CVE-2026-31700
HighCVSS 7.8Exploitation Probability (EPSS)
Low risk1th percentile - higher than 1% of all known CVEs
Summary
A vulnerability has been identified in the Linux kernel in the tpacket_snd() function, allowing for a TOCTOU race condition using mmap'd vnet_hdr. As a result, a user-space thread can modify vnet_hdr fields between validation and use, bypassing all safety checks.
Risk Assessment
This vulnerability could lead to unauthorized memory access and potential system control takeover by malicious code running in user space.
Recommendation
It is recommended to update the Linux kernel to the latest version where a fix has been implemented to copy vnet_hdr from the mmap'd ring buffer to a stack-local variable before validation and use.
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Original NVD description (English source)
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.