tun: fix a crash bug and a memory leak

This patch fixes a crash
tun_chr_close -> netdev_run_todo -> tun_free_netdev -> sk_release_kernel ->
sock_release -> iput(SOCK_INODE(sock))
introduced by commit 1ab5ecb90cb6a3df1476e052f76a6e8f6511cb3d

The problem is that this socket is embedded in struct tun_struct, it has
no inode, iput is called on invalid inode, which modifies invalid memory
and optionally causes a crash.

sock_release also decrements sockets_in_use, this causes a bug that
"sockets: used" field in /proc/*/net/sockstat keeps on decreasing when
creating and closing tun devices.

This patch introduces a flag SOCK_EXTERNALLY_ALLOCATED that instructs
sock_release to not free the inode and not decrement sockets_in_use,
fixing both memory corruption and sockets_in_use underflow.

It should be backported to 3.3 an 3.4 stabke.

Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 961fad1f7053..f3a454c3295a 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -358,6 +358,8 @@ static void tun_free_netdev(struct net_device *dev)
 {
         struct tun_struct *tun = netdev_priv(dev);
 
+        BUG_ON(!test_bit(SOCK_EXTERNALLY_ALLOCATED, &tun->socket.flags));
+
         sk_release_kernel(tun->socket.sk);
 }
 
@@ -1115,6 +1117,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
                 tun->flags = flags;
                 tun->txflt.count = 0;
                 tun->vnet_hdr_sz = sizeof(struct virtio_net_hdr);
+                set_bit(SOCK_EXTERNALLY_ALLOCATED, &tun->socket.flags);
 
                 err = -ENOMEM;
                 sk = sk_alloc(&init_net, AF_UNSPEC, GFP_KERNEL, &tun_proto);