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

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Date:   Thu, 10 Mar 2022 14:42:29 -0800
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Jakub Kicinski <kuba@...nel.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 3/10/22 14:30, Jakub Kicinski wrote:
>> +
>> +			/*
>> +			 *	Check if there is still room for payload
>> +			 */
> TBH I think the check is self-explanatory. Not worth a banner comment,
> for sure.

Ok

> 
>> +			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.
> 

He said to check it before the skb_put() and so I did.
The fragheaderlen is known early, but mtu can be updated inside the loop
by ip6_append_data_mtu() so I'm not sure we can do the check before that.

-- 
Thanks,
Tadeusz