linux-kernel - Re: [PATCH] ip6mr: Fix skb_under_panic in ip6mr_cache

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Date:   Fri, 28 Jul 2023 20:00:20 +0800
From:   YueHaibing <yuehaibing@...wei.com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     <davem@...emloft.net>, <dsahern@...nel.org>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <yoshfuji@...ux-ipv6.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ip6mr: Fix skb_under_panic in ip6mr_cache_report()

On 2023/7/28 18:50, Eric Dumazet wrote:
> On Fri, Jul 28, 2023 at 12:01 PM Yue Haibing <yuehaibing@...wei.com> wrote:
>>
>>  skbuff: skb_under_panic: text:ffffffff88771f69 len:56 put:-4
>>  head:ffff88805f86a800 data:ffff887f5f86a850 tail:0x88 end:0x2c0 dev:pim6reg
>>  ------------[ cut here ]------------
>>
> 
>> When setup a vlan device on dev pim6reg, DAD ns packet may sent on reg_vif_xmit().
>> reg_vif_xmit()
>>     ip6mr_cache_report()
>>         skb_push(skb, -skb_network_offset(pkt));//skb_network_offset(pkt) is 4
>> And skb_push declar as this:
>>         void *skb_push(struct sk_buff *skb, unsigned int len);
>>                 skb->data -= len;
>>                 //0xffff888f5f86a84c - 0xfffffffc = 0xffff887f5f86a850
>> skb->data is set to 0xffff887f5f86a850, which is invalid mem addr, lead to skb_push() fails.
>>
>> Fixes: 14fb64e1f449 ("[IPV6] MROUTE: Support PIM-SM (SSM).")
>> Signed-off-by: Yue Haibing <yuehaibing@...wei.com>
>> ---
>>  net/ipv6/ip6mr.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
>> index cc3d5ad17257..ee9c2ff8b0e4 100644
>> --- a/net/ipv6/ip6mr.c
>> +++ b/net/ipv6/ip6mr.c
>> @@ -1051,9 +1051,9 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
>>         int ret;
>>
>>  #ifdef CONFIG_IPV6_PIMSM_V2
>> +       int nhoff = skb_network_offset(pkt);
>>         if (assert == MRT6MSG_WHOLEPKT || assert == MRT6MSG_WRMIFWHOLE)
>> -               skb = skb_realloc_headroom(pkt, -skb_network_offset(pkt)
>> -                                               +sizeof(*msg));
>> +               skb = skb_realloc_headroom(pkt, -nhoff + sizeof(*msg));
>>         else
>>  #endif
>>                 skb = alloc_skb(sizeof(struct ipv6hdr) + sizeof(*msg), GFP_ATOMIC);
>> @@ -1073,7 +1073,8 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
>>                    And all this only to mangle msg->im6_msgtype and
>>                    to set msg->im6_mbz to "mbz" :-)
>>                  */
>> -               skb_push(skb, -skb_network_offset(pkt));
>> +               skb->data += nhoff;
>> +               skb->len  -= nhoff;
> 
> __skb_pull(skb, nhoff);

Thanks， will do this in v2.
> 
>>
>>                 skb_push(skb, sizeof(*msg));
>>                 skb_reset_transport_header(skb);
>> --
>> 2.34.1
>>
> .
>