Re: [PATCHv4 net 2/2] xfrm/xfrm_policy: fix dst dev null pointer dereference in collect_md mode

["Jonathan Lemon" <jonathan.lemon@gmail.com>]

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

> In decode_session{4,6} there is a possibility that the skb 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_ipv6
>     - __gre6_xmit
>       - ip6_tnl_xmit
>         - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
>     - icmpv6_send
>       - icmpv6_route_lookup
>         - xfrm_decode_session_reverse
>           - decode_session4
>             - oif = skb_dst(skb)->dev->ifindex; <-- here
>           - decode_session6
>             - oif = skb_dst(skb)->dev->ifindex; <-- 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 skb_dst(skb)->dev is actually not needed as we
> called decode_session{4,6} via xfrm_decode_session_reverse(), so oif is not
> used by: fl4->flowi4_oif = reverse ? skb->skb_iif : oif;
>
> So make a dst dev check here should be clean and safe.
>
> v4: No changes.
>
> v3: No changes.
>
> v2: fix the issue in decode_session{4,6} instead of updating shared dst dev
> in {ip_md, ip6}_tunnel_xmit.
>
> Fixes: 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>

Tested-by: Jonathan Lemon <jonathan.lemon@gmail.com>

This does resolve a local crash where the dev pointer is NULL.