff --git a/net/socket.c b/net/socket.c
index 7c4474c966c0..9ad352183fae 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1632,6 +1632,48 @@ int __sock_create_kern(struct net *net, int family, int type, int protocol, stru
 }
 EXPORT_SYMBOL(__sock_create_kern);
 
+/**
+ * sock_create_kern - creates a socket for kernel space
+ *
+ * @net: net namespace
+ * @family: protocol family (AF_INET, ...)
+ * @type: communication type (SOCK_STREAM, ...)
+ * @protocol: protocol (0, ...)
+ * @res: new socket
+ *
+ * Creates a new socket and assigns it to @res.
+ *
+ * The socket is for kernel space and should not be exposed to
+ * userspace via a file descriptor nor BPF hooks except for LSM
+ * (see inet_create(), inet_release(), etc).
+ *
+ * The socket bypasses some LSMs that take care of @kern in
+ * security_socket_create() and security_socket_post_create().
+ *
+ * The socket holds a reference count of @net so that the caller
+ * does not need to care about @net's lifetime.
+ *
+ * This MUST NOT be called from the __net_init path and @net MUST
+ * be alive as of calling sock_create_kern().
+ *
+ * Context: Process context. This function internally uses GFP_KERNEL.
+ * Return: 0 or an error.
+ */
+int sock_create_kern(struct net *net, int family, int type, int protocol,
+		     struct socket **res)
+{
+	int err;
+
+	DEBUG_NET_WARN_ON_ONCE(!net_initialized(net));
+
+	err = __sock_create(net, family, type, protocol, res, 1);
+	if (!err)
+		sk_net_refcnt_upgrade((*res)->sk);
+
+	return err;
+}
+EXPORT_SYMBOL(sock_create_kern);
+
 static struct socket *__sys_socket_create(int family, int type, int protocol)
 {
 	struct socket *sock;
-- 
2.49.0