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Date:   Thu, 8 Oct 2020 16:45:05 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        syzbot <syzbot+e96a7ba46281824cc46a@...kaller.appspotmail.com>,
        Jon Maloy <jmaloy@...hat.com>,
        Ying Xue <ying.xue@...driver.com>
Subject: Re: [Patch net] tipc: fix the skb_unshare() in tipc_buf_append()

On Thu, Oct 8, 2020 at 12:12 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> skb_unshare() drops a reference count on the old skb unconditionally,
> so in the failure case, we end up freeing the skb twice here.
> And because the skb is allocated in fclone and cloned by caller
> tipc_msg_reassemble(), the consequence is actually freeing the
> original skb too, thus triggered the UAF by syzbot.
Do you mean:
                frag = skb_clone(skb, GFP_ATOMIC);
frag = skb_unshare(frag) will free the 'skb' too?

>
> Fix this by replacing this skb_unshare() with skb_cloned()+skb_copy().
>
> Fixes: ff48b6222e65 ("tipc: use skb_unshare() instead in tipc_buf_append()")
> Reported-and-tested-by: syzbot+e96a7ba46281824cc46a@...kaller.appspotmail.com
> Cc: Xin Long <lucien.xin@...il.com>
> Cc: Jon Maloy <jmaloy@...hat.com>
> Cc: Ying Xue <ying.xue@...driver.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/tipc/msg.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/tipc/msg.c b/net/tipc/msg.c
> index 52e93ba4d8e2..681224401871 100644
> --- a/net/tipc/msg.c
> +++ b/net/tipc/msg.c
> @@ -150,7 +150,8 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
>         if (fragid == FIRST_FRAGMENT) {
>                 if (unlikely(head))
>                         goto err;
> -               frag = skb_unshare(frag, GFP_ATOMIC);
> +               if (skb_cloned(frag))
> +                       frag = skb_copy(frag, GFP_ATOMIC);
>                 if (unlikely(!frag))
>                         goto err;
>                 head = *headbuf = frag;
> --
> 2.28.0
>

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