Re: [syzbot] WARNING in vma_merge

[Liam Howlett <liam.howlett@oracle.com>]

* Liam R. Howlett <Liam.Howlett@Oracle.com> [220427 13:52]:
> * Andrew Morton <akpm@linux-foundation.org> [220427 13:20]:
> > On Wed, 27 Apr 2022 06:15:20 -0700 syzbot <syzbot+7728e4ab5371cebc9c00@syzkaller.appspotmail.com> wrote:
> > 
> > > syzbot has found a reproducer for the following issue on:
> > 
> > Thanks.
> > 
> > userfaultfd_release() appears to have offended vma_merge().
> 
> Potentially apply_mlockall_flags() and not userfaultfd_release(). There
> is a potential that the vma iterator is stale if a split/merge succeeds,
> although KASAN should pick this up and KASAN is active according to the
> config below. I've sent a patch for the mlockall issue after Qian Cai's
> reported it to linux-mm.
> 
> > 
> > I'm not seeing any changes in that area from Peter, but Liam's
> > mapletree work changed things around a bit.  Gents, could you please
> > take a look?
> > 
> > 
> > > HEAD commit:    f02ac5c95dfd Add linux-next specific files for 20220427
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=105a33a2f00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=e9256c70f586da8a
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=7728e4ab5371cebc9c00
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15f76f82f00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12b42c3cf00000
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+7728e4ab5371cebc9c00@syzkaller.appspotmail.com
> > > 

...

> 
> I cannot get this issue to trigger with the given tree and git repo with
> my config.  I will try again with the kernel config above.


I was able to reproduce the issue using the config from syzkaller in my
VM and can confirm it was the apply_mlockall_flags() bug also reported
by Qian Cai [1].  The patch I sent earlier [2] fixes this issue as well.


1. https://lore.kernel.org/linux-mm/20220427161033.GA1935@qian/
2. https://lore.kernel.org/linux-mm/20220427165139.5s3qcj2u5vqrvwlc@revolver/

Thanks,
Liam