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Date:   Sat, 23 Feb 2019 12:38:09 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        bpf <bpf@...r.kernel.org>, Lawrence Brakmo <brakmo@...com>
Subject: Re: [PATCH bpf] bpf: properly check TCP_CONGESTION optlen

On Sat, Feb 23, 2019 at 11:07:09AM -0800, Eric Dumazet wrote:
> If caller of bpf_setsockopt() is silly passing a negative optlen
> bad things happen.
> 
> Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Lawrence Brakmo <brakmo@...com>
> ---
>  net/core/filter.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index f7d0004fc16096eb42ece3a6acf645540ee2326b..6a5d89464168f2f35f43986c1dbc0446c9390a3c 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4194,8 +4194,9 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
>  			char name[TCP_CA_NAME_MAX];
>  			bool reinit = bpf_sock->op > BPF_SOCK_OPS_NEEDS_ECN;
>  
> -			strncpy(name, optval, min_t(long, optlen,
> -						    TCP_CA_NAME_MAX-1));
> +			if (optlen < 0)
> +				return -EINVAL;
> +			strncpy(name, optval, min(optlen, TCP_CA_NAME_MAX - 1));

Unnecessary.
The verifier guarantees that optlen > 0 because
static const struct bpf_func_proto bpf_setsockopt_proto = {
        .func           = bpf_setsockopt,
...
        .arg5_type      = ARG_CONST_SIZE,
};

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