Subject: Re: [PATCH net-next v4 07/16] bpf: Add setsockopt helper function to bpf

From: Daniel Borkmann <[email-masked]>
Date: Thu, 29 Jun 2017 12:08:45 +0200

On 06/28/2017 07:31 PM, Lawrence Brakmo wrote:
> Added support for calling a subset of socket setsockopts from
> BPF_PROG_TYPE_SOCK_OPS programs. The code was duplicated rather
> than making the changes to call the socket setsockopt function because
> the changes required would have been larger.
>
> The ops supported are:
>    SO_RCVBUF
>    SO_SNDBUF
>    SO_MAX_PACING_RATE
>    SO_PRIORITY
>    SO_RCVLOWAT
>    SO_MARK
>
> Signed-off-by: Lawrence Brakmo <[email-masked]>
[...]
> @@ -2672,6 +2673,69 @@ static const struct bpf_func_proto bpf_get_socket_uid_proto = {
>   	.arg1_type      = ARG_PTR_TO_CTX,
>   };
>
> +BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
> +	   int, level, int, optname, char *, optval, int, optlen)

Nit: I would rather make optlen a u32. But more below.

> +{
> +	struct sock *sk = bpf_sock->sk;
> +	int ret = 0;
> +	int val;
> +
> +	if (bpf_sock->is_req_sock)
> +		return -EINVAL;
> +
> +	if (level == SOL_SOCKET) {

		if (optlen != sizeof(int))
			return -EINVAL;

> +		/* Only some socketops are supported */
> +		val = *((int *)optval);
> +
> +		switch (optname) {
> +		case SO_RCVBUF:
> +			sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
> +			sk->sk_rcvbuf = max_t(int, val * 2, SOCK_MIN_RCVBUF);
> +			break;
> +		case SO_SNDBUF:
> +			sk->sk_userlocks |= SOCK_SNDBUF_LOCK;
> +			sk->sk_sndbuf = max_t(int, val * 2, SOCK_MIN_SNDBUF);
> +			break;
> +		case SO_MAX_PACING_RATE:
> +			sk->sk_max_pacing_rate = val;
> +			sk->sk_pacing_rate = min(sk->sk_pacing_rate,
> +						 sk->sk_max_pacing_rate);
> +			break;
> +		case SO_PRIORITY:
> +			sk->sk_priority = val;
> +			break;
> +		case SO_RCVLOWAT:
> +			if (val < 0)
> +				val = INT_MAX;
> +			sk->sk_rcvlowat = val ? : 1;
> +			break;
> +		case SO_MARK:
> +			sk->sk_mark = val;
> +			break;
> +		default:
> +			ret = -EINVAL;
> +		}
> +	} else if (level == SOL_TCP &&
> +		   sk->sk_prot->setsockopt == tcp_setsockopt) {
> +		/* Place holder */
> +		ret = -EINVAL;
> +	} else {
> +		ret = -EINVAL;
> +	}
> +	return ret;
> +}
> +
> +static const struct bpf_func_proto bpf_setsockopt_proto = {
> +	.func		= bpf_setsockopt,
> +	.gpl_only	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +	.arg2_type	= ARG_ANYTHING,
> +	.arg3_type	= ARG_ANYTHING,
> +	.arg4_type	= ARG_PTR_TO_MEM,
> +	.arg5_type	= ARG_CONST_SIZE_OR_ZERO,

Any reason you went with the ARG_CONST_SIZE_OR_ZERO type? Semantics
of this are that allowed [arg4, arg5] pair can be i) [NULL, 0] or
ii) [non-NULL, non-zero], where in case ii) verifier checks that the
area is initialized when coming from BPF stack.

So above 'val = *((int *)optval);' would give a NULL pointer deref
with NULL passed as arg or in case optlen was < sizeof(int) we access
stack out of bounds potentially. If the [NULL, 0] pair is not required,
I would just make that a ARG_CONST_SIZE and then check for size before
accessing optval.

> +};
> +
>   static const struct bpf_func_proto *
>   bpf_base_func_proto(enum bpf_func_id func_id)
>   {