Subject: Re: [PATCH] tun: Fix/rewrite packet filtering logic

From: Jeff Garzik <[email-masked]>
Date: Tue, 22 Jul 2008 19:41:47 -0400

Max Krasnyansky wrote:
> Please see the following thread to get some context on this
> 	http://marc.info/?l=linux-netdev&m=121564433018903&w=2
> 
> Basically the issue is that current multi-cast filtering stuff in
> the TUN/TAP driver is seriously broken.
> Original patch went in without proper review and ACK. It was broken and
> confusing to start with and subsequent patches broke it completely.
> To give you an idea of what's broken here are some of the issues:
> 
> - Very confusing comments throughout the code that imply that the
> character device is a network interface in its own right, and that packets
> are passed between the two nics. Which is completely wrong.
> 
> - Wrong set of ioctls is used for setting up filters. They look like
> shortcuts for manipulating state of the tun/tap network interface but
> in reality manipulate the state of the TX filter.
> 
> - ioctls that were originally used for setting address of the the TX filter
> got "fixed" and now set the address of the network interface itself. Which
> made filter totaly useless.
> 
> - Filtering is done too late. Instead of filtering early on, to avoid
> unnecessary wakeups, filtering is done in the read() call.
> 
> The list goes on and on :)
> 
> So the patch cleans all that up. It introduces simple and clean interface for
> setting up TX filters (TUNSETTXFILTER + tun_filter spec) and does filtering
> before enqueuing the packets.
> 
> TX filtering is useful in the scenarios where TAP is part of a bridge, in
> which case it gets all broadcast, multicast and potentially other packets when
> the bridge is learning. So for example Ethernet tunnelling app may want to
> setup TX filters to avoid tunnelling multicast traffic. QEMU and other
> hypervisors can push RX filtering that is currently done in the guest into the
> host context therefore saving wakeups and unnecessary data transfer.
> 
> This is on top of the latest and greatest :). Assuming virt folks are ok with
> the API this should go into 2.6.27.
> 
> Signed-off-by: Max Krasnyansky <[email-masked]>
> ---
>  drivers/net/tun.c      |  316 +++++++++++++++++++++++-------------------------
>  include/linux/if_tun.h |   24 +++-
>  2 files changed, 174 insertions(+), 166 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 2693f88..901551c 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -18,15 +18,11 @@
>  /*
>   *  Changes:
>   *
> - *  Brian Braunstein <[email-masked]> 2007/03/23
> - *    Fixed hw address handling.  Now net_device.dev_addr is kept consistent
> - *    with tun.dev_addr when the address is set by this module.
> - *
>   *  Mike Kershaw <[email-masked]> 2005/08/14
>   *    Add TUNSETLINK ioctl to set the link encapsulation
>   *
>   *  Mark Smith <[email-masked]>
> - *   Use random_ether_addr() for tap MAC address.
> + *    Use random_ether_addr() for tap MAC address.
>   *
>   *  Harald Roelle <[email-masked]>  2004/04/20
>   *    Fixes in packet dropping, queue length setting and queue wakeup.
> @@ -83,9 +79,16 @@ static int debug;
>  #define DBG1( a... )
>  #endif
>  
> +#define FLT_EXACT_COUNT 8
> +struct tap_filter {
> +	unsigned int    count;    /* Number of addrs. Zero means disabled */
> +	u32             mask[2];  /* Mask of the hashed addrs */
> +	unsigned char	addr[FLT_EXACT_COUNT][ETH_ALEN];
> +};
> +
>  struct tun_struct {
>  	struct list_head        list;
> -	unsigned long 		flags;
> +	unsigned int 		flags;
>  	int			attached;
>  	uid_t			owner;
>  	gid_t			group;
> @@ -94,19 +97,119 @@ struct tun_struct {
>  	struct sk_buff_head	readq;
>  
>  	struct net_device	*dev;
> +	struct fasync_struct	*fasync;
>  
> -	struct fasync_struct    *fasync;
> -
> -	unsigned long if_flags;
> -	u8 dev_addr[ETH_ALEN];
> -	u32 chr_filter[2];
> -	u32 net_filter[2];
> +	struct tap_filter       txflt;
>  
>  #ifdef TUN_DEBUG
>  	int debug;
>  #endif
>  };
>  
> +/* TAP filterting */
> +static void addr_hash_set(u32 *mask, const u8 *addr)
> +{
> +	int n = ether_crc(ETH_ALEN, addr) >> 26;
> +	mask[n >> 5] |= (1 << (n & 31));
> +}
> +
> +static unsigned int addr_hash_test(const u32 *mask, const u8 *addr)
> +{
> +	int n = ether_crc(ETH_ALEN, addr) >> 26;
> +	return mask[n >> 5] & (1 << (n & 31));
> +}
> +
> +static int update_filter(struct tap_filter *filter, void __user *arg)
> +{
> +	struct { u8 u[ETH_ALEN]; } *addr;
> +	struct tun_filter uf;
> +	int err, alen, n, nexact;
> +
> +	if (copy_from_user(&uf, arg, sizeof(uf)))
> +		return -EFAULT;
> +
> +	if (!uf.count) {
> +		/* Disabled */
> +		filter->count = 0;
> +		return 0;
> +	}
> +
> +	alen = ETH_ALEN * uf.count;
> +	addr = kmalloc(alen, GFP_KERNEL);
> +	if (!addr)
> +		return -ENOMEM;
> +
> +	if (copy_from_user(addr, arg + sizeof(uf), alen)) {
> +		err = -EFAULT;
> +		goto done;
> +	}
> +
> +	/* The filter is updated without holding any locks. Which is
> +	 * perfectly safe. We disable it first and in the worst
> +	 * case we'll accept a few undesired packets. */
> +	filter->count = 0;
> +	wmb();
> +
> +	/* Use first set of addresses as an exact filter */
> +	for (n = 0; n < uf.count && n < FLT_EXACT_COUNT; n++)
> +		memcpy(filter->addr[n], addr[n].u, ETH_ALEN);
> +
> +	nexact = n;
> +
> +	/* The rest is hashed */
> +	memset(filter->mask, 0, sizeof(filter->mask));
> +	for (; n < uf.count; n++)
> +		addr_hash_set(filter->mask, addr[n].u);
> +
> +	/* For ALLMULTI just set the mask to all ones.
> +	 * This overrides the mask populated above. */
> +	if ((uf.flags & TUN_FLT_ALLMULTI))
> +		memset(filter->mask, ~0, sizeof(filter->mask));
> +
> +	/* Now enable the filter */
> +	wmb();
> +	filter->count = nexact;
> +
> +	/* Return the number of exact filters */
> +	err = nexact;
> +
> +done:
> +	kfree(addr);
> +	return err;
> +}
> +
> +/* Returns: 0 - drop, !=0 - accept */
> +static int run_filter(struct tap_filter *filter, const struct sk_buff *skb)
> +{
> +	/* Cannot use eth_hdr(skb) here because skb_mac_hdr() is incorrect
> +	 * at this point. */
> +	struct ethhdr *eh = (struct ethhdr *) skb->data;
> +	int i;
> +
> +	/* Exact match */
> +	for (i = 0; i < filter->count; i++)
> +		if (!compare_ether_addr(eh->h_dest, filter->addr[i]))
> +			return 1;
> +
> +	/* Inexact match (multicast only) */
> +	if (is_multicast_ether_addr(eh->h_dest))
> +		return addr_hash_test(filter->mask, eh->h_dest);
> +
> +	return 0;
> +}
> +
> +/*
> + * Checks whether the packet is accepted or not.
> + * Returns: 0 - drop, !=0 - accept
> + */
> +static int check_filter(struct tap_filter *filter, const struct sk_buff *skb)
> +{
> +	if (!filter->count)
> +		return 1;
> +
> +	return run_filter(filter, skb);
> +}
> +
>  /* Network device part of the driver */
>  
>  static unsigned int tun_net_id;
> @@ -141,7 +244,12 @@ static int tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	if (!tun->attached)
>  		goto drop;
>  
> -	/* Packet dropping */
> +	/* Drop if the filter does not like it.
> +	 * This is a noop if the filter is disabled.
> +	 * Filter can be enabled only for the TAP devices. */
> +	if (!check_filter(&tun->txflt, skb))
> +		goto drop;
> +
>  	if (skb_queue_len(&tun->readq) >= dev->tx_queue_len) {
>  		if (!(tun->flags & TUN_ONE_QUEUE)) {
>  			/* Normal queueing mode. */
> @@ -158,7 +266,7 @@ static int tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  		}
>  	}
>  
> -	/* Queue packet */
> +	/* Enqueue packet */
>  	skb_queue_tail(&tun->readq, skb);
>  	dev->trans_start = jiffies;
>  
> @@ -174,41 +282,14 @@ drop:
>  	return 0;
>  }
>  
> -/** Add the specified Ethernet address to this multicast filter. */
> -static void
> -add_multi(u32* filter, const u8* addr)
> -{
> -	int bit_nr = ether_crc(ETH_ALEN, addr) >> 26;
> -	filter[bit_nr >> 5] |= 1 << (bit_nr & 31);
> -}
> -
> -/** Remove the specified Ethernet addres from this multicast filter. */
> -static void
> -del_multi(u32* filter, const u8* addr)
> +static void tun_net_mclist(struct net_device *dev)
>  {
> -	int bit_nr = ether_crc(ETH_ALEN, addr) >> 26;
> -	filter[bit_nr >> 5] &= ~(1 << (bit_nr & 31));
> -}
> -
> -/** Update the list of multicast groups to which the network device belongs.
> - * This list is used to filter packets being sent from the character device to
> - * the network device. */
> -static void
> -tun_net_mclist(struct net_device *dev)
> -{
> -	struct tun_struct *tun = netdev_priv(dev);
> -	const struct dev_mc_list *mclist;
> -	int i;
> -	DECLARE_MAC_BUF(mac);
> -	DBG(KERN_DEBUG "%s: tun_net_mclist: mc_count %d\n",
> -			dev->name, dev->mc_count);
> -	memset(tun->chr_filter, 0, sizeof tun->chr_filter);
> -	for (i = 0, mclist = dev->mc_list; i < dev->mc_count && mclist != NULL;
> -			i++, mclist = mclist->next) {
> -		add_multi(tun->net_filter, mclist->dmi_addr);
> -		DBG(KERN_DEBUG "%s: tun_net_mclist: %s\n",
> -		    dev->name, print_mac(mac, mclist->dmi_addr));
> -	}
> +	/*
> +	 * This callback is supposed to deal with mc filter in
> +	 * _rx_ path and has nothing to do with the _tx_ path.
> +	 * In rx path we always accept everything userspace gives us.
> +	 */
> +	return;
>  }
>  
>  #define MIN_MTU 68
> @@ -244,13 +325,11 @@ static void tun_net_init(struct net_device *dev)
>  
>  	case TUN_TAP_DEV:
>  		/* Ethernet TAP Device */
> -		dev->set_multicast_list = tun_net_mclist;
> -
>  		ether_setup(dev);
> -		dev->change_mtu = tun_net_change_mtu;
> +		dev->change_mtu         = tun_net_change_mtu;
> +		dev->set_multicast_list = tun_net_mclist;
>  
> -		/* random address already created for us by tun_set_iff, use it */
> -		memcpy(dev->dev_addr, tun->dev_addr, min(sizeof(tun->dev_addr), sizeof(dev->dev_addr)) );
> +		random_ether_addr(dev->dev_addr);
>  
>  		dev->tx_queue_len = TUN_READQ_SIZE;  /* We prefer our own queue length */
>  		break;
> @@ -486,7 +565,6 @@ static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
>  	DECLARE_WAITQUEUE(wait, current);
>  	struct sk_buff *skb;
>  	ssize_t len, ret = 0;
> -	DECLARE_MAC_BUF(mac);
>  
>  	if (!tun)
>  		return -EBADFD;
> @@ -499,10 +577,6 @@ static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
>  
>  	add_wait_queue(&tun->read_wait, &wait);
>  	while (len) {
> -		const u8 ones[ ETH_ALEN] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
> -		u8 addr[ ETH_ALEN];
> -		int bit_nr;
> -
>  		current->state = TASK_INTERRUPTIBLE;
>  
>  		/* Read frames from the queue */
> @@ -522,36 +596,9 @@ static ssize_t tun_chr_aio_read(struct kiocb *iocb, const struct iovec *iv,
>  		}
>  		netif_wake_queue(tun->dev);
>  
> -		/** Decide whether to accept this packet. This code is designed to
> -		 * behave identically to an Ethernet interface. Accept the packet if
> -		 * - we are promiscuous.
> -		 * - the packet is addressed to us.
> -		 * - the packet is broadcast.
> -		 * - the packet is multicast and
> -		 *   - we are multicast promiscous.
> -		 *   - we belong to the multicast group.
> -		 */
> -		skb_copy_from_linear_data(skb, addr, min_t(size_t, sizeof addr,
> -								   skb->len));
> -		bit_nr = ether_crc(sizeof addr, addr) >> 26;
> -		if ((tun->if_flags & IFF_PROMISC) ||
> -				memcmp(addr, tun->dev_addr, sizeof addr) == 0 ||
> -				memcmp(addr, ones, sizeof addr) == 0 ||
> -				(((addr[0] == 1 && addr[1] == 0 && addr[2] == 0x5e) ||
> -				  (addr[0] == 0x33 && addr[1] == 0x33)) &&
> -				 ((tun->if_flags & IFF_ALLMULTI) ||
> -				  (tun->chr_filter[bit_nr >> 5] & (1 << (bit_nr & 31)))))) {
> -			DBG(KERN_DEBUG "%s: tun_chr_readv: accepted: %s\n",
> -					tun->dev->name, print_mac(mac, addr));
> -			ret = tun_put_user(tun, skb, (struct iovec *) iv, len);
> -			kfree_skb(skb);
> -			break;
> -		} else {
> -			DBG(KERN_DEBUG "%s: tun_chr_readv: rejected: %s\n",
> -					tun->dev->name, print_mac(mac, addr));
> -			kfree_skb(skb);
> -			continue;
> -		}
> +		ret = tun_put_user(tun, skb, (struct iovec *) iv, len);
> +		kfree_skb(skb);
> +		break;
>  	}
>  
>  	current->state = TASK_RUNNING;
> @@ -647,12 +694,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  		tun = netdev_priv(dev);
>  		tun->dev = dev;
>  		tun->flags = flags;
> -		/* Be promiscuous by default to maintain previous behaviour. */
> -		tun->if_flags = IFF_PROMISC;
> -		/* Generate random Ethernet address. */
> -		*(__be16 *)tun->dev_addr = htons(0x00FF);
> -		get_random_bytes(tun->dev_addr + sizeof(u16), 4);
> -		memset(tun->chr_filter, 0, sizeof tun->chr_filter);
> +		tun->txflt.count = 0;
>  
>  		tun_net_init(dev);
>  
> @@ -751,6 +793,7 @@ static int tun_chr_ioctl(struct inode *inode, struct file *file,
>  	struct tun_struct *tun = file->private_data;
>  	void __user* argp = (void __user*)arg;
>  	struct ifreq ifr;
> +	int ret;
>  	DECLARE_MAC_BUF(mac);
>  
>  	if (cmd == TUNSETIFF || _IOC_TYPE(cmd) == 0x89)
> @@ -826,9 +869,6 @@ static int tun_chr_ioctl(struct inode *inode, struct file *file,
>  		break;
>  
>  	case TUNSETLINK:
> -	{
> -		int ret;
> -
>  		/* Only allow setting the type when the interface is down */
>  		rtnl_lock();
>  		if (tun->dev->flags & IFF_UP) {
> @@ -842,94 +882,44 @@ static int tun_chr_ioctl(struct inode *inode, struct file *file,
>  		}
>  		rtnl_unlock();
>  		return ret;
> -	}
>  
>  #ifdef TUN_DEBUG
>  	case TUNSETDEBUG:
>  		tun->debug = arg;
>  		break;
>  #endif
> -
>  	case TUNSETOFFLOAD:
> -	{
> -		int ret;
>  		rtnl_lock();
>  		ret = set_offload(tun->dev, arg);
>  		rtnl_unlock();
>  		return ret;
> -	}
>  
> -	case SIOCGIFFLAGS:
> -		ifr.ifr_flags = tun->if_flags;
> -		if (copy_to_user( argp, &ifr, sizeof ifr))
> -			return -EFAULT;
> -		return 0;
> -
> -	case SIOCSIFFLAGS:
> -		/** Set the character device's interface flags. Currently only
> -		 * IFF_PROMISC and IFF_ALLMULTI are used. */
> -		tun->if_flags = ifr.ifr_flags;
> -		DBG(KERN_INFO "%s: interface flags 0x%lx\n",
> -				tun->dev->name, tun->if_flags);
> -		return 0;
> +	case TUNSETTXFILTER:
> +		/* Can be set only for TAPs */
> +		if ((tun->flags & TUN_TYPE_MASK) != TUN_TAP_DEV)
> +			return -EINVAL;
> +		rtnl_lock();
> +		ret = update_filter(&tun->txflt, (void *) __user arg);

looks mostly OK, but stuff like the above should be

	(void __user *) arg

Did you check this with sparse (Documentation/sparse.txt)?

	Jeff