CVE-2026-74567: keys: fix out-of-bounds read in keyring_get_key_chunk()

In the Linux kernel, the following vulnerability has been resolved:

keys: fix out-of-bounds read in keyringgetkeychunk()

For description-level chunks keyringgetkeychunk() advances the read pointer by level sizeof(long) past the inline prefix but only bounds-checks the prefix, so a long enough key description is read past its kmemdup(desc, desclen + 1) allocation. Compute the full byte offset and bounds-check the description against it before reading.

The walk only reaches a description-level chunk when two keys collide through the hash, x, type and domaintag chunks, so this is reached from an unprivileged addkey(2) with a crafted pair of same-type keys whose index hashes collide; KASAN reports a slab-out-of-bounds read.