Re: [PATCH] mm/slab: sample slab_state once in kmem_cache_destroy()

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On Wed, Sep 30, 2026 at 09:51:09PM +0200, Imre Kaloz wrote:
> kmem_cache_destroy() tests slab_state >= FULL for sysfs_slab_unlink()
> and again in kmem_cache_release() for sysfs_slab_release(), with the
> cache already off slab_caches in between. If slab_late_init() runs in
> that gap it sets slab_state to FULL but never calls sysfs_slab_add() for
> the unlinked cache, so kmem_cache_release() ends up in kobject_put() on
> a kobject that was never initialized:
>
>   WARNING: lib/kobject.c:734 at kobject_put+0x64/0x2c0, CPU#1: kworker/u8:3/55
>   kobject: '(null)' ((____ptrval____)): is not initialized, yet kobject_put() is being called.
>   refcount_t: underflow; use-after-free.
>   kobject_put+0x64/0x2c0
>   sysfs_slab_release+0xc/0x20
>   kmem_cache_destroy+0x104/0x1e0
>   bioset_exit+0x13c/0x1e0
>   disk_release+0x54/0x140
>   put_disk+0x18/0x40
>   floppy_async_init+0xbec/0xd10
>
> Seen on sparc64 at boot, where the asynchronous floppy init tears down
> its bio slab while the late initcalls are running.
>
> Read slab_state once, under slab_mutex which slab_late_init() holds when
> it sets FULL, and use the result for both the unlink and the release.
> The kobject flags (state_initialized, state_in_sysfs) were considered as
> the key instead, but slab_state is what cache creation and
> slab_late_init() decide on.

the enum has nothing between UP and FULL, and the create path skips
sysfs_slab_add() while slab_state <= UP, so both tests are the one that
decides whether the kobject exists, and a false value cannot skip a
kobject_put() that was owed.

it would read better to me if the changelog said that. otherwise LGTM.

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

> debugfs_slab_release() is not part of the race: it only looks the cache
> up by name in the debugfs root and does nothing before that root exists.
>
> Fixes: 4ec10268ed98 ("mm, slab: unlink slabinfo, sysfs and debugfs immediately")
> Cc: stable@vger.kernel.org
> Signed-off-by: Imre Kaloz <kaloz@kernel.org>

--
cheers,
jose a. p-a