Re: [PATCH bpf] bpf: properly check TCP_CONGESTION optlen

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	netdev <netdev@vger.kernel.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, bpf <bpf@vger.kernel.org>,
	Lawrence Brakmo <brakmo@fb.com>
Subject: Re: [PATCH bpf] bpf: properly check TCP_CONGESTION optlen
Date: Sat, 23 Feb 2019 12:38:09 -0800	[thread overview]
Message-ID: <20190223203808.f2r54xz2z7j2gecg@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20190223190709.61558-1-edumazet@google.com>

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@google.com>
> Cc: Lawrence Brakmo <brakmo@fb.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,
};