netdev - Re: [PATCHv4 net 1/2] ipv4/icmp: fix rt dst dev null pointer dereference

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Date:   Fri, 23 Aug 2019 15:32:35 -0700
From:   "Jonathan Lemon" <jlemon@...gsvamp.com>
To:     "Hangbin Liu" <liuhangbin@...il.com>
Cc:     netdev@...r.kernel.org, "Stefano Brivio" <sbrivio@...hat.com>,
        wenxu <wenxu@...oud.cn>, "Alexei Starovoitov" <ast@...com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric Dumazet" <eric.dumazet@...il.com>,
        "Julian Anastasov" <ja@....bg>
Subject: Re: [PATCHv4 net 1/2] ipv4/icmp: fix rt dst dev null pointer
 dereference

On 22 Aug 2019, at 7:19, Hangbin Liu wrote:

> In __icmp_send() there is a possibility that the rt->dst.dev is NULL,
> e,g, with tunnel collect_md mode, which will cause kernel crash.
> Here is what the code path looks like, for GRE:
>
> - ip6gre_tunnel_xmit
>   - ip6gre_xmit_ipv4
>     - __gre6_xmit
>       - ip6_tnl_xmit
>         - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
>     - icmp_send
>       - net = dev_net(rt->dst.dev); <-- here
>
> The reason is __metadata_dst_init() init dst->dev to NULL by default.
> We could not fix it in __metadata_dst_init() as there is no dev supplied.
> On the other hand, the reason we need rt->dst.dev is to get the net.
> So we can just try get it from skb->dev when rt->dst.dev is NULL.
>
> v4: Julian Anastasov remind skb->dev also could be NULL. We'd better
> still use dst.dev and do a check to avoid crash.
>
> v3: No changes.
>
> v2: fix the issue in __icmp_send() instead of updating shared dst dev
> in {ip_md, ip6}_tunnel_xmit.
>
> Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>

Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>