[PATCH net] net: bridge: use one headroom snapshot for ND replies

[PATCH net] net: bridge: use one headroom snapshot for ND replies

[Jun Yang]

From: Jun Yang <junvyyang@tencent.com>

br_nd_send() calls LL_RESERVED_SPACE(dev) once to size the reply skb and
again to reserve its headroom. When a port requiring more headroom is
added to the bridge, br_add_if() can increase dev->needed_headroom
between the two calls. The reserved headroom can then exceed the space
accounted for by alloc_skb().

Take one headroom snapshot and use it for both the allocation size and
skb_reserve().

Fixes: ed842faeb2bd ("bridge: suppress nd pkts on BR_NEIGH_SUPPRESS ports")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Assisted-by: tencentos-corvus-ai:hy4-preview
Signed-off-by: Jun Yang <junvyyang@tencent.com>
---
A KASAN reproducer for this issue is available if requested.

 net/bridge/br_arp_nd_proxy.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_arp_nd_proxy.c b/net/bridge/br_arp_nd_proxy.c
index 87a40e2f1c50..985ed43f7e30 100644
--- a/net/bridge/br_arp_nd_proxy.c
+++ b/net/bridge/br_arp_nd_proxy.c
@@ -262,6 +262,7 @@ static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
 	struct sk_buff *reply;
 	struct ipv6hdr *pip6;
 	int na_olen = 8; /* opt hdr + ETH_ALEN for target */
+	int headroom;
 	int ns_olen;
 	u8 *daddr;
 	bool dad;
@@ -271,7 +272,8 @@ static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
 	if (!dev)
 		return;
 
-	len = LL_RESERVED_SPACE(dev) + sizeof(struct ipv6hdr) +
+	headroom = LL_RESERVED_SPACE(dev);
+	len = headroom + sizeof(struct ipv6hdr) +
 		sizeof(*na) + na_olen + dev->needed_tailroom;
 
 	reply = alloc_skb(len, GFP_ATOMIC);
@@ -280,7 +282,7 @@ static void br_nd_send(struct net_bridge *br, struct net_bridge_port *p,
 
 	reply->protocol = htons(ETH_P_IPV6);
 	reply->dev = dev;
-	skb_reserve(reply, LL_RESERVED_SPACE(dev));
+	skb_reserve(reply, headroom);
 	skb_push(reply, sizeof(struct ethhdr));
 	skb_set_mac_header(reply, 0);
 

base-commit: ffdf678dd16610e3f006dac904bd37c09684673c
-- 
2.54.0 (Apple Git-157)

Re: [PATCH net] net: bridge: use one headroom snapshot for ND replies

[Ido Schimmel]

On Tue, Nov 03, 2026 at 03:54:37PM +0800, Jun Yang wrote:
> From: Jun Yang <junvyyang@tencent.com>
> 
> br_nd_send() calls LL_RESERVED_SPACE(dev) once to size the reply skb and
> again to reserve its headroom. When a port requiring more headroom is
> added to the bridge, br_add_if() can increase dev->needed_headroom
> between the two calls. The reserved headroom can then exceed the space
> accounted for by alloc_skb().
> 
> Take one headroom snapshot and use it for both the allocation size and
> skb_reserve().
> 
> Fixes: ed842faeb2bd ("bridge: suppress nd pkts on BR_NEIGH_SUPPRESS ports")
> Cc: stable@kernel.org
> Reported-by: TencentOS Corvus AI <corvus@tencent.com>
> Assisted-by: tencentos-corvus-ai:hy4-preview

This tag is deprecated since commit 1e035297ffda ("docs: remove
Assisted-by references").

> Signed-off-by: Jun Yang <junvyyang@tencent.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>