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Date: Thu, 14 Jan 2021 00:07:14 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc: netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
John Fastabend <john.fastabend@...il.com>,
Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
colrack@...il.com
Subject: Re: [PATCH bpf-next V11 4/7] bpf: add BPF-helper for MTU checking
On 1/12/21 6:45 PM, Jesper Dangaard Brouer wrote:
> This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.
[...]
> + * int bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)
> + * Description
> + * Check ctx packet size against MTU of net device (based on
> + * *ifindex*). This helper will likely be used in combination with
> + * helpers that adjust/change the packet size. The argument
> + * *len_diff* can be used for querying with a planned size
> + * change. This allows to check MTU prior to changing packet ctx.
> + *
> + * Specifying *ifindex* zero means the MTU check is performed
> + * against the current net device. This is practical if this isn't
> + * used prior to redirect.
> + *
> + * The Linux kernel route table can configure MTUs on a more
> + * specific per route level, which is not provided by this helper.
> + * For route level MTU checks use the **bpf_fib_lookup**\ ()
> + * helper.
> + *
> + * *ctx* is either **struct xdp_md** for XDP programs or
> + * **struct sk_buff** for tc cls_act programs.
> + *
> + * The *flags* argument can be a combination of one or more of the
> + * following values:
> + *
> + * **BPF_MTU_CHK_SEGS**
> + * This flag will only works for *ctx* **struct sk_buff**.
> + * If packet context contains extra packet segment buffers
> + * (often knows as GSO skb), then MTU check is harder to
> + * check at this point, because in transmit path it is
> + * possible for the skb packet to get re-segmented
> + * (depending on net device features). This could still be
> + * a MTU violation, so this flag enables performing MTU
> + * check against segments, with a different violation
> + * return code to tell it apart. Check cannot use len_diff.
> + *
> + * On return *mtu_len* pointer contains the MTU value of the net
> + * device. Remember the net device configured MTU is the L3 size,
> + * which is returned here and XDP and TX length operate at L2.
> + * Helper take this into account for you, but remember when using
> + * MTU value in your BPF-code. On input *mtu_len* must be a valid
> + * pointer and be initialized (to zero), else verifier will reject
> + * BPF program.
> + *
> + * Return
> + * * 0 on success, and populate MTU value in *mtu_len* pointer.
> + *
> + * * < 0 if any input argument is invalid (*mtu_len* not updated)
> + *
> + * MTU violations return positive values, but also populate MTU
> + * value in *mtu_len* pointer, as this can be needed for
> + * implementing PMTU handing:
> + *
> + * * **BPF_MTU_CHK_RET_FRAG_NEEDED**
> + * * **BPF_MTU_CHK_RET_SEGS_TOOBIG**
> + *
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3998,6 +4053,7 @@ union bpf_attr {
> FN(ktime_get_coarse_ns), \
> FN(ima_inode_hash), \
> FN(sock_from_file), \
> + FN(check_mtu), \
> /* */
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> @@ -5030,6 +5086,17 @@ struct bpf_redir_neigh {
> };
> };
>
> +/* bpf_check_mtu flags*/
> +enum bpf_check_mtu_flags {
> + BPF_MTU_CHK_SEGS = (1U << 0),
> +};
> +
> +enum bpf_check_mtu_ret {
> + BPF_MTU_CHK_RET_SUCCESS, /* check and lookup successful */
> + BPF_MTU_CHK_RET_FRAG_NEEDED, /* fragmentation required to fwd */
> + BPF_MTU_CHK_RET_SEGS_TOOBIG, /* GSO re-segmentation needed to fwd */
> +};
> +
> enum bpf_task_fd_type {
> BPF_FD_TYPE_RAW_TRACEPOINT, /* tp name */
> BPF_FD_TYPE_TRACEPOINT, /* tp name */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index db59ab55572c..3f2e593244ca 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5604,6 +5604,124 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
> .arg4_type = ARG_ANYTHING,
> };
>
> +static struct net_device *__dev_via_ifindex(struct net_device *dev_curr,
> + u32 ifindex)
> +{
> + struct net *netns = dev_net(dev_curr);
> +
> + /* Non-redirect use-cases can use ifindex=0 and save ifindex lookup */
> + if (ifindex == 0)
> + return dev_curr;
> +
> + return dev_get_by_index_rcu(netns, ifindex);
> +}
> +
> +BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
> + u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
> +{
> + int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> + struct net_device *dev = skb->dev;
> + int skb_len, dev_len;
> + int mtu;
> +
> + if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
> + return -EINVAL;
> +
> + dev = __dev_via_ifindex(dev, ifindex);
> + if (unlikely(!dev))
> + return -ENODEV;
> +
> + mtu = READ_ONCE(dev->mtu);
> +
> + dev_len = mtu + dev->hard_header_len;
> + skb_len = skb->len + len_diff; /* minus result pass check */
> + if (skb_len <= dev_len) {
> + ret = BPF_MTU_CHK_RET_SUCCESS;
> + goto out;
> + }
> + /* At this point, skb->len exceed MTU, but as it include length of all
> + * segments, it can still be below MTU. The SKB can possibly get
> + * re-segmented in transmit path (see validate_xmit_skb). Thus, user
> + * must choose if segs are to be MTU checked. Last SKB "headlen" is
> + * checked against MTU.
> + */
> + if (skb_is_gso(skb)) {
> + ret = BPF_MTU_CHK_RET_SUCCESS;
> +
> + if (!(flags & BPF_MTU_CHK_SEGS))
> + goto out;
> +
> + if (!skb_gso_validate_network_len(skb, mtu)) {
> + ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
> + goto out;
> + }
> +
> + skb_len = skb_headlen(skb) + len_diff;
> + if (skb_len > dev_len) {
This is still not universally correct given drivers could cook up non-linear
skbs (e.g. page frags) on rx. So the result from BPF_MTU_CHK_SEGS flag cannot
be relied on. Do you have a particular use case for the BPF_MTU_CHK_SEGS? I
also don't see the flag being used anywhere in your selftests, so I presume
not as otherwise you would have added an example there? I would just drop the
flag altogether for the tc helper..
> + ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> + goto out;
> + }
> + }
> +out:
> + /* BPF verifier guarantees valid pointer */
> + *mtu_len = mtu;
> +
> + return ret;
> +}
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