[PATCH net-next 2/2] tun: Add ioctl() SIOCGSKNS cmd to allow obtaining net ns of tun device

[Kirill Tkhai]

This patch adds possibility to get tun device's net namespace fd
in the same way we allow to do that for sockets.

Socket ioctl numbers do not intersect with tun-specific, and there
is already SIOCSIFHWADDR used in tun code. So, SIOCGSKNS number
is choosen instead of custom-made for this functionality.

Note, that open_related_ns() uses plain get_net_ns() and it's safe
(net can't be already dead at this moment):

  tun socket is allocated via sk_alloc() with zero last arg (kern = 0).
  So, each alive socket increments net::count, and the socket is definitely
  alive during ioctl syscall.

Also, common variable net is introduced, so small cleanup in TUNSETIFF
is made.

Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
---
 drivers/net/tun.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 81e6cc951e7f..8e9a0ac644d2 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -78,6 +78,7 @@
 #include <linux/mutex.h>
 
 #include <linux/uaccess.h>
+#include <linux/proc_fs.h>
 
 /* Uncomment to enable debugging */
 /* #define TUN_DEBUG 1 */
@@ -2793,6 +2794,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
        struct tun_struct *tun;
        void __user* argp = (void __user*)arg;
        struct ifreq ifr;
+       struct net *net;
        kuid_t owner;
        kgid_t group;
        int sndbuf;
@@ -2801,7 +2803,8 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
        int le;
        int ret;
 
-       if (cmd == TUNSETIFF || cmd == TUNSETQUEUE || _IOC_TYPE(cmd) == 
SOCK_IOC_TYPE) {
+       if (cmd == TUNSETIFF || cmd == TUNSETQUEUE ||
+           (_IOC_TYPE(cmd) == SOCK_IOC_TYPE && cmd != SIOCGSKNS)) {
                if (copy_from_user(&ifr, argp, ifreq_len))
                        return -EFAULT;
        } else {
@@ -2821,6 +2824,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
        rtnl_lock();
 
        tun = tun_get(tfile);
+       net = sock_net(&tfile->sk);
        if (cmd == TUNSETIFF) {
                ret = -EEXIST;
                if (tun)
@@ -2828,7 +2832,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
 
                ifr.ifr_name[IFNAMSIZ-1] = '\0';
 
-               ret = tun_set_iff(sock_net(&tfile->sk), file, &ifr);
+               ret = tun_set_iff(net, file, &ifr);
 
                if (ret)
                        goto unlock;
@@ -2850,6 +2854,14 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
                tfile->ifindex = ifindex;
                goto unlock;
        }
+       if (cmd == SIOCGSKNS) {
+               ret = -EPERM;
+               if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+                       goto unlock;
+
+               ret = open_related_ns(&net->ns, get_net_ns);
+               goto unlock;
+       }
 
        ret = -EBADFD;
        if (!tun)