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

[Imre Kaloz]

Hi Harry,

On Thu, 1 Oct 2026, Harry Yoo wrote:

> Hi Imre,
> Thanks for catching and fixing this!
>
> 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.
> >
> > 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")
>
> Overall looks good to me, but could you please explain why it's
> not relevant before this commit? pre-4ec10268 still reads slab_state
> outside slab_mutex.

The outside-mutex read is older, yes. What 4ec10268ed98 changed is that
unlink and release no longer share it.

Before that commit, kmem_cache_release() did one test and used it for
both:

    if (slab_state >= FULL) {
        sysfs_slab_unlink(s);
        sysfs_slab_release(s);
    } else {
        slab_kmem_cache_release(s);
    }

So the two could not disagree. 4ec10268 moved sysfs_slab_unlink() into
kmem_cache_destroy(), after list_del() and after dropping slab_mutex,
and left a second slab_state test in kmem_cache_release() for
sysfs_slab_release(). The cache is already off slab_caches in between.

That is the window in the warning: the first test sees < FULL, so the
kobject is never linked; slab_late_init() then sets FULL under
slab_mutex and calls sysfs_slab_add() only for caches still on the
list; the second test sees FULL and kobject_put()s a kobject that
kobject_init() never ran on.

A single read cannot produce that split decision, which is why Fixes:
points at 4ec10268ed98.

There is a related older window: after list_del() and before that
single read, slab_sysfs_init() could set FULL, skip this cache, and
the single read would then call both unlink and release on an
uninitialized kobject. I have not hit that, and this patch does not
close it.


Best,
Imre