CVE-2025-39840: audit: fix out-of-bounds read in audit_compare_dname_path()

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

audit: fix out-of-bounds read in auditcomparednamepath()

When a watch on dir=/ is combined with an fsnotify event for a single-character name directly under / (e.g., creating /a), an out-of-bounds read can occur in auditcomparednamepath().

The helper parentlen() returns 1 for "/". In auditcomparednamepath(), when parentlen equals the full path length (1), the code sets p = path + 1 and pathlen = 1 - 1 = 0. The subsequent loop then dereferences p[pathlen - 1] (i.e., p[-1]), causing an out-of-bounds read.

Fix this by adding a pathlen > 0 check to the while loop condition to prevent the out-of-bounds access.

[PM: subject tweak, sign-off email fixes]

— NVD