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

[Imre Kaloz <kaloz@kernel.org>]

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