Re: [PATCH] mm/slab_common: fix possiable double free of kmem_cache

[Hyeonggon Yoo]

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