[PATCH v3] net: make get_net_ns return error if NET_NS is disabled

[Changbin Du <changbin.du@xxxxxxxxx>]

There is a panic in socket ioctl cmd SIOCGSKNS when NET_NS is not enabled.
The reason is that nsfs tries to access ns->ops but the proc_ns_operations
is not implemented in this case.

[7.670023] Unable to handle kernel NULL pointer dereference at virtual address 00000010
[7.670268] pgd = 32b54000
[7.670544] [00000010] *pgd=00000000
[7.671861] Internal error: Oops: 5 [#1] SMP ARM
[7.672315] Modules linked in:
[7.672918] CPU: 0 PID: 1 Comm: systemd Not tainted 5.13.0-rc3-00375-g6799d4f2da49 #16
[7.673309] Hardware name: Generic DT based system
[7.673642] PC is at nsfs_evict+0x24/0x30
[7.674486] LR is at clear_inode+0x20/0x9c

The same to tun SIOCGSKNS command.

To fix this problem, we make get_net_ns() return -EINVAL when NET_NS is
disabled. Meanwhile move it to right place net/core/net_namespace.c.

Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>
Cc: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>

---
Patch "net: make get_net_ns_by_fd inline if NET_NS is disabled" must be
applied first.
---
 include/linux/socket.h      |  2 --
 include/net/net_namespace.h |  6 ++++++
 net/core/net_namespace.c    | 12 ++++++++++++
 net/socket.c                | 13 -------------
 4 files changed, 18 insertions(+), 15 deletions(-)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index b8fc5c53ba6f..0d8e3dcb7f88 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -438,6 +438,4 @@ extern int __sys_socketpair(int family, int type, int protocol,
 			    int __user *usockvec);
 extern int __sys_shutdown_sock(struct socket *sock, int how);
 extern int __sys_shutdown(int fd, int how);
-
-extern struct ns_common *get_net_ns(struct ns_common *ns);
 #endif /* _LINUX_SOCKET_H */
diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 0a25f95691d9..bdc0459a595e 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -185,6 +185,7 @@ void net_ns_get_ownership(const struct net *net, kuid_t *uid, kgid_t *gid);
 
 void net_ns_barrier(void);
 
+struct ns_common *get_net_ns(struct ns_common *ns);
 struct net *get_net_ns_by_fd(int fd);
 #else /* CONFIG_NET_NS */
 #include <linux/sched.h>
@@ -206,6 +207,11 @@ static inline void net_ns_get_ownership(const struct net *net,
 
 static inline void net_ns_barrier(void) {}
 
+static inline struct ns_common *get_net_ns(struct ns_common *ns)
+{
+	return ERR_PTR(-EINVAL);
+}
+
 static inline struct net *get_net_ns_by_fd(int fd)
 {
 	return ERR_PTR(-EINVAL);
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 6a0d9583d69c..9b5a767eddd5 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -641,6 +641,18 @@ void __put_net(struct net *net)
 }
 EXPORT_SYMBOL_GPL(__put_net);
 
+/**
+ * get_net_ns - increment the refcount of the network namespace
+ * @ns: common namespace (net)
+ *
+ * Returns the net's common namespace.
+ */
+struct ns_common *get_net_ns(struct ns_common *ns)
+{
+	return &get_net(container_of(ns, struct net, ns))->ns;
+}
+EXPORT_SYMBOL_GPL(get_net_ns);
+
 struct net *get_net_ns_by_fd(int fd)
 {
 	struct file *file;
diff --git a/net/socket.c b/net/socket.c
index 27e3e7d53f8e..4f2c6d2795d0 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1072,19 +1072,6 @@ static long sock_do_ioctl(struct net *net, struct socket *sock,
  *	what to do with it - that's up to the protocol still.
  */
 
-/**
- *	get_net_ns - increment the refcount of the network namespace
- *	@ns: common namespace (net)
- *
- *	Returns the net's common namespace.
- */
-
-struct ns_common *get_net_ns(struct ns_common *ns)
-{
-	return &get_net(container_of(ns, struct net, ns))->ns;
-}
-EXPORT_SYMBOL_GPL(get_net_ns);
-
 static long sock_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 {
 	struct socket *sock;
-- 
2.30.2