Subject: Re: [PATCH] Crash in tun

From: Max Krasnyansky <[email-masked]>
Date: Thu, 19 Jul 2012 10:44:01 -0700

On 07/19/2012 09:13 AM, Mikulas Patocka wrote:
> 
> 
> On Thu, 19 Jul 2012, Eric Dumazet wrote:
> 
>> Hi Mikulas
>>
>> A fix for this problem is : http://patchwork.ozlabs.org/patch/170440/
> 
> If you call tun_free_netdev beacuse of a jump to an error label 
> err_free_sk, your patch still calls it with NULL file, causing a memory 
> corruption and a possible crash.
> 
> Your patch doesn't fix sockets_in_use underflow.
> 
> Maybe we can commit this patch --- it introduces a new flag 
> SOCK_EXTERNALLY_ALLOCATED to work around both problems. (it looks quite 
> nicer than my previous patch with file = (void *)1).

I definitely like this second version better. Less hacky an all.

btw I don't remember now who added the socket business to tun_struct and why.
It seems to be messy in general. Originally version, back when I was still paying attention to it,
didn't have sockets at all. Only char and net devices. It was much cleaner.

Max

> ---
> 
> 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 <[email-masked]>
> Cc: [email-masked]
> 
> ---
>  drivers/net/tun.c   |    3 +++
>  include/linux/net.h |    1 +
>  net/socket.c        |    3 +++
>  3 files changed, 7 insertions(+)
> 
> Index: linux-3.4.5-fast/drivers/net/tun.c
> ===================================================================
> --- linux-3.4.5-fast.orig/drivers/net/tun.c	2012-07-19 17:55:16.000000000 +0200
> +++ linux-3.4.5-fast/drivers/net/tun.c	2012-07-19 17:58:30.000000000 +0200
> @@ -358,6 +358,8 @@ static void tun_free_netdev(struct net_d
>  {
>  	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,
>  		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);
> Index: linux-3.4.5-fast/include/linux/net.h
> ===================================================================
> --- linux-3.4.5-fast.orig/include/linux/net.h	2012-07-19 17:54:31.000000000 +0200
> +++ linux-3.4.5-fast/include/linux/net.h	2012-07-19 17:55:03.000000000 +0200
> @@ -72,6 +72,7 @@ struct net;
>  #define SOCK_NOSPACE		2
>  #define SOCK_PASSCRED		3
>  #define SOCK_PASSSEC		4
> +#define SOCK_EXTERNALLY_ALLOCATED 5
>  
>  #ifndef ARCH_HAS_SOCKET_TYPES
>  /**
> Index: linux-3.4.5-fast/net/socket.c
> ===================================================================
> --- linux-3.4.5-fast.orig/net/socket.c	2012-07-19 17:56:55.000000000 +0200
> +++ linux-3.4.5-fast/net/socket.c	2012-07-19 17:57:50.000000000 +0200
> @@ -522,6 +522,9 @@ void sock_release(struct socket *sock)
>  	if (rcu_dereference_protected(sock->wq, 1)->fasync_list)
>  		printk(KERN_ERR "sock_release: fasync list not empty!\n");
>  
> +	if (test_bit(SOCK_EXTERNALLY_ALLOCATED, &sock->flags))
> +		return;
> +
>  	percpu_sub(sockets_in_use, 1);
>  	if (!sock->file) {
>  		iput(SOCK_INODE(sock));
>