linux-kernel - Re: [syzbot] WARNING in __kfence

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Message-ID: <YmI4d8xR3tafv2Cq@FVFYT0MHHV2J.usts.net>
Date:   Fri, 22 Apr 2022 13:09:11 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Marco Elver <elver@...gle.com>
Cc:     syzbot <syzbot+ffe71f1ff7f8061bcc98@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, dvyukov@...gle.com, glider@...gle.com,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        cgroups@...r.kernel.org
Subject: Re: [syzbot] WARNING in __kfence_free

On Thu, Apr 21, 2022 at 11:12:17AM +0200, Marco Elver wrote:
> On Thu, Apr 21, 2022 at 01:58AM -0700, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    559089e0a93d vmalloc: replace VM_NO_HUGE_VMAP with VM_ALLO..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=10853220f00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=2e1f9b9947966f42
> > dashboard link: https://syzkaller.appspot.com/bug?extid=ffe71f1ff7f8061bcc98
> > compiler:       aarch64-linux-gnu-gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > userspace arch: arm64
> > 
> > Unfortunately, I don't have any reproducer for this issue yet.
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+ffe71f1ff7f8061bcc98@...kaller.appspotmail.com
> > 
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 2216 at mm/kfence/core.c:1022 __kfence_free+0x84/0xc0 mm/kfence/core.c:1022
> 
> That's this warning in __kfence_free:
> 
> 	#ifdef CONFIG_MEMCG
> 		KFENCE_WARN_ON(meta->objcg);
> 	#endif
> 
> introduced in 8f0b36497303 ("mm: kfence: fix objcgs vector allocation").
> 
> Muchun, are there any circumstances where the assumption may be broken?
> Or a new bug elsewhere?

meta->objcg always should be NULL when reaching __kfence_free().
In theory, meta->objcg should be cleared via memcg_slab_free_hook().

I found the following code snippet in do_slab_free().

  /* memcg_slab_free_hook() is already called for bulk free. */
  if (!tail)
  	memcg_slab_free_hook(s, &head, 1); 

The only posibility is @tail is not NULL, which is the case of
kmem_cache_free_bulk(). However, here the call trace is kfree(),
it seems to be impossible that missing call memcg_slab_free_hook().

Thanks.