tun/tap: Fix crashes if open() /dev/net/tun and then poll() it.

Fix NULL pointer dereference in tun_chr_pool() introduced by commit
33dccbb050bbe35b88ca8cf1228dcf3e4d4b3554 ("tun: Limit amount of queued
packets per device") and triggered by this code:

	int fd;
	struct pollfd pfd;
	fd = open("/dev/net/tun", O_RDWR);
	pfd.fd = fd;
	pfd.events = POLLIN | POLLOUT;
	poll(&pfd, 1, 0);

Reported-by: Eugene Kapun <abacabadabacaba@gmail.com>
Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index b393536012f..027f7aba26a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -486,12 +486,14 @@ static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
 {
         struct tun_file *tfile = file->private_data;
         struct tun_struct *tun = __tun_get(tfile);
-        struct sock *sk = tun->sk;
+        struct sock *sk;
         unsigned int mask = 0;
 
         if (!tun)
                 return POLLERR;
 
+        sk = tun->sk;
+
         DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name);
 
         poll_wait(file, &tun->socket.wait, wait);