CVE-2026-53250

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.