netdev - Re: [PATCH v4 bpf-next 1/2] bpf: add cg_skb_is_valid_access for BPF_PROG_TYPE_CGROUP

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Date:   Fri, 19 Oct 2018 02:14:06 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Song Liu <songliubraving@...com>, netdev@...r.kernel.org
Cc:     ast@...nel.org, kernel-team@...com
Subject: Re: [PATCH v4 bpf-next 1/2] bpf: add cg_skb_is_valid_access for
 BPF_PROG_TYPE_CGROUP_SKB

On 10/18/2018 06:06 PM, Song Liu wrote:
> BPF programs of BPF_PROG_TYPE_CGROUP_SKB need to access headers in the
> skb. This patch enables direct access of skb for these programs.
> 
> Two helper functions bpf_compute_and_save_data_pointers() and
> bpf_restore_data_pointers() are introduced. There are used in
> __cgroup_bpf_run_filter_skb(), to compute proper data_end for the
> BPF program, and restore original data afterwards.
> 
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  include/linux/filter.h | 24 ++++++++++++++++++++++++
>  kernel/bpf/cgroup.c    |  6 ++++++
>  net/core/filter.c      | 36 +++++++++++++++++++++++++++++++++++-
>  3 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 5771874bc01e..96b3ee7f14c9 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -548,6 +548,30 @@ static inline void bpf_compute_data_pointers(struct sk_buff *skb)
>  	cb->data_end  = skb->data + skb_headlen(skb);
>  }
>  
> +/* Similar to bpf_compute_data_pointers(), except that save orginal
> + * data in cb->data and cb->meta_data for restore.
> + */
> +static inline void bpf_compute_and_save_data_pointers(
> +	struct sk_buff *skb, void *saved_pointers[2])
> +{
> +	struct bpf_skb_data_end *cb = (struct bpf_skb_data_end *)skb->cb;
> +
> +	saved_pointers[0] = cb->data_meta;
> +	saved_pointers[1] = cb->data_end;
> +	cb->data_meta = skb->data - skb_metadata_len(skb);
> +	cb->data_end  = skb->data + skb_headlen(skb);

Hmm, can you elaborate why populating data_meta here ...

> +}
> +
> +/* Restore data saved by bpf_compute_data_pointers(). */
> +static inline void bpf_restore_data_pointers(
> +	struct sk_buff *skb, void *saved_pointers[2])
> +{
> +	struct bpf_skb_data_end *cb = (struct bpf_skb_data_end *)skb->cb;
> +
> +	cb->data_meta = saved_pointers[0];
> +	cb->data_end = saved_pointers[1];;
> +}
> +
>  static inline u8 *bpf_skb_cb(struct sk_buff *skb)
>  {
>  	/* eBPF programs may read/write skb->cb[] area to transfer meta
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 00f6ed2e4f9a..5f5180104ddc 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -554,6 +554,7 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
>  	unsigned int offset = skb->data - skb_network_header(skb);
>  	struct sock *save_sk;
>  	struct cgroup *cgrp;
> +	void *saved_pointers[2];
>  	int ret;
>  
>  	if (!sk || !sk_fullsock(sk))
> @@ -566,8 +567,13 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
>  	save_sk = skb->sk;
>  	skb->sk = sk;
>  	__skb_push(skb, offset);
> +
> +	/* compute pointers for the bpf prog */
> +	bpf_compute_and_save_data_pointers(skb, saved_pointers);
> +
>  	ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], skb,
>  				 bpf_prog_run_save_cb);
> +	bpf_restore_data_pointers(skb, saved_pointers);
>  	__skb_pull(skb, offset);
>  	skb->sk = save_sk;
>  	return ret == 1 ? 0 : -EPERM;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 1a3ac6c46873..e3ca30bd6840 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5346,6 +5346,40 @@ static bool sk_filter_is_valid_access(int off, int size,
>  	return bpf_skb_is_valid_access(off, size, type, prog, info);
>  }
>  
> +static bool cg_skb_is_valid_access(int off, int size,
> +				   enum bpf_access_type type,
> +				   const struct bpf_prog *prog,
> +				   struct bpf_insn_access_aux *info)
> +{
> +	switch (off) {
> +	case bpf_ctx_range(struct __sk_buff, tc_classid):
> +	case bpf_ctx_range(struct __sk_buff, data_meta):
> +	case bpf_ctx_range(struct __sk_buff, flow_keys):
> +		return false;

... if it's disallowed anyway (disallowing it is the right thing to do,
but no need to save/restore then..)?

> +	}
> +	if (type == BPF_WRITE) {
> +		switch (off) {
> +		case bpf_ctx_range(struct __sk_buff, mark):
> +		case bpf_ctx_range(struct __sk_buff, priority):
> +		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
> +			break;
> +		default:
> +			return false;
> +		}
> +	}
> +
> +	switch (off) {
> +	case bpf_ctx_range(struct __sk_buff, data):
> +		info->reg_type = PTR_TO_PACKET;
> +		break;
> +	case bpf_ctx_range(struct __sk_buff, data_end):
> +		info->reg_type = PTR_TO_PACKET_END;
> +		break;
> +	}
> +
> +	return bpf_skb_is_valid_access(off, size, type, prog, info);
> +}
> +
>  static bool lwt_is_valid_access(int off, int size,
>  				enum bpf_access_type type,
>  				const struct bpf_prog *prog,
> @@ -7038,7 +7072,7 @@ const struct bpf_prog_ops xdp_prog_ops = {
>  
>  const struct bpf_verifier_ops cg_skb_verifier_ops = {
>  	.get_func_proto		= cg_skb_func_proto,
> -	.is_valid_access	= sk_filter_is_valid_access,
> +	.is_valid_access	= cg_skb_is_valid_access,
>  	.convert_ctx_access	= bpf_convert_ctx_access,
>  };
>  
>