From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C49AB3EEAF4; Fri, 9 Oct 2026 21:52:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791582738; cv=none; b=V1r2szxiLkOZ1rttlCsBDkJb1wNNsVNth91CfmiL6VcsHliOORC4RExVqsizVY0eXIy9iKxrRWlLb0nEoKK4NbnZyvCh+Hn8/Qv+DLgLkO1J5Swb29B86XJsBl8R4ouNRgYr3xs39piWb+1/ft8bCrCuCiMF5JmYehBFyASVQ84= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791582738; c=relaxed/simple; bh=nBEmiiEoAfQb3/RPfTImmOxn8PBxV0P99lY6N2vclYw=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=LkmUgImLMUbFa36PItvd6+0d/Z6WfkzvLL7CcJPbzLlg9btKy1hFuIuPA+0kFXVDspOjNEOI2hTTmJ0VnQuvm3i4Yeq2BH061H9LYm1tzwK+sQQJLo6W0VoCbR0OW8D3ea5H2J76sLWZDUXraOirqK7ELFS4iSJPHc+XMgwVGyg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Bkb5mNQr; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Bkb5mNQr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 56D311F00893; Fri, 9 Oct 2026 21:52:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791582737; bh=M7QuAsYvBBUyOh4F7fA0Cqn6o4QP0E6xrLJ+y+EX3qw=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=Bkb5mNQrENr/KzawpnhRJOic9aO+817sW3hto+0q3XDSDYcrXmatcsiiCwPO5d1Nr HHGR0rgnjJw+WquXKKY0UW3/89q3eEngQu6wfSo0/r/vCLHYTaGFuvcffvI6dqGE23 GN/gGP5V/XrHufuoSBH9oZfVXBqoid7JeWZC6UQUPkm2LflHYyaFaatsWkKiTVxB2o OcdDcGDq5LM4T7KnBOeq38h1OhYM8gxJEx0kWbOsgrhxTaDGSBjl6rn/9j4aYDlI84 g4mVPFpbCYkGVzdcNF1raCpA/L/w6jXY6oLmRbAsxUyBvaMpsmpMc1f2UrQfpdB8KK rsmMLd3hJ/AMg== From: Christian Brauner Date: Fri, 09 Oct 2026 23:51:56 +0200 Subject: [PATCH 6/8] unicode: free the map after a grace period Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261009-work-fixes-rcu-v1-6-7446d2d335e6@kernel.org> References: <20261009-work-fixes-rcu-v1-0-7446d2d335e6@kernel.org> In-Reply-To: <20261009-work-fixes-rcu-v1-0-7446d2d335e6@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: Alexander Viro , Jan Kara , linux-ext4@vger.kernel.org, linux-afs@lists.infradead.org, v9fs@lists.linux.dev, linux-trace-kernel@vger.kernel.org, ntfs3@lists.linux.dev, "Christian Brauner (Amutable)" , stable@vger.kernel.org X-Mailer: b4 0.17-dev-3d1b8 X-Developer-Signature: v=1; a=openpgp-sha256; l=6529; i=brauner@kernel.org; h=from:subject:message-id; bh=nBEmiiEoAfQb3/RPfTImmOxn8PBxV0P99lY6N2vclYw=; b=owGbwMvMwCU28Zj0gdSKO4sYT6slMWSdTPwrolTeJyk8baJ4W5Pr1hM7RHy7ckwSd0jKhNekf ynNnGzeUcrCIMbFICumyOLQbhIut5ynYrNRpgbMHFYmkCEMXJwCMJGVCxkZtus9l53zwya/VJcz 0f5ZlfwjW72DKmrXfiW2Wm+4IrCdjZGh961EkdpWt9y/RlqbrvjILT7QfCX1TM2RvRcypCz2slx gAwA= X-Developer-Key: i=brauner@kernel.org; a=openpgp; fpr=4880B8C9BD0E5106FC070F4F7B3C391EFEA93624 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 Signed-off-by: Christian Brauner (Amutable) --- Reproducer: delay.c holds a task named walker in utf8_casefold_hash() in rcu pathwalk mode for two seconds and repro.c drops the last reference to a lazily unmounted casefolded tmpfs meanwhile. mount -t tmpfs -o casefold tmpfs /mnt chattr +F /mnt echo hello > /mnt/Foo insmod delay.ko ./repro Without this patch that gives the splat above, with it it runs clean. delay.c: #include #include #include /* keep a task named "walker" in utf8_casefold_hash() in rcu pathwalk */ static int pre(struct kprobe *p, struct pt_regs *regs) { if (!strcmp(current->comm, "walker")) mdelay(2000); return 0; } static struct kprobe kp = { .symbol_name = "utf8_casefold_hash", .pre_handler = pre, }; static int __init delay_init(void) { return register_kprobe(&kp); } static void __exit delay_exit(void) { unregister_kprobe(&kp); } module_init(delay_init); module_exit(delay_exit); MODULE_LICENSE("GPL"); repro.c: #define _GNU_SOURCE #include #include #include #include #include #include #include #include static void pin(int cpu) { cpu_set_t set; CPU_ZERO(&set); CPU_SET(cpu, &set); sched_setaffinity(0, sizeof(set), &set); } static void *walker(void *arg) { struct stat st; char *p; /* * The delay spins. Empty this cpu's lru batches so that an * lru_add_drain_all() on the other side has no work to wait for here. */ pin(1); p = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); memset(p, 1, 4096); madvise(p, 4096, MADV_COLD); prctl(PR_SET_NAME, "walker"); stat("FOO", &st); return NULL; } int main(void) { struct stat st; pthread_t t; pin(0); chdir("/mnt"); stat("FOO", &st); /* the cwd now holds the last reference to the mount */ umount2("/mnt", MNT_DETACH); pthread_create(&t, NULL, walker, NULL); sleep(1); /* drop it, ->kill_sb() runs before chdir() returns */ chdir("/"); pthread_join(t, NULL); return 0; } --- 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); -- 2.53.0