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

fsnotify: Fix stale object mask after concurrent mark updates

When a mark gets a new event bit, fanotify and inotify may avoid
recalculating the object mask if the cached aggregate already contains that
bit. This is racy with a recalculation triggered by a concurrent update to
another mark on the same connector.

The concurrent scan can read the mark before the new bit is added, while
the updater reads the old aggregate before that scan publishes its result.
The updater then skips recalculation and the scan publishes a mask without
the bit, leaving the object mask stale after both updates complete.

This can be reproduced with two fanotify groups watching the same inode:
one thread removes FAN_MODIFY from one existing mark while another thread
adds FAN_MODIFY to the other mark. After both fanotify_mark() calls return,
writes can fail to produce FAN_MODIFY for the group whose mark now contains
the bit. This was reproduced on an unmodified v6.12.95 kernel. The
equivalent inotify interleaving loses IN_MODIFY events.

For normal fanotify additions, recalculate whenever the raw mark mask
changes. The normal mask is not cleared asynchronously, so an unchanged
addition cannot introduce missing interest. Always recalculate ignore-mask
updates because FS_MODIFY handling may clear the ignore mask without taking
mark->lock, making snapshot comparisons unreliable.

Always recalculate after updating an existing inotify watch. Its replace
path temporarily sets mark->mask to zero, so a concurrent scan can observe
zero even when the old and final masks are equal. Assigning the replacement
mask directly would avoid the transient zero, but existing-watch updates
are infrequent, so unconditional recalculation is simpler.