Re: [PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append_data

From: Jakub Kicinski <kuba@kernel.org>
To: Tadeusz Struk <tadeusz.struk@linaro.org>
Cc: davem@davemloft.net,
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com
Subject: Re: [PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append_data
Date: Thu, 10 Mar 2022 14:30:11 -0800	[thread overview]
Message-ID: <20220310143011.00c21f53@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20220310221328.877987-1-tadeusz.struk@linaro.org>

On Thu, 10 Mar 2022 14:13:28 -0800 Tadeusz Struk wrote:
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 4788f6b37053..6d45112322a0 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -1649,6 +1649,16 @@ static int __ip6_append_data(struct sock *sk,
>  			skb->protocol = htons(ETH_P_IPV6);
>  			skb->ip_summed = csummode;
>  			skb->csum = 0;
> +
> +			/*
> +			 *	Check if there is still room for payload
> +			 */

TBH I think the check is self-explanatory. Not worth a banner comment,
for sure.

> +			if (fragheaderlen >= mtu) {
> +				err = -EMSGSIZE;
> +				kfree_skb(skb);
> +				goto error;
> +			}

Not sure if Willem prefers this placement, but seems like we can lift
this check out of the loop, as soon as fragheaderlen and mtu are known.

>  			/* reserve for fragmentation and ipsec header */
>  			skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
>  				    dst_exthdrlen);