[PATCH 1/2] tun: Add ioctl() TUNGETDEVNETNS cmd to allow obtaining real net ns of tun device

[Kirill Tkhai <ktkhai@virtuozzo.com>]

In commit f2780d6d7475 "tun: Add ioctl() SIOCGSKNS cmd to allow
obtaining net ns of tun device" it was missed that tun may change
its net ns, while net ns of socket remains the same as it was
created initially. SIOCGSKNS returns net ns of socket, so it is
not suitable for obtaining net ns of device.

We may have two tun devices with the same names in two net ns,
and in this case it's not possible to determ, which of them
fd refers to (TUNGETIFF will return the same name).

This patch adds new ioctl() cmd for obtaining net ns of a device.

Reported-by: Harald Albrecht <harald.albrecht@gmx.net>
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 drivers/net/tun.c           |    8 ++++++++
 include/uapi/linux/if_tun.h |    1 +
 2 files changed, 9 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index e9ca1c088d0b..b7137edff624 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3103,6 +3103,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 
 	tun_debug(KERN_INFO, tun, "tun_chr_ioctl cmd %u\n", cmd);
 
+	net = dev_net(tun->dev);
 	ret = 0;
 	switch (cmd) {
 	case TUNGETIFF:
@@ -3328,6 +3329,13 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 		ret = tun_net_change_carrier(tun->dev, (bool)carrier);
 		break;
 
+	case TUNGETDEVNETNS:
+		ret = -EPERM;
+		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+			goto unlock;
+		ret = open_related_ns(&net->ns, get_net_ns);
+		break;
+
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
index 23a6753b37df..454ae31b93c7 100644
--- a/include/uapi/linux/if_tun.h
+++ b/include/uapi/linux/if_tun.h
@@ -60,6 +60,7 @@
 #define TUNSETSTEERINGEBPF _IOR('T', 224, int)
 #define TUNSETFILTEREBPF _IOR('T', 225, int)
 #define TUNSETCARRIER _IOW('T', 226, int)
+#define TUNGETDEVNETNS _IO('T', 227)
 
 /* TUNSETIFF ifr flags */
 #define IFF_TUN		0x0001