netdev - Re: [PATCH net] net: clear the dst when changing skb protocol

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Message-ID: <d0793e6f-401f-40d0-9199-a039fafd779c@iogearbox.net>
Date: Wed, 4 Jun 2025 23:45:32 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc: netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com,
 andrew+netdev@...n.ch, horms@...nel.org, martin.lau@...ux.dev,
 john.fastabend@...il.com, eddyz87@...il.com, sdf@...ichev.me,
 haoluo@...gle.com, willemb@...gle.com, william.xuanziyang@...wei.com,
 alan.maguire@...cle.com, bpf@...r.kernel.org, maze@...gle.com
Subject: Re: [PATCH net] net: clear the dst when changing skb protocol

On 6/4/25 11:06 PM, Jakub Kicinski wrote:
> A not-so-careful NAT46 BPF program can crash the kernel
> if it indiscriminately flips ingress packets from v4 to v6:
> 
>    BUG: kernel NULL pointer dereference, address: 0000000000000000
>      ip6_rcv_core (net/ipv6/ip6_input.c:190:20)
>      ipv6_rcv (net/ipv6/ip6_input.c:306:8)
>      process_backlog (net/core/dev.c:6186:4)
>      napi_poll (net/core/dev.c:6906:9)
>      net_rx_action (net/core/dev.c:7028:13)
>      do_softirq (kernel/softirq.c:462:3)
>      netif_rx (net/core/dev.c:5326:3)
>      dev_loopback_xmit (net/core/dev.c:4015:2)
>      ip_mc_finish_output (net/ipv4/ip_output.c:363:8)
>      NF_HOOK (./include/linux/netfilter.h:314:9)
>      ip_mc_output (net/ipv4/ip_output.c:400:5)
>      dst_output (./include/net/dst.h:459:9)
>      ip_local_out (net/ipv4/ip_output.c:130:9)
>      ip_send_skb (net/ipv4/ip_output.c:1496:8)
>      udp_send_skb (net/ipv4/udp.c:1040:8)
>      udp_sendmsg (net/ipv4/udp.c:1328:10)
> 
> The output interface has a 4->6 program attached at ingress.
> We try to loop the multicast skb back to the sending socket.
> Ingress BPF runs as part of netif_rx(), pushes a valid v6 hdr
> and changes skb->protocol to v6. We enter ip6_rcv_core which
> tries to use skb_dst(). But the dst is still an IPv4 one left
> after IPv4 mcast output.
> 
> Clear the dst in all BPF helpers which change the protcol.

(nit: protcol)

> Try to preserve metadata dsts, those won't hurt.
> 
> Fixes: d219df60a70e ("bpf: Add ipip6 and ip6ip decap support for bpf_skb_adjust_room()")
> Fixes: 1b00e0dfe7d0 ("bpf: update skb->protocol in bpf_skb_net_grow")
> Fixes: 6578171a7ff0 ("bpf: add bpf_skb_change_proto helper")
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>

lgtm!

Acked-by: Daniel Borkmann <daniel@...earbox.net>