linux-kernel - Re: [PATCH 1/2] net: ipv6: Fix a possible null-pointer dereference in ip6

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Date:   Fri, 26 Jul 2019 07:44:54 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] net: ipv6: Fix a possible null-pointer dereference in ip6_xmit()

On Fri, Jul 26, 2019 at 4:03 AM Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>
> In ip6_xmit(), there is an if statement on line 245 to check whether
> np is NULL:
>     if (np)
>
> When np is NULL, it is used on line 251:
>     ip6_autoflowlabel(net, np)
>         if (!np->autoflowlabel_set)
>
> Thus, a possible null-pointer dereference may occur.
>
> To fix this bug, np is checked before calling
> ip6_autoflowlabel(net,np).
>
> This bug is found by a static analysis tool STCheck written by us.
>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  net/ipv6/ip6_output.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 8e49fd62eea9..07db5ab6e970 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -247,8 +247,10 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
>         if (hlimit < 0)
>                 hlimit = ip6_dst_hoplimit(dst);
>
> -       ip6_flow_hdr(hdr, tclass, ip6_make_flowlabel(net, skb, fl6->flowlabel,
> -                               ip6_autoflowlabel(net, np), fl6));
> +       if (np) {
> +               ip6_flow_hdr(hdr, tclass, ip6_make_flowlabel(net, skb, fl6->flowlabel,
> +                                       ip6_autoflowlabel(net, np), fl6));
> +       }

I don't know when np can be NULL in ip6_xmit. But if so, must still
setup the ipv6 header.

A more narrow change would be in ip6_autoflowlabel

-        if (!np->autoflowlabel_set)
+       if (!np || !np->autoflowlabel_set)