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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 1 Aug 2023 09:51:29 +0200
Message-ID: <CANn89iJO44CiUjftDZHEjOCy5Q3-PDB12uWTkrbA5JJNXMoeDA@mail.gmail.com>
Subject: Re: [PATCH v3] ip6mr: Fix skb_under_panic in ip6mr_cache_report()
To:     Yue Haibing <yuehaibing@huawei.com>
Cc:     davem@davemloft.net, dsahern@kernel.org, kuba@kernel.org,
        pabeni@redhat.com, yoshfuji@linux-ipv6.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, simon.horman@corigine.com
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On Tue, Aug 1, 2023 at 8:45=E2=80=AFAM Yue Haibing <yuehaibing@huawei.com> =
wrote:
>
>  skbuff: skb_under_panic: text:ffffffff88771f69 len:56 put:-4
>  head:ffff88805f86a800 data:ffff887f5f86a850 tail:0x88 end:0x2c0 dev:pim6=
reg
>

> When setup a vlan device on dev pim6reg, DAD ns packet may sent on reg_vi=
f_xmit().
> reg_vif_xmit()
>     ip6mr_cache_report()
>         skb_push(skb, -skb_network_offset(pkt));//skb_network_offset(pkt)=
 is 4
> And skb_push declared as:
>         void *skb_push(struct sk_buff *skb, unsigned int len);
>                 skb->data -=3D len;
>                 //0xffff88805f86a84c - 0xfffffffc =3D 0xffff887f5f86a850
> skb->data is set to 0xffff887f5f86a850, which is invalid mem addr, lead t=
o skb_push() fails.
>
> Fixes: 14fb64e1f449 ("[IPV6] MROUTE: Support PIM-SM (SSM).")
> Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
> ---
> v3: drop unnecessary nhoff change
> v2: Use __skb_pull() and fix commit log.
> ---
>  net/ipv6/ip6mr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
> index cc3d5ad17257..67a3b8f6e72b 100644
> --- a/net/ipv6/ip6mr.c
> +++ b/net/ipv6/ip6mr.c
> @@ -1073,7 +1073,7 @@ 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_pull(skb, skb_network_offset(pkt));
>
>                 skb_push(skb, sizeof(*msg));
>                 skb_reset_transport_header(skb);

Presumably this code has never been tested :/

Reviewed-by: Eric Dumazet <edumazet@google.com>