CVE-2026-31700

About vulnerability

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. A flaw was found in the Linux kernel. A Time-of-check to Time-of-use (TOCTOU) race condition exists in the tpacket_snd() function when PACKET_VNET_HDR is enabled. A local user can exploit this by modifying the vnet_hdr fields in the mmap’d TX ring buffer between validation and use, thereby bypassing safety checks. This could allow a local attacker to achieve a security bypass.

Live-patch status from KernelCare for each operating system.