CVE-2026-31700

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

net/packet: fix TOCTOU race on mmap'd vnet_hdr in tpacket_snd()

In tpacket_snd(), when PACKET_VNET_HDR is enabled, vnet_hdr points

directly into the mmap'd TX ring buffer shared with userspace. The

kernel validates the header via __packet_snd_vnet_parse() but then

re-reads all fields later in virtio_net_hdr_to_skb(). A concurrent

userspace thread can modify the vnet_hdr fields between validation

and use, bypassing all safety checks.

The non-TPACKET path (packet_snd()) already correctly copies vnet_hdr

to a stack-local variable. All other vnet_hdr consumers in the kernel

(tun.c, tap.c, virtio_net.c) also use stack copies. The TPACKET TX

path is the only caller of virtio_net_hdr_to_skb() that reads directly

from user-controlled shared memory.

Fix this by copying vnet_hdr from the mmap'd ring buffer to a

stack-local variable before validation and use, consistent with the

approach used in packet_snd() and all other callers.