> 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?yep, right. I have already proposed a fix for the driver here: https://lore.kernel.org/netdev/[email protected]/ but I guess it worths to fix neigh_hh_bridge() as well. > > > [ 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 <[email protected]> > > --- > > 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? ack, I will fix it in v2. > > > > > 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? ack, I will fix it in v2. Regards, Lorenzo > > > 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 <[email protected]> > >
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