CVE-2026-99175: fsnotify: prevent mark additions during connector teardown

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

fsnotify: prevent mark additions during connector teardown

fsnotifydestroymarks() keeps a reference to its current mark while it drops conn->lock to destroy the mark. This stabilizes the forward-walk cursor, but does not prevent a new mark from being inserted before it. For example, an inotify watch re-added after INIGNORED sorts before the old, detached mark for the same group. The walk can miss the replacement and then detach the entire connector from the object.

The replacement is still on its group's markslist and, for inotify, in the watch IDR. The group mutex no longer protects the connector's object from disappearing. An unprivileged fdinfo read racing the final detach can consequently call igrab(NULL):

BUG: KASAN: null-ptr-deref in igrab+0x24/0x100 Write of size 4 at addr 0000000000000148 Call Trace: igrab+0x24/0x100 inotifyshowfdinfo+0x1dc/0x5e0 seqshow+0x397/0x690

Set a DESTROYING flag under conn->lock before starting the walk, and reject new marks on that connector with -ENOENT. The existing add-mark error paths undo the temporary group-list entry and the backend's watch allocation. Do not wait for teardown while holding the group mutex, which the destroyer needs. Once the old connector is detached, an add can install a fresh connector as before.

This closes the insertion race for all object types using the common connector teardown, without changing the mark reference or SRCU lifetime rules.