Re: [PATCH bpf-next V11 4/7] bpf: add BPF-helper for MTU checking

[Daniel Borkmann <daniel@iogearbox.net>]

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;
> +}