netdev - Re: [PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append

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Date:   Thu, 10 Mar 2022 14:30:11 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Tadeusz Struk <tadeusz.struk@...aro.org>
Cc:     davem@...emloft.net,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        syzbot+e223cf47ec8ae183f2a0@...kaller.appspotmail.com
Subject: Re: [PATCH v2] net: ipv6: fix skb_over_panic in __ip6_append_data

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);