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Date: Fri, 25 Jun 2021 15:41:27 +0800
From: Jason Wang <jasowang@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>, netdev@...r.kernel.org
Cc: Eugenio Pérez <eperezma@...hat.com>,
Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v3 4/5] net: tun: fix tun_xdp_one() for IFF_TUN mode
在 2021/6/24 下午8:30, David Woodhouse 写道:
> From: David Woodhouse <dwmw@...zon.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@...gle.com>
> Fixes: 043d222f93ab ("tuntap: accept an array of XDP buffs through sendmsg()")
> Signed-off-by: David Woodhouse <dwmw@...zon.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);
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