Re: [PATCH net] net: neigh: Reallocate headroom if necessary in neigh_hh_bridge()

[Ido Schimmel <idosch@nvidia.com>]

On Fri, May 08, 2026 at 01:25:14PM +0200, Lorenzo Bianconi wrote:
> neigh_hh_bridge() assumes the skb always has sufficient headroom to copy
> the aligned  L2 header. This assumption can trigger the crash reported
> below using the following netfilter setup:
> 
> $modprobe br_netfilter
> $sysctl -w net.bridge.bridge-nf-call-iptables=1
> 
> $root@OpenWrt:~# nft list ruleset
> table ip nat {
>         chain prerouting {
>                 type nat hook prerouting priority dstnat; policy accept;
>                 ip daddr 192.168.83.123 dnat to 192.168.83.120
>         }
> }
> 
> - iperf3 client (192.168.83.119) --> bridge (192.168.83.118) --> iperf3 server (192.168.83.120)
> 
> the iperf3 client is sending packet for 192.168.83.123 to the bridge device.
> 
> [ 1579.036575] Unable to handle kernel write to read-only memory at virtual address ffffff8004d76ffe
> [ 1579.045482] Mem abort info:
> [ 1579.048273]   ESR = 0x000000009600004f
> [ 1579.052024]   EC = 0x25: DABT (current EL), IL = 32 bits
> [ 1579.057363]   SET = 0, FnV = 0
> [ 1579.060417]   EA = 0, S1PTW = 0
> [ 1579.063550]   FSC = 0x0f: level 3 permission fault
> [ 1579.068345] Data abort info:
> [ 1579.071224]   ISV = 0, ISS = 0x0000004f, ISS2 = 0x00000000
> [ 1579.076720]   CM = 0, WnR = 1, TnD = 0, TagAccess = 0
> [ 1579.081770]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> [ 1579.087092] swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000080dc4000
> [ 1579.093794] [ffffff8004d76ffe] pgd=180000009ffff003, p4d=180000009ffff003, pud=180000009ffff003, pmd=180000009ffe3003, pte=0060000084d76787
> [ 1579.106343] Internal error: Oops: 000000009600004f [#1] SMP
> [ 1579.193824] CPU: 0 UID: 0 PID: 235 Comm: napi/qdma_eth-3 Tainted: G           O       6.12.57 #0

AFAICT this driver does not reserve any headroom in skbs that it's
injecting to the Rx path. Is there a reason for that?

> [ 1579.202614] Tainted: [O]=OOT_MODULE
> [ 1579.206102] Hardware name: Airoha AN7581 Evaluation Board (DT)
> [ 1579.211929] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [ 1579.218889] pc : br_nf_pre_routing_finish_bridge+0x1ac/0xcc8 [br_netfilter]
> [ 1579.225859] lr : br_nf_pre_routing_finish_bridge+0x18c/0xcc8 [br_netfilter]
> [ 1579.232822] sp : ffffffc0817cba20
> [ 1579.236128] x29: ffffffc0817cba20 x28: 0000000000000000 x27: ffffff8002b89000
> [ 1579.243273] x26: ffffff8004d7700e x25: 0000000000000008 x24: 0000000000000000
> [ 1579.250416] x23: ffffffc08179d4c0 x22: 0000000000000000 x21: ffffffc08179d4c0
> [ 1579.257561] x20: ffffff8004d9b800 x19: ffffff8015010000 x18: 0000000000000014
> [ 1579.264704] x17: ffffffbf9e930000 x16: ffffffc0817c8000 x15: 0000000000000070
> [ 1579.271848] x14: 0000000000000080 x13: 0000000000000001 x12: 0000000000000000
> [ 1579.278993] x11: ffffffc0798caae0 x10: ffffff8014db6fd8 x9 : 0000000000000000
> [ 1579.286136] x8 : 0000000000000003 x7 : ffffffc08171f628 x6 : 000000001a3b83d3
> [ 1579.293281] x5 : 0000000000000000 x4 : 1beb76f22fee0000 x3 : ffffff8004d7700e
> [ 1579.300425] x2 : 0000000000000000 x1 : ffffff8004d9b8bc x0 : ffffff80026ed000
> [ 1579.307570] Call trace:
> [ 1579.310018]  br_nf_pre_routing_finish_bridge+0x1ac/0xcc8 [br_netfilter]
> [ 1579.316632]  br_nf_hook_thresh+0xd4/0x14bc [br_netfilter]
> [ 1579.322032]  br_nf_hook_thresh+0x250/0x14bc [br_netfilter]
> [ 1579.327517]  br_nf_hook_thresh+0x76c/0x14bc [br_netfilter]
> [ 1579.333003]  br_handle_frame+0x180/0x480
> [ 1579.336935]  __netif_receive_skb_core.constprop.0+0x540/0xf40
> [ 1579.342682]  __netif_receive_skb_one_core+0x28/0x50
> [ 1579.347561]  process_backlog+0x98/0x1e0
> [ 1579.351398]  __napi_poll+0x34/0x1c4
> [ 1579.354887]  net_rx_action+0x178/0x330
> [ 1579.358638]  handle_softirqs+0x108/0x2d4
> [ 1579.362560]  __do_softirq+0x10/0x18
> [ 1579.366051]  ____do_softirq+0xc/0x20
> [ 1579.369627]  call_on_irq_stack+0x30/0x4c
> [ 1579.373550]  do_softirq_own_stack+0x18/0x20
> [ 1579.377734]  do_softirq+0x4c/0x60
> [ 1579.381050]  __local_bh_enable_ip+0x88/0x98
> [ 1579.385234]  napi_threaded_poll_loop+0x188/0x21c
> [ 1579.389853]  napi_threaded_poll+0x70/0x80
> [ 1579.393863]  kthread+0xd8/0xdc
> [ 1579.396918]  ret_from_fork+0x10/0x20
> [ 1579.400499] Code: 88dffc22 3707ffc2 f9406663 f9406684 (f81f0064)
> [ 1579.406589] ---[ end trace 0000000000000000 ]---
> [ 1579.411209] Kernel panic - not syncing: Oops: Fatal exception in interrupt
> [ 1579.418083] SMP: stopping secondary CPUs
> [ 1579.422012] Kernel Offset: disabled
> 
> Fix the issue reallocating the skb headroom if necessary in neigh_hh_bridge routine.
> 
> Fixes: e179e6322ac33 ("netfilter: bridge-netfilter: Fix MAC header handling with IP DNAT")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  include/net/neighbour.h         | 15 +++++++++++----
>  net/bridge/br_netfilter_hooks.c |  5 ++++-
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index 2dfee6d4258a..4e1222968753 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -487,16 +487,23 @@ static inline int neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
>  }
>  
>  #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
> -static inline int neigh_hh_bridge(struct hh_cache *hh, struct sk_buff *skb)
> +static inline struct sk_buff *
> +neigh_hh_bridge(struct hh_cache *hh, struct sk_buff *skb)
>  {
> -	unsigned int seq, hh_alen;
> +	unsigned int seq, hh_alen = HH_DATA_ALIGN(ETH_HLEN);
> +
> +	if (unlikely(skb_headroom(skb) < hh_alen)) {
> +		skb = skb_expand_head(skb, hh_alen);
> +		if (!skb)
> +			return NULL;
> +	}

The comment from Sashiko looks relevant:

Does this adequately protect against writing to shared or cloned SKBs?

If a cloned SKB already has sufficient headroom, this check evaluates to
false, and the code proceeds to overwrite the MAC header via memcpy().
Modifying a cloned SKB without unsharing it could corrupt the data for
other users of the buffer, or still trigger the read-only memory panic
this patch aims to fix.

Should this use skb_cow_head() or explicitly check skb_shared() and
skb_cloned() before modifying the buffer data?

>  
>  	do {
>  		seq = read_seqbegin(&hh->hh_lock);
> -		hh_alen = HH_DATA_ALIGN(ETH_HLEN);
>  		memcpy(skb->data - hh_alen, hh->hh_data, ETH_ALEN + hh_alen - ETH_HLEN);
>  	} while (read_seqretry(&hh->hh_lock, seq));
> -	return 0;
> +
> +	return skb;
>  }
>  #endif
>  
> diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
> index 0ab1c94db4b9..6b59d7eb7906 100644
> --- a/net/bridge/br_netfilter_hooks.c
> +++ b/net/bridge/br_netfilter_hooks.c
> @@ -297,7 +297,10 @@ int br_nf_pre_routing_finish_bridge(struct net *net, struct sock *sk, struct sk_
>  				goto free_skb;
>  			}
>  
> -			neigh_hh_bridge(&neigh->hh, skb);
> +			skb = neigh_hh_bridge(&neigh->hh, skb);
> +			if (!skb)
> +				return -ENOMEM;
> +

Also from Sashiko:

Does returning early here leak the neighbour reference?

Earlier in br_nf_pre_routing_finish_bridge(), a reference to neigh is
obtained via dst_neigh_lookup_skb(dst, skb). By returning -ENOMEM here,
we bypass the neigh_release(neigh) call at the end of the if (neigh) block.

Could this cause the neighbour reference count to leak, eventually preventing
the network device from being unregistered?

>  			skb->dev = br_indev;
>  
>  			ret = br_handle_frame_finish(net, sk, skb);
> 
> ---
> base-commit: fcee7d82f27d6a8b1ddc5bbefda59b4e441e9bc0
> change-id: 20260508-nf-neigh_hh_bridge-fix-9ab775ee23c6
> 
> Best regards,
> -- 
> Lorenzo Bianconi <lorenzo@kernel.org>
>