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llc: reserve device headroom for allocated frames

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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: bf9ae5386b ("llc: use dev_hard_header")
Cc: stable@vger.kernel.org
Reported-by: VEGA <vega@nebusec.ai>
Signed-off-by: Zixuan Chai <petalzu987@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260924012613.2533934-1-weir@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Zixuan Chai 2026-09-24 09:26:05 +08:00 committed by Jakub Kicinski
parent 488089055b
commit 72b5b9a28b
1 changed files with 4 additions and 4 deletions
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8
net/llc/llc_sap.c
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@ -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) {