From 3e271a27c46fd591d1011c7de5a644a41e2010f1 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?G=C3=BCnther=20Noack?= Date: Tue, 6 Oct 2026 12:58:47 +0200 Subject: [PATCH] landlock: Fix superblock use-after-free in release_inode() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Without this fix, release_inode() (task 1) can race with a umount operation in hook_sb_delete() (task 2). The order of operations in which the race occurs is: * task 2: hook_sb_delete() arrives at the end of the function, waiting for landlock_superblock(sb)->inode_refs to become zero. * task 1: release_inode(): does atomic_long_dec_and_test(), decrementing inode_refs to zero. * task 2: gets woken up through some other means, sees that inode_refs is zero and returns. The caller is the unmount operation and proceeds to release the superblock. * task 1: Runs wake_up_var(&landlock_superblock(sb)->inode_refs), *dereferencing the sb_security on the freed-up superblock*. Notably, wake_up_var() does not dereference the pointer passed to it, and only uses it to determine an overapproximated set of tasks to wake up. So while it's potentially possible for an attacker to provide a different value for that pointer, this only results in the wrong set of tasks getting woken up, and in this scenario, no further tasks are actually waiting. Nevertheless, KASAN detects it as a slab-use-after-free read on sb->s_security: BUG: KASAN: slab-use-after-free in release_inode+0x196/0x290 Read of size 8 at addr ffff888103b6c0a8 by task repro/217 release_inode+0x196/0x290 landlock_put_object+0x71/0x90 landlock_free_rules+0x38/0xc0 landlock_put_ruleset+0x47/0x130 fop_ruleset_release+0x33/0x40 __fput+0x305/0x780 Freed by task 54: kfree+0x11c/0x390 process_scheduled_works+0x766/0xe50 worker_thread+0x7bc/0xb20 The fix is to determine &landlock_superblock(inode->i_sb)->inode_refs only once earlier in release_inode(), and later use that precalculated address for the wake_up_var() call, without having to dereference through the superblock. Cc: stable@vger.kernel.org Fixes: cb2c7d1a1776 ("landlock: Support filesystem access-control") Assisted-by: LLM Signed-off-by: Günther Noack Link: https://patch.msgid.link/20261006105847.2541190-1-gnoack@google.com Signed-off-by: Mickaël Salaün --- security/landlock/fs.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/security/landlock/fs.c b/security/landlock/fs.c index a8fa8f77e775..2ab67bcad198 100644 --- a/security/landlock/fs.c +++ b/security/landlock/fs.c @@ -61,7 +61,7 @@ static void release_inode(struct landlock_object *const object) __releases(object->lock) { struct inode *const inode = object->underobj; - struct super_block *sb; + atomic_long_t *inode_refs; if (!inode) { spin_unlock(&object->lock); @@ -77,8 +77,8 @@ static void release_inode(struct landlock_object *const object) * Makes sure that if the filesystem is concurrently unmounted, * hook_sb_delete() will wait for us to finish iput(). */ - sb = inode->i_sb; - atomic_long_inc(&landlock_superblock(sb)->inode_refs); + inode_refs = &landlock_superblock(inode->i_sb)->inode_refs; + atomic_long_inc(inode_refs); spin_unlock(&object->lock); /* * Because object->underobj was not NULL, hook_sb_delete() and @@ -92,8 +92,8 @@ static void release_inode(struct landlock_object *const object) */ iput(inode); - if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs)) - wake_up_var(&landlock_superblock(sb)->inode_refs); + if (atomic_long_dec_and_test(inode_refs)) + wake_up_var(inode_refs); } static const struct landlock_object_underops landlock_fs_underops = {