netdev - Re: [PATCH bpf 03/11] bpf: Add write access to tcp

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Date:   Wed, 20 Dec 2017 01:51:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Lawrence Brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>
Cc:     Kernel Team <kernel-team@...com>, Blake Matheny <bmatheny@...com>,
        Alexei Starovoitov <ast@...com>
Subject: Re: [PATCH bpf 03/11] bpf: Add write access to tcp_sock and sock
 fields

On 12/19/2017 07:21 AM, Lawrence Brakmo wrote:
> This patch adds a macro, SOCK_OPS_SET_FIELD, for writing to
> struct tcp_sock or struct sock fields. This required adding a new
> field "temp" to struct bpf_sock_ops_kern for temporary storage that
> is used by sock_ops_convert_ctx_access. It is used to store and recover
> the contents of a register, so the register can be used to store the
> address of the sk. Since we cannot overwrite the dst_reg because it
> contains the pointer to ctx, nor the src_reg since it contains the value
> we want to store, we need an extra register to contain the address
> of the sk.
> 
> Also adds the macro SOCK_OPS_GET_OR_SET_FIELD that calls one of the
> GET or SET macros depending on the value of the TYPE field.
> 
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  include/linux/filter.h |  3 +++
>  include/net/tcp.h      |  2 +-
>  net/core/filter.c      | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 5feb441..8929162 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -987,6 +987,9 @@ struct bpf_sock_ops_kern {
>  		u32 replylong[4];
>  	};
>  	u32	is_fullsock;
> +	u64	temp;			/* Used by sock_ops_convert_ctx_access
> +					 * as temporary storaage of a register
> +					 */
>  };
>  
>  #endif /* __LINUX_FILTER_H__ */
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 6cc205c..e0213f1 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2011,7 +2011,7 @@ static inline int tcp_call_bpf(struct sock *sk, int op)
>  	struct bpf_sock_ops_kern sock_ops;
>  	int ret;
>  
> -	memset(&sock_ops, 0, sizeof(sock_ops));
> +	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, is_fullsock));

I don't think this is correct. sock_ops is on stack, so above you only
zero up to the offset of is_fullsock, but not including it, so when
you have !sk_fullsock(sk), then your BPF prog will still act as if the
sock_ops.is_fullsock was set in case prior stack garbage said so.

>  	if (sk_fullsock(sk)) {
>  		sock_ops.is_fullsock = 1;
>  		sock_owned_by_me(sk);

Thanks,
Daniel