High [CVE-2026-53250] cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata

In the Linux kernel, the following vulnerability has been resolved: xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() The TX metadata area resides in the UMEM buffer which is memory-mapped and concurrently writable by userspace.

In xsk_skb_metadata(), csum_start and csum_offset are read from shared memory for bounds validation, then read again for skb assignment. A malicious userspace application can race to overwrite these values between the two reads, bypassing the bounds check and causing out-of-bounds memory access during checksum computation in the transmit path.

Fix this by reading csum_start and csum_offset into local variables once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launch_time) and the cached csum fields may be mutually inconsistent due to concurrent userspace writes, but this is benign: the only security-critical invariant is that each field's validated value is the same one used, which local caching guarantees.

A flaw was found in the Linux kernel's xsk (AF_XDP socket) component. A malicious local userspace application can exploit a Time-of-check to time-of-use (TOCTOU) race condition in the `xsk_skb_metadata()` function.

By modifying checksum-related values in shared memory between validation and use, an attacker can bypass security checks.