CVE-2026-99180

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

unicode: free the map after a grace period

generic ci d hash() and generic ci d compare() read sb->s encoding and dereference the unicode map while a pathwalk in rcu mode holds nothing but rcu read lock(). The filesystems that use them, ext4, f2fs and tmpfs, drop the map with utf8 unload() from ->put super(), which runs when the last reference to the mount goes away. A lookup that is still inside such a filesystem at that point reads a freed map.

With a pathwalk held up in utf8 casefold hash() while the last reference to a casefolded tmpfs goes away:

BUG: KASAN: slab-use-after-free in utf8nlookup+0x74c/0x950 Read of size 8 at addr ff110001030fe658 by task walker/98 Call Trace: utf8nlookup+0x74c/0x950 utf8byte+0x160/0x1560 utf8 casefold hash+0x167/0x230 generic ci d hash+0xdd/0x1c0 link path walk+0x11ad/0x1d30 path lookupat+0x63/0xb00 filename lookup+0x1f1/0x5b0 vfs statx+0xd7/0x3b0 [...] Allocated by task 94: kmalloc cache noprof+0x249/0x560 utf8 load+0x4e/0x570 shmem parse opt casefold.isra.0+0x92/0x270 [...] Freed by task 97: kfree+0x1db/0x5b0 shmem put super+0x72/0x170 generic shutdown super+0x14c/0x530 kill anon super+0x3a/0x60 deactivate locked super+0x9c/0xf0 cleanup mnt+0x9f/0x130

So far that has been hard to hit. umount(2) waits for a grace period in namespace unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker. So it is reachable.

Delay the free until a grace period has passed. The map and the data table it holds a reference to then outlive any pathwalk that found the superblock before the mount went away. The reference on the data table module is dropped from the rcu callback as well. utf8-core is built in whenever CONFIG UNICODE is set, so there is no module exit that would have to wait for the callbacks.

Overlayfs points the s encoding of an overlay at the map of its first layer without taking a reference of its own. That stays correct. The layer is held by the overlay, so its map is dropped only after the overlay is gone.