Linux Kernel TOCTOU Race Condition Vulnerability in TPACKET Transmission Path

A time-of-check to time-of-use (TOCTOU) race condition vulnerability has been identified in the Linux kernel's TPACKET transmission path. When the PACKET_VNET_HDR option is enabled, the virtual network header (vnet_hdr) is mapped directly to the user-space shared memory of the transmit (TX) ring buffer. The kernel initially validates this header but subsequently re-reads the fields from the shared memory, creating a window where a concurrent user-space thread could alter the header information, bypassing safety checks. This issue is unique to the TPACKET TX path, as other components that use vnet_hdr rely on stack copies, not direct memory mappings.

Impact

Exploitation of this vulnerability allows for a TOCTOU race condition, where a user-space thread can manipulate the vnet_hdr fields between validation and usage, potentially leading to unintended behavior in packet transmission.

Reproduction

The vulnerability can be reproduced by enabling PACKET_VNET_HDR and sending packets through a TPACKET socket while concurrently modifying the vnet_hdr fields in the user-space application. This can be done by creating a race condition between the packet validation and the subsequent processing, taking advantage of the direct memory mapping from the user-space to the kernel.

Remediation

The vulnerability has been fixed by modifying the TPACKET transmission function to copy the virtual network header from the mmap'd ring buffer to a local stack variable before validation and use. Users should update to the latest version of the Linux kernel where this fix has been applied.