Re: [PATCH v3 4/5] net: tun: fix tun_xdp_one() for IFF_TUN mode

[Jason Wang <jasowang@redhat.com>]

在 2021/6/24 下午8:30, David Woodhouse 写道:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> In tun_get_user(), skb->protocol is either taken from the tun_pi header
> or inferred from the first byte of the packet in IFF_TUN mode, while
> eth_type_trans() is called only in the IFF_TAP mode where the payload
> is expected to be an Ethernet frame.
>
> The equivalent code path in tun_xdp_one() was unconditionally using
> eth_type_trans(), which is the wrong thing to do in IFF_TUN mode and
> corrupts packets.
>
> Pull the logic out to a separate tun_skb_set_protocol() function, and
> call it from both tun_get_user() and tun_xdp_one().
>
> XX: It is not entirely clear to me why it's OK to call eth_type_trans()
> in some cases without first checking that enough of the Ethernet header
> is linearly present by calling pskb_may_pull().


Looks like a bug.


>   Such a check was never
> present in the tun_xdp_one() code path, and commit 96aa1b22bd6bb ("tun:
> correct header offsets in napi frags mode") deliberately added it *only*
> for the IFF_NAPI_FRAGS mode.


We had already checked this in tun_get_user() before:

         if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP) {
                 align += NET_IP_ALIGN;
                 if (unlikely(len < ETH_HLEN ||
                              (gso.hdr_len && tun16_to_cpu(tun, 
gso.hdr_len) < ETH_HLEN)))
                         return -EINVAL;
         }


>
> I would like to see specific explanations of *why* it's ever valid and
> necessary (is it so much faster?) to skip the pskb_may_pull() by setting
> the 'no_pull_check' flag to tun_skb_set_protocol(), but for now I'll
> settle for faithfully preserving the existing behaviour and pretending
> it's someone else's problem.
>
> Cc: Willem de Bruijn <willemb@google.com>
> Fixes: 043d222f93ab ("tuntap: accept an array of XDP buffs through sendmsg()")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>


Thanks


> ---
>   drivers/net/tun.c | 95 +++++++++++++++++++++++++++++++----------------
>   1 file changed, 63 insertions(+), 32 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 1b553f79adb0..9379fa86fae9 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1641,6 +1641,47 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>   	return NULL;
>   }
>   
> +static int tun_skb_set_protocol(struct tun_struct *tun, struct sk_buff *skb,
> +				__be16 pi_proto, bool no_pull_check)
> +{
> +	switch (tun->flags & TUN_TYPE_MASK) {
> +	case IFF_TUN:
> +		if (tun->flags & IFF_NO_PI) {
> +			u8 ip_version = skb->len ? (skb->data[0] >> 4) : 0;
> +
> +			switch (ip_version) {
> +			case 4:
> +				pi_proto = htons(ETH_P_IP);
> +				break;
> +			case 6:
> +				pi_proto = htons(ETH_P_IPV6);
> +				break;
> +			default:
> +				return -EINVAL;
> +			}
> +		}
> +
> +		skb_reset_mac_header(skb);
> +		skb->protocol = pi_proto;
> +		skb->dev = tun->dev;
> +		break;
> +	case IFF_TAP:
> +		/* As an optimisation, no_pull_check can be set in the cases
> +		 * where the caller *knows* that eth_type_trans() will be OK
> +		 * because the Ethernet header is linearised at skb->data.
> +		 *
> +		 * XX: Or so I was reliably assured when I moved this code
> +		 * and didn't make it unconditional. dwmw2.
> +		 */
> +		if (!no_pull_check && !pskb_may_pull(skb, ETH_HLEN))
> +			return -ENOMEM;
> +
> +		skb->protocol = eth_type_trans(skb, tun->dev);
> +		break;
> +	}
> +	return 0;
> +}
> +
>   /* Get packet from user space buffer */
>   static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>   			    void *msg_control, struct iov_iter *from,
> @@ -1784,37 +1825,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>   		return -EINVAL;
>   	}
>   
> -	switch (tun->flags & TUN_TYPE_MASK) {
> -	case IFF_TUN:
> -		if (tun->flags & IFF_NO_PI) {
> -			u8 ip_version = skb->len ? (skb->data[0] >> 4) : 0;
> -
> -			switch (ip_version) {
> -			case 4:
> -				pi.proto = htons(ETH_P_IP);
> -				break;
> -			case 6:
> -				pi.proto = htons(ETH_P_IPV6);
> -				break;
> -			default:
> -				atomic_long_inc(&tun->dev->rx_dropped);
> -				kfree_skb(skb);
> -				return -EINVAL;
> -			}
> -		}
> -
> -		skb_reset_mac_header(skb);
> -		skb->protocol = pi.proto;
> -		skb->dev = tun->dev;
> -		break;
> -	case IFF_TAP:
> -		if (frags && !pskb_may_pull(skb, ETH_HLEN)) {
> -			err = -ENOMEM;
> -			goto drop;
> -		}
> -		skb->protocol = eth_type_trans(skb, tun->dev);
> -		break;
> -	}
> +	err = tun_skb_set_protocol(tun, skb, pi.proto, !frags);
> +	if (err)
> +		goto drop;
>   
>   	/* copy skb_ubuf_info for callback when skb has no error */
>   	if (zerocopy) {
> @@ -2335,12 +2348,24 @@ static int tun_xdp_one(struct tun_struct *tun,
>   	struct virtio_net_hdr *gso = NULL;
>   	struct bpf_prog *xdp_prog;
>   	struct sk_buff *skb = NULL;
> +	__be16 proto = 0;
>   	u32 rxhash = 0, act;
>   	int buflen = hdr->buflen;
>   	int err = 0;
>   	bool skb_xdp = false;
>   	struct page *page;
>   
> +	if (!(tun->flags & IFF_NO_PI)) {
> +		struct tun_pi *pi = tun_hdr;
> +		tun_hdr += sizeof(*pi);
> +
> +		if (tun_hdr > xdp->data) {
> +			atomic_long_inc(&tun->rx_frame_errors);
> +			return -EINVAL;
> +		}
> +		proto = pi->proto;
> +	}


As replied in patch 2, I think it's better to make XDP path work only 
for TAP but not TUN.

Then the series would be much simpler.

Thanks


> +
>   	if (tun->flags & IFF_VNET_HDR) {
>   		gso = tun_hdr;
>   		tun_hdr += sizeof(*gso);
> @@ -2413,7 +2438,13 @@ static int tun_xdp_one(struct tun_struct *tun,
>   		goto out;
>   	}
>   
> -	skb->protocol = eth_type_trans(skb, tun->dev);
> +	err = tun_skb_set_protocol(tun, skb, proto, true);
> +	if (err) {
> +		atomic_long_inc(&tun->dev->rx_dropped);
> +		kfree_skb(skb);
> +		goto out;
> +	}
> +
>   	skb_reset_network_header(skb);
>   	skb_probe_transport_header(skb);
>   	skb_record_rx_queue(skb, tfile->queue_index);