Re: [PATCH v2] configfs: fix slab-UAF Write in config_item_put

[DRIF ABDELMALEK MOHAMED SAID]

Hello,
Just following up on this patch to see if anyone has had a chance to
review it or if any changes are needed since we're at 7.4-rc1.
Thanks,
Drif Abdelmalek Mohamed Said

On Fri, Oct 23, 2026 at 5:29 PM Drif Abdelmalek Mohamed Said
<drifabdelmalekmohamedsaid@xxxxxxxxx> wrote:
>
> 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.
>
> Re-looking up parent_item is not sufficient on its own.
> configfs_get_config_item() only fails if the parent dentry has been
> unhashed, and the rmdir path in another thread may skip that: if
> simple_rmdir() failed, the earlier d_drop() was skipped, so the dentry
> can stay hashed even though the directory has already been detached.
>
> Such a directory was created as a default group (CONFIGFS_USET_DIR), so
> configfs_rmdir() takes the CONFIGFS_USET_DIR branch and calls
> configfs_detach_group(). detach_groups() then marks each child inode
> S_DEAD (d_inode(child)->i_flags |= S_DEAD). Check IS_DEADDIR(dir)
> after retaking the inode lock and before taking a reference on the
> parent, and return -ENOENT. This also rejects a detached directory
> whose dentry is still hashed.
>
> Reported-by: syzbot+63336d70e1af377d7388@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=63336d70e1af377d7388
> Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@xxxxxxxxx>
> ---
> v2: Added check for dead dir, this would be helpful when configfs_remove_dir skips
> unhashing the dentry.
> v1: https://lore.kernel.org/all/20261017123748.4430-1-drifabdelmalekmohamedsaid@xxxxxxxxx/T/
>  fs/configfs/symlink.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
> index 89178f5d371a..7d746a10c369 100644
> --- a/fs/configfs/symlink.c
> +++ b/fs/configfs/symlink.c
> @@ -196,11 +196,26 @@ 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 */
> +
> +       if (IS_DEADDIR(dir)) {
> +               ret = -ENOENT;
> +               goto out_put_target;
> +       }
> +
> +       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 +233,7 @@ 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
>