netdev - Re: [PATCH v3 net] icmp6: Fix null-ptr-deref of ip6_null_entry->rt6i_idev in icmp6

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Date: Sat, 8 Jul 2023 10:07:24 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	Wang Yufen <wangyufen@...wei.com>
Subject: Re: [PATCH v3 net] icmp6: Fix null-ptr-deref of ip6_null_entry->rt6i_idev
 in icmp6_dev().

On Sat, Jul 8, 2023 at 3:43 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> With some IPv6 Ext Hdr (RPL, SRv6, etc.), we can send a packet that
> has the link-local address as src and dst IP and will be forwarded to
> an external IP in the IPv6 Ext Hdr.
>
> For example, the script below generates a packet whose src IP is the
> link-local address and dst is updated to 11::.
>
>   # for f in $(find /proc/sys/net/ -name *seg6_enabled*); do echo 1 > $f; done
>   # python3
>   >>> from socket import *
>   >>> from scapy.all import *
>   >>>
>   >>> SRC_ADDR = DST_ADDR = "fe80::5054:ff:fe12:3456"
>   >>>
>   >>> pkt = IPv6(src=SRC_ADDR, dst=DST_ADDR)
>   >>> pkt /= IPv6ExtHdrSegmentRouting(type=4, addresses=["11::", "22::"], segleft=1)
>   >>>
>   >>> sk = socket(AF_INET6, SOCK_RAW, IPPROTO_RAW)
>   >>> sk.sendto(bytes(pkt), (DST_ADDR, 0))
>
> For such a packet, we call ip6_route_input() to look up a route for the
> next destination in these three functions depending on the header type.
>
>   * ipv6_rthdr_rcv()
>   * ipv6_rpl_srh_rcv()
>   * ipv6_srh_rcv()
>
> If no route is found, ip6_null_entry is set to skb, and the following
> dst_input(skb) calls ip6_pkt_drop().
>
> Finally, in icmp6_dev(), we dereference skb_rt6_info(skb)->rt6i_idev->dev
> as the input device is the loopback interface.  Then, we have to check if
> skb_rt6_info(skb)->rt6i_idev is NULL or not to avoid NULL pointer deref
> for ip6_null_entry.
>
>
> Fixes: 4832c30d5458 ("net: ipv6: put host and anycast routes on device with address")
> Reported-by: Wang Yufen <wangyufen@...wei.com>
> Closes: https://lore.kernel.org/netdev/c41403a9-c2f6-3b7e-0c96-e1901e605cd0@huawei.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> Reviewed-by: David Ahern <dsahern@...nel.org>
> ---
> v3:
>   * Fix Closes: link
>
> v2: https://lore.kernel.org/netdev/20230708002145.64069-1-kuniyu@amazon.com/
>   * Add Reviewed-by
>   * s/fib6_null_entry/ip6_null_entry/g
>
> v1: https://lore.kernel.org/netdev/20230706233024.63730-1-kuniyu@amazon.com/
> ---
>  net/ipv6/icmp.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
> index 9edf1f45b1ed..65fa5014bc85 100644
> --- a/net/ipv6/icmp.c
> +++ b/net/ipv6/icmp.c
> @@ -424,7 +424,10 @@ static struct net_device *icmp6_dev(const struct sk_buff *skb)
>         if (unlikely(dev->ifindex == LOOPBACK_IFINDEX || netif_is_l3_master(skb->dev))) {
>                 const struct rt6_info *rt6 = skb_rt6_info(skb);
>
> -               if (rt6)
> +               /* The destination could be an external IP in Ext Hdr (SRv6, RPL, etc.),
> +                * and ip6_null_entry could be set to skb if no route is found.
> +                */
> +               if (rt6 && rt6->rt6i_idev)
>                         dev = rt6->rt6i_idev->dev;
>         }

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