Re: [PATCH net] ipv6: Fix dangling pointer when ipv6 fragment

[Eric Dumazet]

On 03/30/2019 12:29 AM, hujunwei wrote:
> From: Junwei Hu <hujunwei4@xxxxxxxxxx>
> 
> At the beginning of ip6_fragment func, the prevhdr pointer is
> obtained in the ip6_find_1stfragopt func.
> However, all the pointers pointing into skb header may change
> when calling skb_checksum_help func with
> skb->ip_summed = CHECKSUM_PARTIAL condition.
> The prevhdr pointe will be dangling if it is not reloaded after
> calling __skb_linearize func in skb_checksum_help func.
> 
> Here, I add a variable, nexthdr_offset, to evaluate the offset,
> which does not changes even after calling __skb_linearize func.
> 
> Fixes: 405c92f7a541 ("ipv6: add defensive check for CHECKSUM_PARTIAL skbs in ip_fragment")
> Signed-off-by: Junwei Hu <hujunwei4@xxxxxxxxxx>
> Reported-by: Wenhao Zhang <zhangwenhao8@xxxxxxxxxx>
> Reviewed-by: Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>

Interesting.

We got a syzbot report yesterday about this issue. (email thread : BUG: unable to handle kernel paging request in ip6_fragment)

<quote>
syzbot found the following crash on:

HEAD commit:    8c838f53 dpaa2-eth: fix race condition with bql frame acco..
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=12b83a9b200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f05902bca21d8935
dashboard link: https://syzkaller.appspot.com/bug?extid=e8ce541d095e486074fc
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+e8ce541d095e486074fc@xxxxxxxxxxxxxxxxxxxxxxxxx
</quote>


> 
> ---
> Ânet/ipv6/ip6_output.c | 4 ++++
> Â1 file changed, 4 insertions(+)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index edbd12067170..6db3c60b3b66 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -606,12 +606,14 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
> ÂÂÂÂ __be32 frag_id;
> ÂÂÂÂ int ptr, offset = 0, err = 0;
> ÂÂÂÂ u8 *prevhdr, nexthdr = 0;
> +ÂÂÂ u8 nexthdr_offset;
> Â
> ÂÂÂÂ err = ip6_find_1stfragopt(skb, &prevhdr);
> ÂÂÂÂ if (err < 0)
> ÂÂÂÂ ÂÂÂ goto fail;
> ÂÂÂÂ hlen = err;
> ÂÂÂÂ nexthdr = *prevhdr;
> +ÂÂÂ nexthdr_offset = prevhdr - skb_network_header(skb);
> Â
> ÂÂÂÂ mtu = ip6_skb_dst_mtu(skb);
> Â
> @@ -646,6 +648,8 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
> ÂÂÂÂ ÂÂÂ (err = skb_checksum_help(skb)))
> ÂÂÂÂ ÂÂÂ goto fail;
> Â
> +ÂÂÂ prevhdr = skb_network_header(skb) + nexthdr_offset;
> +
> ÂÂÂÂ hroom = LL_RESERVED_SPACE(rt->dst.dev);
> ÂÂÂÂ if (skb_has_frag_list(skb)) {
> ÂÂÂÂ ÂÂÂ unsigned int first_len = skb_pagelen(skb);
>