CVE-2026-99180

Status: UPDATED  |  Advisory ID: CVE-2026-99180

Key Details

Affected Products, Subsystems & Sectors

What to Know

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:

<TASK>

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. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References