[PATCH net v2 1/1] llc: reserve device headroom for allocated frames

[Ren Wei <weir@nebusec.ai>]

From: Zixuan Chai <petalzu987@gmail.com>

llc_alloc_frame() reserves link-layer headroom using the device type.
This is insufficient for stacked Ethernet devices such as VLAN devices,
where vlan_dev_hard_header() pushes a VLAN header before the lower
device's Ethernet header. An LLC response on such a device can
therefore underflow skb headroom in eth_header().

Use LL_RESERVED_SPACE() to account for the device's actual required
headroom while preserving the existing LLC device-type check.

Fixes: bf9ae5386bca ("llc: use dev_hard_header")
Cc: stable@vger.kernel.org
Reported-by: VEGA <vega@nebusec.ai>
Assisted-by: LLM
Signed-off-by: Zixuan Chai <petalzu987@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/llc/llc_sap.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/llc/llc_sap.c b/net/llc/llc_sap.c
index 1bd446a21092..3904a1b4ba84 100644
--- a/net/llc/llc_sap.c
+++ b/net/llc/llc_sap.c
@@ -19,12 +19,12 @@
 #include <linux/llc.h>
 #include <linux/slab.h>
 
-static int llc_mac_header_len(unsigned short devtype)
+static int llc_mac_header_len(struct net_device *dev)
 {
-	switch (devtype) {
+	switch (dev->type) {
 	case ARPHRD_ETHER:
 	case ARPHRD_LOOPBACK:
-		return sizeof(struct ethhdr);
+		return LL_RESERVED_SPACE(dev);
 	}
 	return 0;
 }
@@ -45,7 +45,7 @@ struct sk_buff *llc_alloc_frame(struct sock *sk, struct net_device *dev,
 	int hlen = type == LLC_PDU_TYPE_U ? 3 : 4;
 	struct sk_buff *skb;
 
-	hlen += llc_mac_header_len(dev->type);
+	hlen += llc_mac_header_len(dev);
 	skb = alloc_skb(hlen + data_size, GFP_ATOMIC);
 
 	if (skb) {
-- 
2.34.1