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Date:   Tue, 8 Mar 2022 07:43:25 -0800
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To:     David Laight <David.Laight@ACULAB.COM>,
        "davem@davemloft.net" <davem@davemloft.net>
Cc:     Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@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" <netdev@vger.kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com" 
        <syzbot+e223cf47ec8ae183f2a0@syzkaller.appspotmail.com>
References: <20220308000146.534935-1-tadeusz.struk@linaro.org>
 <14626165dad64bbaabed58ba7d59e523@AcuMS.aculab.com>
From:   Tadeusz Struk <tadeusz.struk@linaro.org>
Subject: Re: [PATCH] net: ipv6: fix invalid alloclen in __ip6_append_data
In-Reply-To: <14626165dad64bbaabed58ba7d59e523@AcuMS.aculab.com>
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Hi David,
On 3/7/22 18:58, David Laight wrote:
>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>> index 4788f6b37053..622345af323e 100644
>> --- a/net/ipv6/ip6_output.c
>> +++ b/net/ipv6/ip6_output.c
>> @@ -1629,6 +1629,13 @@ static int __ip6_append_data(struct sock *sk,
>>   				err = -EINVAL;
>>   				goto error;
>>   			}
>> +			if (unlikely(alloclen < fraglen)) {
>> +				if (printk_ratelimit())
>> +					pr_warn("%s: wrong alloclen: %d, fraglen: %d",
>> +						__func__, alloclen, fraglen);
>> +				alloclen = fraglen;
>> +			}
>> +
> Except that is a valid case, see a few lines higher:
> 
> 				alloclen = min_t(int, fraglen, MAX_HEADER);
> 				pagedlen = fraglen - alloclen;
> 
> You need to report the input values that cause the problem later on.

OK, but in this case it falls into the first if block:
https://elixir.bootlin.com/linux/v5.17-rc7/source/net/ipv6/ip6_output.c#L1606
where alloclen is assigned the value of mtu.
The values in this case are just before the alloc_skb() are:

alloclen = 1480
alloc_extra = 136
datalen = 64095
fragheaderlen = 1480
fraglen = 65575
transhdrlen = 0
mtu = 1480

-- 
Thanks,
Tadeusz