Re: [PATCH] bpf: Unshare cloned skb before devmap egress XDP program

[Menglong Dong]

On 2026/6/9 18:02 Sun Jian <sun.jian.kdev@xxxxxxxxx> write:
> dev_map_redirect_clone() uses skb_clone() when redirecting a generic XDP
> skb to multiple devmap destinations. The cloned skb can share packet data
> with other clones.
> 
> If the destination devmap entry has an egress XDP program, that program
> can modify packet data. Such modifications can then be observed by other
> clones sharing the same packet data.
> 
> This can be reproduced by strengthening xdp_veth_egress to configure a
> different source MAC for each egress device and checking that store_mac_1/2
> observe the MAC configured for their own egress devices. Without the fix,
> the SKB_MODE subtest observes store_mac_1 receiving the MAC configured for
> the next egress device.
> 
> Fix this by unsharing the cloned skb before running the devmap egress XDP
> program. Limit the extra copy to destinations with an attached egress
> program.

Hi, Jian.

This sounds like a good idea in this case. When I have a look at bpf_clone_redirect(),
I found that it use skb_clone() too, which means it has the same problem. The
data can be modified by other xdp prog in the destination NIC if we use
bpf_clone_redirect().

So maybe this is the default logic, and I'm not sure if this patch can break the
existing users :/

Thanks!
Menglong Dong

> 
> Tested with:
>   ./test_progs -t xdp_veth_egress
>   ./test_progs -t xdp_veth
>   ./test_progs -t xdp
[...]
>  
>  destroy_xdp_redirect_map:
> -- 
> 2.43.0
> 
> 
>