CVE-2026-99033

In the Linux kernel, the following vulnerability has been resolved:

net/packet: guard the ll header push in packet_rcv_spkt()

packet_rcv_spkt() restores the link layer header with

skb_push(skb, skb->data - skb_mac_header(skb));

That subtraction is only meaningful when the device actually has a link layer header. packet_rcv() and tpacket_rcv() both wrap it in dev_has_header(), which is also the predicate the block comment at the top of the file states the restore in terms of; packet_rcv_spkt() does not. commit d549699048b4 (net/packet: fix packet receive on L3 devices without visible hard header) introduced the helper and changed the two call sites, and this one stayed behind.

A device without a visible ll header can leave skb->mac_header at the 0xFFFF sentinel that _alloc_skb() initialises it to. A CAN skb does: init_can_skb() sets pkt_type and ip_summed but does not reset the headers, and commit 9f10374bb024 (can: remove private CAN skb headroom infrastructure) dropped the skb_reset*_header() calls that used to be there. skb_mac_header() is then 0xFFFF, the length becomes a large negative number and skb_push() reports it through skb_under_panic() – from softirq context, so it is a full system panic even with panic_on_oops=0:

skbuff: skb_under_panic: text:ffffffff8bd21bc1 len:-65455 put:-65471 head:… data:… tail:0x50 end:0x180 dev:can0 kernel BUG at net/core/skbuff.c:214! RIP: 0010:skb_panic+0x50/0x60 Call Trace: skb_push+0x38/0x40 packet_rcv_spkt+0xe1/0x170 __netif_receive_skb_core.constprop.0+0x7e8/0xd30 … Kernel panic - not syncing: Fatal exception in interrupt

The socket type is reachable: packet_create() accepts SOCK_PACKET alongside SOCK_RAW and SOCK_DGRAM behind the same CAP_NET_RAW check, and neither the socket length nor a capability check keeps it away from a CAN interface.

The missing skb_reset_*_header() calls in init_can_skb() are a regression in their own right and are being fixed separately, but a packet socket should not turn a link layer that did not initialise its mac header into a kernel panic. Guard the push the way the other two receive paths do.

Tested on v7.3-rc5 under QEMU with a slcan device on a pty, which is the driver RX path: vcan does not reproduce it, because can_send() resets the headers on the way out. One SOCK_PACKET socket bound to can0 and one frame written into the line discipline panics an unpatched kernel with the trace above; the same image with this patch prints no panic and powers off normally. Both kernels are this tree, defconfig plus CONFIG_CAN_SLCAN=y, differing only in this hunk.

Affected Software

References

• https://git.kernel.org/stable/c/00fbf71e74115f7628fc0880b855434b1e70c40c
• https://git.kernel.org/stable/c/28bc1ef699610ee09ce3d46a00552f5a0a0144bd
• https://git.kernel.org/stable/c/31c7b15b3716e8d0dad58d2c3cdf321dfd06f4be
• https://git.kernel.org/stable/c/3efe0e0944157fdd2072b4dc7eb0491a317ed272
• https://git.kernel.org/stable/c/48aa1155fa8d83362fc33d02d527a3c3fe40ce1c
• https://git.kernel.org/stable/c/7f84ed94ccfe36091234a2891b88356dc06e6d66
• https://git.kernel.org/stable/c/b880df0e487caa85c3c7ffda37dcd05334b1aa80
• https://git.kernel.org/stable/c/d8a24b9e1774a93943caf68d48c5e17be432220b