Re: [PATCH v3 4/5] net: tun: fix tun_xdp_one() for IFF_TUN mode
From: Jason Wang <hidden>
Date: 2021-06-25 07:41:54
在 2021/6/24 下午8:30, David Woodhouse 写道:
From: David Woodhouse <redacted> 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 <redacted>Thanks
quoted hunk ↗ jump to hunk
--- 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
quoted hunk ↗ jump to hunk
+ 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);