[PATCH v2] net: do not invoke open_related_ns when NET_NS is disabled

[Changbin Du <changbin.du@gmail.com>]

When NET_NS is not enabled, socket ioctl cmd SIOCGSKNS should do nothing
but acknowledge userspace it is not supported. Otherwise, kernel would
panic wherever nsfs trys to access ns->ops since 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.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
---
 drivers/net/tun.c | 4 ++++
 net/socket.c      | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 84f832806313..8ec5977d2f34 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3003,9 +3003,13 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 	} else if (cmd == TUNSETQUEUE) {
 		return tun_set_queue(file, &ifr);
 	} else if (cmd == SIOCGSKNS) {
+#ifdef CONFIG_NET_NS
 		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
 			return -EPERM;
 		return open_related_ns(&net->ns, get_net_ns);
+#else
+		return -EOPNOTSUPP;
+#endif
 	}
 
 	rtnl_lock();
diff --git a/net/socket.c b/net/socket.c
index 27e3e7d53f8e..bc644030d2e8 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1149,11 +1149,15 @@ static long sock_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 			mutex_unlock(&vlan_ioctl_mutex);
 			break;
 		case SIOCGSKNS:
+#ifdef CONFIG_NET_NS
 			err = -EPERM;
 			if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
 				break;
 
 			err = open_related_ns(&net->ns, get_net_ns);
+#else
+			err = -EOPNOTSUPP;
+#endif
 			break;
 		case SIOCGSTAMP_OLD:
 		case SIOCGSTAMPNS_OLD:
-- 
2.30.2