CVE-2026-99180

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> unicode: free the map after a grace period<br /> <br /> generic_ci_d_hash() and generic_ci_d_compare() read sb-&gt;s_encoding and<br /> dereference the unicode map while a pathwalk in rcu mode holds nothing<br /> but rcu_read_lock(). The filesystems that use them, ext4, f2fs and<br /> tmpfs, drop the map with utf8_unload() from -&gt;put_super(), which runs<br /> when the last reference to the mount goes away. A lookup that is still<br /> inside such a filesystem at that point reads a freed map.<br /> <br /> With a pathwalk held up in utf8_casefold_hash() while the last reference<br /> to a casefolded tmpfs goes away:<br /> <br /> BUG: KASAN: slab-use-after-free in utf8nlookup+0x74c/0x950<br /> Read of size 8 at addr ff110001030fe658 by task walker/98<br /> Call Trace:<br /> <br /> utf8nlookup+0x74c/0x950<br /> utf8byte+0x160/0x1560<br /> utf8_casefold_hash+0x167/0x230<br /> generic_ci_d_hash+0xdd/0x1c0<br /> link_path_walk+0x11ad/0x1d30<br /> path_lookupat+0x63/0xb00<br /> filename_lookup+0x1f1/0x5b0<br /> vfs_statx+0xd7/0x3b0<br /> [...]<br /> Allocated by task 94:<br /> __kmalloc_cache_noprof+0x249/0x560<br /> utf8_load+0x4e/0x570<br /> shmem_parse_opt_casefold.isra.0+0x92/0x270<br /> [...]<br /> Freed by task 97:<br /> kfree+0x1db/0x5b0<br /> shmem_put_super+0x72/0x170<br /> generic_shutdown_super+0x14c/0x530<br /> kill_anon_super+0x3a/0x60<br /> deactivate_locked_super+0x9c/0xf0<br /> cleanup_mnt+0x9f/0x130<br /> <br /> So far that has been hard to hit. umount(2) waits for a grace period<br /> in namespace_unlock() before it drops the mount, so only a mount whose<br /> last reference goes away elsewhere, like the close of a file or a chdir<br /> out of a lazily unmounted directory, can race a walker. So it is<br /> reachable.<br /> <br /> Delay the free until a grace period has passed. The map and the data<br /> table it holds a reference to then outlive any pathwalk that found the<br /> superblock before the mount went away. The reference on the data table<br /> module is dropped from the rcu callback as well. utf8-core is built in<br /> whenever CONFIG_UNICODE is set, so there is no module exit that would<br /> have to wait for the callbacks.<br /> <br /> Overlayfs points the s_encoding of an overlay at the map of its first<br /> layer without taking a reference of its own. That stays correct. The<br /> layer is held by the overlay, so its map is dropped only after the<br /> overlay is gone.