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    Subject[PATCH 5.19 053/155] mm/slab_common: Deleting kobject in kmem_cache_destroy() without holding slab_mutex/cpu_hotplug_lock
    Date
    From: Waiman Long <longman@redhat.com>

    [ Upstream commit 0495e337b7039191dfce6e03f5f830454b1fae6b ]

    A circular locking problem is reported by lockdep due to the following
    circular locking dependency.

    +--> cpu_hotplug_lock --> slab_mutex --> kn->active --+
    | |
    +-----------------------------------------------------+

    The forward cpu_hotplug_lock ==> slab_mutex ==> kn->active dependency
    happens in

    kmem_cache_destroy(): cpus_read_lock(); mutex_lock(&slab_mutex);
    ==> sysfs_slab_unlink()
    ==> kobject_del()
    ==> kernfs_remove()
    ==> __kernfs_remove()
    ==> kernfs_drain(): rwsem_acquire(&kn->dep_map, ...);

    The backward kn->active ==> cpu_hotplug_lock dependency happens in

    kernfs_fop_write_iter(): kernfs_get_active();
    ==> slab_attr_store()
    ==> cpu_partial_store()
    ==> flush_all(): cpus_read_lock()

    One way to break this circular locking chain is to avoid holding
    cpu_hotplug_lock and slab_mutex while deleting the kobject in
    sysfs_slab_unlink() which should be equivalent to doing a write_lock
    and write_unlock pair of the kn->active virtual lock.

    Since the kobject structures are not protected by slab_mutex or the
    cpu_hotplug_lock, we can certainly release those locks before doing
    the delete operation.

    Move sysfs_slab_unlink() and sysfs_slab_release() to the newly
    created kmem_cache_release() and call it outside the slab_mutex &
    cpu_hotplug_lock critical sections. There will be a slight delay
    in the deletion of sysfs files if kmem_cache_release() is called
    indirectly from a work function.

    Fixes: 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context")
    Signed-off-by: Waiman Long <longman@redhat.com>
    Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
    Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
    Acked-by: David Rientjes <rientjes@google.com>
    Link: https://lore.kernel.org/all/YwOImVd+nRUsSAga@hyeyoo/
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/slab_common.c | 45 +++++++++++++++++++++++++++++----------------
    1 file changed, 29 insertions(+), 16 deletions(-)

    diff --git a/mm/slab_common.c b/mm/slab_common.c
    index 77c3adf40e504..dbd4b6f9b0e79 100644
    --- a/mm/slab_common.c
    +++ b/mm/slab_common.c
    @@ -420,6 +420,28 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
    }
    EXPORT_SYMBOL(kmem_cache_create);

    +#ifdef SLAB_SUPPORTS_SYSFS
    +/*
    + * For a given kmem_cache, kmem_cache_destroy() should only be called
    + * once or there will be a use-after-free problem. The actual deletion
    + * and release of the kobject does not need slab_mutex or cpu_hotplug_lock
    + * protection. So they are now done without holding those locks.
    + *
    + * Note that there will be a slight delay in the deletion of sysfs files
    + * if kmem_cache_release() is called indrectly from a work function.
    + */
    +static void kmem_cache_release(struct kmem_cache *s)
    +{
    + sysfs_slab_unlink(s);
    + sysfs_slab_release(s);
    +}
    +#else
    +static void kmem_cache_release(struct kmem_cache *s)
    +{
    + slab_kmem_cache_release(s);
    +}
    +#endif
    +
    static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
    {
    LIST_HEAD(to_destroy);
    @@ -446,11 +468,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
    list_for_each_entry_safe(s, s2, &to_destroy, list) {
    debugfs_slab_release(s);
    kfence_shutdown_cache(s);
    -#ifdef SLAB_SUPPORTS_SYSFS
    - sysfs_slab_release(s);
    -#else
    - slab_kmem_cache_release(s);
    -#endif
    + kmem_cache_release(s);
    }
    }

    @@ -465,20 +483,11 @@ static int shutdown_cache(struct kmem_cache *s)
    list_del(&s->list);

    if (s->flags & SLAB_TYPESAFE_BY_RCU) {
    -#ifdef SLAB_SUPPORTS_SYSFS
    - sysfs_slab_unlink(s);
    -#endif
    list_add_tail(&s->list, &slab_caches_to_rcu_destroy);
    schedule_work(&slab_caches_to_rcu_destroy_work);
    } else {
    kfence_shutdown_cache(s);
    debugfs_slab_release(s);
    -#ifdef SLAB_SUPPORTS_SYSFS
    - sysfs_slab_unlink(s);
    - sysfs_slab_release(s);
    -#else
    - slab_kmem_cache_release(s);
    -#endif
    }

    return 0;
    @@ -493,14 +502,16 @@ void slab_kmem_cache_release(struct kmem_cache *s)

    void kmem_cache_destroy(struct kmem_cache *s)
    {
    + int refcnt;
    +
    if (unlikely(!s) || !kasan_check_byte(s))
    return;

    cpus_read_lock();
    mutex_lock(&slab_mutex);

    - s->refcount--;
    - if (s->refcount)
    + refcnt = --s->refcount;
    + if (refcnt)
    goto out_unlock;

    WARN(shutdown_cache(s),
    @@ -509,6 +520,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
    out_unlock:
    mutex_unlock(&slab_mutex);
    cpus_read_unlock();
    + if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
    + kmem_cache_release(s);
    }
    EXPORT_SYMBOL(kmem_cache_destroy);

    --
    2.35.1


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