CVE-2026-31700

Summary

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/0f4c9754956b86de158a4af5278c5cf5bda9439e
• https://git.kernel.org/stable/c/714aa973da8163925eda7efd49361ccbee21ee46
• https://git.kernel.org/stable/c/1490f82353bdabc09265a74e645b07f05cf4188e
• https://git.kernel.org/stable/c/74e2db36fe50e3ad9d5300d7fd0e6e2a15a6d121
• https://git.kernel.org/stable/c/3a1bf9116ea31470b89692585c3910dfe830dcdd
• https://git.kernel.org/stable/c/28324a3b62d9ce7f9bdd65a8ce63f382041d1b27
• https://git.kernel.org/stable/c/48a6ef291a17639e1b6ae0fbe9c8b2bb87d7804b
• https://git.kernel.org/stable/c/2c054e17d9d41f1020376806c7f750834ced4dc5

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