From a96c5299bd7305eb984377681eb3222a57376405 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Fri, 9 Oct 2026 23:51:56 +0200 Subject: [PATCH] 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. Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-6-7446d2d335e6@kernel.org Signed-off-by: Christian Brauner (Amutable) --- fs/unicode/utf8-core.c | 21 +++++++++++++++++---- include/linux/unicode.h | 1 + 2 files changed, 18 insertions(+), 4 deletions(-) diff --git a/fs/unicode/utf8-core.c b/fs/unicode/utf8-core.c index f313532f5e80..74a74a48dd8b 100644 --- a/fs/unicode/utf8-core.c +++ b/fs/unicode/utf8-core.c @@ -3,6 +3,7 @@ #include #include #include +#include #include #include #include @@ -183,12 +184,24 @@ struct unicode_map *utf8_load(unsigned int version) } EXPORT_SYMBOL(utf8_load); +static void utf8_unload_rcu(struct rcu_head *head) +{ + struct unicode_map *um = container_of(head, struct unicode_map, rcu); + + symbol_put(utf8_data_table); + kfree(um); +} + +/* + * A pathwalk in rcu mode may still be in ->d_hash() or ->d_compare() of a + * filesystem that is being torn down and read the map through its + * superblock, so the map and the tables it points to have to outlive the + * grace period. + */ void utf8_unload(struct unicode_map *um) { - if (um) { - symbol_put(utf8_data_table); - kfree(um); - } + if (um) + call_rcu(&um->rcu, utf8_unload_rcu); } EXPORT_SYMBOL(utf8_unload); diff --git a/include/linux/unicode.h b/include/linux/unicode.h index 64fa44fe180c..3a8cb603eb93 100644 --- a/include/linux/unicode.h +++ b/include/linux/unicode.h @@ -53,6 +53,7 @@ struct unicode_map { unsigned int version; const struct utf8data *ntab[UTF8_NMAX]; const struct utf8data_table *tables; + struct rcu_head rcu; }; int utf8_validate(const struct unicode_map *um, const struct qstr *str);