Cloudanix VulnDB

CVE-2026-99180

None

CVSS

—

EPSS

0.2%

Published

Oct 28, 2026

CISA due date

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Cloudanix analysis

Schedule a fix SSVC: Track

A Linux kernel bug can let a program read memory that has already been freed when a special kind of folder is being removed while another program is still looking inside it.

Exploitation
Local Privilege
Attacker needs
Local User
Complexity
Moderate
Impact
Privilege Gain
Component
OS kernel
Class
Use After Free
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Use-after-free when generic_ci_d_hash() and generic_ci_d_compare() dereference an sb->s_encoding unicode map under rcu_read_lock() after utf8_unload() has already freed it from put_super() in ext4, f2fs or tmpfs.

Why it matters to the business

Systems running recent Linux kernels with casefolded mounts (especially tmpfs) face a narrow window for kernel memory corruption during unmount races; widespread kernel usage makes prompt patching important even though the race is hard to trigger.

How an attack unfolds

An attacker triggers a path lookup that enters utf8_casefold_hash() while simultaneously causing the last reference to a casefolded filesystem to be dropped via close, chdir, or lazy unmount, racing the RCU grace period so the walker reads the now-freed map.

How to fix it

Update to a Linux kernel containing the RCU grace-period delay for utf8_unload() and the associated data-table module reference drop; rebuild and reboot affected systems.

How to confirm the fix worked

Confirm the kernel source contains the grace-period callback around utf8_unload() and that KASAN no longer reports slab-use-after-free in utf8nlookup() during concurrent unmount and lookup stress tests.

Typical attack path

  1. Mount a casefolded tmpfs or other supported filesystem
  2. Hold an open file descriptor or current working directory inside it
  3. Trigger unmount that drops the final superblock reference outside namespace_unlock()
  4. Perform a filename lookup that reaches generic_ci_d_hash() during the race window
  5. Read freed unicode map data via utf8nlookup()

How to detect it

Search kernel logs for KASAN or KMSAN reports citing utf8nlookup, utf8_casefold_hash or generic_ci_d_hash during filesystem teardown; examine audit logs for concurrent umount and open/lookup activity on casefolded mounts.

What your security team should do

  • Prioritize kernel updates on any system exposing casefolded tmpfs or ext4/f2fs mounts to untrusted users
  • Audit for custom kernels or embedded devices that enable CONFIG_UNICODE and ship casefolding support
  • Monitor upstream stable branches for the backported fix given the one-day publication age

The record

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.

Affected packages

Ecosystem Package Introduced Fixed