netdev - Re: [PATCH net-next v6 19/21] pfcp: always set pfcp metadata

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Message-Id: <701f8f93-f5fb-408b-822a-37a1d5c424ba@app.fastmail.com>
Date: Wed, 03 Apr 2024 22:59:36 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Alexander Lobakin" <aleksander.lobakin@...el.com>,
 "David S . Miller" <davem@...emloft.net>,
 "Eric Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>,
 "Paolo Abeni" <pabeni@...hat.com>
Cc: "Yury Norov" <yury.norov@...il.com>,
 "Alexander Potapenko" <glider@...gle.com>,
 nex.sw.ncis.osdt.itp.upstreaming@...el.com,
 "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
 Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
 "Michal Swiatkowski" <michal.swiatkowski@...ux.intel.com>,
 "Marcin Szycik" <marcin.szycik@...ux.intel.com>,
 "Simon Horman" <horms@...nel.org>, "Kees Cook" <keescook@...omium.org>
Subject: Re: [PATCH net-next v6 19/21] pfcp: always set pfcp metadata

On Wed, Mar 27, 2024, at 16:23, Alexander Lobakin wrote:

> +static int pfcp_encap_recv(struct sock *sk, struct sk_buff *skb)
> +{
> +	IP_TUNNEL_DECLARE_FLAGS(flags) = { };
> +	struct metadata_dst *tun_dst;
> +	struct pfcp_metadata *md;
> +	struct pfcphdr *unparsed;
> +	struct pfcp_dev *pfcp;
> +
> +	if (unlikely(!pskb_may_pull(skb, PFCP_HLEN)))
> +		goto drop;
> +
> +	pfcp = rcu_dereference_sk_user_data(sk);
> +	if (unlikely(!pfcp))
> +		goto drop;
> +
> +	unparsed = pfcp_hdr(skb);
> +
> +	ip_tunnel_flags_zero(flags);
> +	tun_dst = udp_tun_rx_dst(skb, sk->sk_family, flags, 0,
> +				 sizeof(*md));
> +	if (unlikely(!tun_dst))
> +		goto drop;
> +
> +	md = ip_tunnel_info_opts(&tun_dst->u.tun_info);
> +	if (unlikely(!md))
> +		goto drop;
> +
> +	if (unparsed->flags & PFCP_SEID_FLAG)
> +		pfcp_session_recv(pfcp, skb, md);
> +	else
> +		pfcp_node_recv(pfcp, skb, md);
> +
> +	__set_bit(IP_TUNNEL_PFCP_OPT_BIT, flags);
> +	ip_tunnel_info_opts_set(&tun_dst->u.tun_info, md, sizeof(*md),
> +				flags);
> +

The memcpy() in the ip_tunnel_info_opts_set() causes
a string.h fortification warning, with at least gcc-13:

    In function 'fortify_memcpy_chk',
        inlined from 'ip_tunnel_info_opts_set' at include/net/ip_tunnels.h:619:3,
        inlined from 'pfcp_encap_recv' at drivers/net/pfcp.c:84:2:
    include/linux/fortify-string.h:553:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
      553 |                         __write_overflow_field(p_size_field, size);

As far as I can tell, the warning is caused by the
ambiguity of the union, but what I noticed is that
it also seems to copy a buffer to itself, as 'md'
is initialized to tun_dst->u.tun_info as well.

Is this intentional?

      Arnd