[PATCH] configfs: fix slab-UAF Write in config_item_put
From: Drif Abdelmalek Mohamed Said
Date: Sat Oct 17 2026 - 08:52:08 EST
syzbot reported:
BUG: KASAN: slab-use-after-free in instrument_atomic_read_write
include/linux/instrumented.h:112 [inline]
BUG: KASAN: slab-use-after-free in atomic_fetch_sub_release
include/linux/atomic/atomic-instrumented.h:400 [inline]
BUG: KASAN: slab-use-after-free in __refcount_sub_and_test
include/linux/refcount.h:389 [inline]
BUG: KASAN: slab-use-after-free in __refcount_dec_and_test
include/linux/refcount.h:432 [inline]
BUG: KASAN: slab-use-after-free in refcount_dec_and_test
include/linux/refcount.h:450 [inline]
BUG: KASAN: slab-use-after-free in kref_put
include/linux/kref.h:64 [inline]
BUG: KASAN: slab-use-after-free in config_item_put+0x26/0x90
fs/configfs/item.c:149
Write of size 4 at addr ffff88803b6444b4 by task syz-executor324/5709
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description+0x55/0x1e0 mm/kasan/report.c:378
print_report+0x58/0x70 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
instrument_atomic_read_write include/linux/instrumented.h:112 [inline]
atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
__refcount_sub_and_test include/linux/refcount.h:389 [inline]
__refcount_dec_and_test include/linux/refcount.h:432 [inline]
refcount_dec_and_test include/linux/refcount.h:450 [inline]
kref_put include/linux/kref.h:64 [inline]
config_item_put+0x26/0x90 fs/configfs/item.c:149
configfs_symlink+0x6be/0x11d0 fs/configfs/symlink.c:225
vfs_symlink+0x18b/0x330 fs/namei.c:5823
filename_symlinkat+0x1cd/0x420 fs/namei.c:5848
__do_sys_symlinkat fs/namei.c:5868 [inline]
__se_sys_symlinkat+0x4e/0x2b0 fs/namei.c:5863
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Allocated by task 5712:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:409 [inline]
__kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:426
kasan_kmalloc include/linux/kasan.h:263 [inline]
__kmalloc_cache_noprof+0x3d5/0x680 mm/slub.c:5563
_kmalloc_noprof include/linux/slab.h:991 [inline]
_kzalloc_noprof include/linux/slab.h:1312 [inline]
nvmet_ports_make+0xe6/0xf00 drivers/nvme/target/configfs.c:2054
configfs_mkdir+0x4f6/0x9e0 fs/configfs/dir.c:1369
vfs_mkdir+0x408/0x620 fs/namei.c:5439
filename_mkdirat+0x289/0x520 fs/namei.c:5472
__do_sys_mkdir fs/namei.c:5499 [inline]
__se_sys_mkdir+0x34/0x150 fs/namei.c:5496
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 5712:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:264 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:296
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2748 [inline]
slab_free mm/slub.c:6509 [inline]
kfree+0x1c5/0x6d0 mm/slub.c:6802
config_item_cleanup fs/configfs/item.c:128 [inline]
config_item_release+0x13a/0x2d0 fs/configfs/item.c:137
configfs_rmdir+0x885/0x950 fs/configfs/dir.c:1580
vfs_rmdir+0x3e9/0x6b0 fs/namei.c:5544
filename_rmdir+0x292/0x520 fs/namei.c:5601
__do_sys_rmdir fs/namei.c:5624 [inline]
__se_sys_rmdir+0x2e/0x140 fs/namei.c:5621
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff88803b644000
which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 1204 bytes inside of
freed 4096-byte region [ffff88803b644000, ffff88803b645000)
This is a race between a thread creating a symlink and another one
doing rmdir on a parent (or ancestor) directory of symlink creation dir.
configfs_symlink() takes a reference on parent_item, the directory the
link is created in, and then drops the directory inode lock around
get_target(). While the lock is dropped, a concurrent rmdir can remove
the group that parent_item belongs to.
If parent_item is a default group embedded in a larger object, such as
nvmet_port.subsys_group, the reference on the embedded item does not
keep the containing object alive. rmdir frees the object, and the final
config_item_put(parent_item) in configfs_symlink() then decrements a
kref inside freed memory.
Fix this by not holding parent_item across the unlocked window. Drop
the reference before inode_unlock() and look it up again from
dentry->d_parent after inode_lock(). configfs_get_config_item() returns
NULL if the parent dentry was unhashed in the meantime, in which case
release the target references and return -ENOENT.
Reported-by: syzbot+63336d70e1af377d7388@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=63336d70e1af377d7388
Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@xxxxxxxxx>
---
fs/configfs/symlink.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 89178f5d371a..51eb58b68b55 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -196,11 +196,21 @@ int configfs_symlink(const struct mnt_idmap *idmap, struct inode *dir,
*
* AV, a thoroughly annoyed bastard.
*/
+
+ /* Put the reference back, retake and check after relocking*/
+ config_item_put(parent_item);
+ parent_item = NULL;
inode_unlock(dir);
ret = get_target(symname, &target_item, &target_sd, dentry->d_sb);
inode_lock(dir);
if (ret)
- goto out_put;
+ goto out_put; /* config_item_put on NULL, no problem */
+
+ parent_item = configfs_get_config_item(dentry->d_parent);
+ if (!parent_item) {
+ ret = -ENOENT;
+ goto out_put_target;
+ }
if (dentry->d_inode || d_unhashed(dentry))
ret = -EEXIST;
@@ -218,6 +228,7 @@ int configfs_symlink(const struct mnt_idmap *idmap, struct inode *dir,
target_item);
}
+out_put_target:
configfs_put(target_sd);
config_item_put(target_item);
--
2.43.0