Re: [PATCH net] netfilter: flowtable: account for Ethernet header in nf_flow_pppoe_proto()

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Eric,

Thanks for your patch.

On Mon, Jul 07, 2025 at 12:45:17PM +0000, Eric Dumazet wrote:
> syzbot found a potential access to uninit-value in nf_flow_pppoe_proto()
> 
> Blamed commit forgot the Ethernet header.

I see, vlan indeed includes the ethernet header.

        case htons(ETH_P_8021Q):
+               if (!pskb_may_pull(skb, skb_mac_offset(skb) + sizeof(*veth)))

validates this, after this patch this looks consistent.

> BUG: KMSAN: uninit-value in nf_flow_offload_inet_hook+0x7e4/0x940 net/netfilter/nf_flow_table_inet.c:27
>   nf_flow_offload_inet_hook+0x7e4/0x940 net/netfilter/nf_flow_table_inet.c:27
>   nf_hook_entry_hookfn include/linux/netfilter.h:157 [inline]
>   nf_hook_slow+0xe1/0x3d0 net/netfilter/core.c:623
>   nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
>   nf_ingress net/core/dev.c:5742 [inline]
>   __netif_receive_skb_core+0x4aff/0x70c0 net/core/dev.c:5837
>   __netif_receive_skb_one_core net/core/dev.c:5975 [inline]
>   __netif_receive_skb+0xcc/0xac0 net/core/dev.c:6090
>   netif_receive_skb_internal net/core/dev.c:6176 [inline]
>   netif_receive_skb+0x57/0x630 net/core/dev.c:6235
>   tun_rx_batched+0x1df/0x980 drivers/net/tun.c:1485
>   tun_get_user+0x4ee0/0x6b40 drivers/net/tun.c:1938
>   tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1984
>   new_sync_write fs/read_write.c:593 [inline]
>   vfs_write+0xb4b/0x1580 fs/read_write.c:686
>   ksys_write fs/read_write.c:738 [inline]
>   __do_sys_write fs/read_write.c:749 [inline]
> 
> Reported-by: syzbot+bf6ed459397e307c3ad2@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/686bc073.a00a0220.c7b3.0086.GAE@google.com/T/#u
> Fixes: 87b3593bed18 ("netfilter: flowtable: validate pppoe header")
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>

Thanks.