On Mon, Sep 19, 2022 at 11:12:38AM +0200, Vlastimil Babka wrote:
> On 9/19/22 05:12, Feng Tang wrote:
> > When doing slub_debug test, kfence's 'test_memcache_typesafe_by_rcu'
> > kunit test case cause a use-after-free error:
> >
If I'm not mistaken, I think the subject should be:
s/double free/use after free/g
> > BUG: KASAN: use-after-free in kobject_del+0x14/0x30
> > Read of size 8 at addr ffff888007679090 by task kunit_try_catch/261
> >
> > CPU: 1 PID: 261 Comm: kunit_try_catch Tainted: G B N 6.0.0-rc5-next-20220916 #17
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> > Call Trace:
> > <TASK>
> > dump_stack_lvl+0x34/0x48
> > print_address_description.constprop.0+0x87/0x2a5
> > print_report+0x103/0x1ed
> > kasan_report+0xb7/0x140
> > kobject_del+0x14/0x30
> > kmem_cache_destroy+0x130/0x170
> > test_exit+0x1a/0x30
> > kunit_try_run_case+0xad/0xc0
> > kunit_generic_run_threadfn_adapter+0x26/0x50
> > kthread+0x17b/0x1b0
> > </TASK>
> >
> > The cause is inside kmem_cache_destroy():
> >
> > kmem_cache_destroy
> > acquire lock/mutex
> > shutdown_cache
> > schedule_work(kmem_cache_release) (if RCU flag set)
> > release lock/mutex
> > kmem_cache_release (if RCU flag set)
>
> ^ not set
>
> I've fixed that up.
>
> >
> > in some certain timing, the scheduled work could be run before
> > the next RCU flag checking which will get a wrong state.
> >
> > Fix it by caching the RCU flag inside protected area, just like 'refcnt'
Very nice catch, thanks!
Otherwise (and with Vlastimil's fix):
Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> >
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
>
> Thanks!
>
> > ---
> >
> > note:
> >
> > The error only happens on linux-next tree, and not in Linus' tree,
> > which already has Waiman's commit:
> > 0495e337b703 ("mm/slab_common: Deleting kobject in kmem_cache_destroy()
> > without holding slab_mutex/cpu_hotplug_lock")
>
> Actually that commit is already in Linus' rc5 too, so I will send your fix
> this week too. Added a Fixes: 0495e337b703 (...) too.
>
> > mm/slab_common.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 07b948288f84..ccc02573588f 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -475,6 +475,7 @@ void slab_kmem_cache_release(struct kmem_cache *s)
> > void kmem_cache_destroy(struct kmem_cache *s)
> > {
> > int refcnt;
> > + bool rcu_set;
> >
> > if (unlikely(!s) || !kasan_check_byte(s))
> > return;
> > @@ -482,6 +483,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > cpus_read_lock();
> > mutex_lock(&slab_mutex);
> >
> > + rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
> > +
> > refcnt = --s->refcount;
> > if (refcnt)
> > goto out_unlock;
> > @@ -492,7 +495,7 @@ 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))
> > + if (!refcnt && !rcu_set)
> > kmem_cache_release(s);
> > }
> > EXPORT_SYMBOL(kmem_cache_destroy);
>
--
Thanks,
Hyeonggon
