From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
aarcange@redhat.com, akpm@linux-foundation.org,
christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com,
elena.reshetova@intel.com, guro@fb.com, hch@infradead.org,
james.bottomley@hansenpartnership.com, jglisse@redhat.com,
keescook@chromium.org, ldv@altlinux.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-parisc@vger.kernel.org, luto@amacapital.net,
mhocko@suse.com, mingo@kernel.org, namit@vmware.com,
peterz@infradead.org, syzkaller-bugs@googlegroups.com,
viro@zeniv.linux.org.uk, wad@chromium.org
Subject: Re: WARNING in __mmdrop
Date: Tue, 23 Jul 2019 03:23:20 -0400 [thread overview]
Message-ID: <20190723032024-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <b4696f2e-678a-bdb2-4b7c-fb4ce040ec2a@redhat.com>
On Tue, Jul 23, 2019 at 01:47:04PM +0800, Jason Wang wrote:
>
> On 2019/7/23 下午1:01, Michael S. Tsirkin wrote:
> > On Tue, Jul 23, 2019 at 12:01:40PM +0800, Jason Wang wrote:
> > > On 2019/7/22 下午4:08, Michael S. Tsirkin wrote:
> > > > On Mon, Jul 22, 2019 at 01:24:24PM +0800, Jason Wang wrote:
> > > > > On 2019/7/21 下午8:18, Michael S. Tsirkin wrote:
> > > > > > On Sun, Jul 21, 2019 at 06:02:52AM -0400, Michael S. Tsirkin wrote:
> > > > > > > On Sat, Jul 20, 2019 at 03:08:00AM -0700, syzbot wrote:
> > > > > > > > syzbot has bisected this bug to:
> > > > > > > >
> > > > > > > > commit 7f466032dc9e5a61217f22ea34b2df932786bbfc
> > > > > > > > Author: Jason Wang<jasowang@redhat.com>
> > > > > > > > Date: Fri May 24 08:12:18 2019 +0000
> > > > > > > >
> > > > > > > > vhost: access vq metadata through kernel virtual address
> > > > > > > >
> > > > > > > > bisection log:https://syzkaller.appspot.com/x/bisect.txt?x=149a8a20600000
> > > > > > > > start commit: 6d21a41b Add linux-next specific files for 20190718
> > > > > > > > git tree: linux-next
> > > > > > > > final crash:https://syzkaller.appspot.com/x/report.txt?x=169a8a20600000
> > > > > > > > console output:https://syzkaller.appspot.com/x/log.txt?x=129a8a20600000
> > > > > > > > kernel config:https://syzkaller.appspot.com/x/.config?x=3430a151e1452331
> > > > > > > > dashboard link:https://syzkaller.appspot.com/bug?extid=e58112d71f77113ddb7b
> > > > > > > > syz repro:https://syzkaller.appspot.com/x/repro.syz?x=10139e68600000
> > > > > > > >
> > > > > > > > Reported-by:syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com
> > > > > > > > Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual
> > > > > > > > address")
> > > > > > > >
> > > > > > > > For information about bisection process see:https://goo.gl/tpsmEJ#bisection
> > > > > > > OK I poked at this for a bit, I see several things that
> > > > > > > we need to fix, though I'm not yet sure it's the reason for
> > > > > > > the failures:
> > > > > > >
> > > > > > >
> > > > > > > 1. mmu_notifier_register shouldn't be called from vhost_vring_set_num_addr
> > > > > > > That's just a bad hack, in particular I don't think device
> > > > > > > mutex is taken and so poking at two VQs will corrupt
> > > > > > > memory.
> > > > > > > So what to do? How about a per vq notifier?
> > > > > > > Of course we also have synchronize_rcu
> > > > > > > in the notifier which is slow and is now going to be called twice.
> > > > > > > I think call_rcu would be more appropriate here.
> > > > > > > We then need rcu_barrier on module unload.
> > > > > > > OTOH if we make pages linear with map then we are good
> > > > > > > with kfree_rcu which is even nicer.
> > > > > > >
> > > > > > > 2. Doesn't map leak after vhost_map_unprefetch?
> > > > > > > And why does it poke at contents of the map?
> > > > > > > No one should use it right?
> > > > > > >
> > > > > > > 3. notifier unregister happens last in vhost_dev_cleanup,
> > > > > > > but register happens first. This looks wrong to me.
> > > > > > >
> > > > > > > 4. OK so we use the invalidate count to try and detect that
> > > > > > > some invalidate is in progress.
> > > > > > > I am not 100% sure why do we care.
> > > > > > > Assuming we do, uaddr can change between start and end
> > > > > > > and then the counter can get negative, or generally
> > > > > > > out of sync.
> > > > > > >
> > > > > > > So what to do about all this?
> > > > > > > I am inclined to say let's just drop the uaddr optimization
> > > > > > > for now. E.g. kvm invalidates unconditionally.
> > > > > > > 3 should be fixed independently.
> > > > > > Above implements this but is only build-tested.
> > > > > > Jason, pls take a look. If you like the approach feel
> > > > > > free to take it from here.
> > > > > >
> > > > > > One thing the below does not have is any kind of rate-limiting.
> > > > > > Given it's so easy to restart I'm thinking it makes sense
> > > > > > to add a generic infrastructure for this.
> > > > > > Can be a separate patch I guess.
> > > > > I don't get why must use kfree_rcu() instead of synchronize_rcu() here.
> > > > synchronize_rcu has very high latency on busy systems.
> > > > It is not something that should be used on a syscall path.
> > > > KVM had to switch to SRCU to keep it sane.
> > > > Otherwise one guest can trivially slow down another one.
> > >
> > > I think you mean the synchronize_rcu_expedited()? Rethink of the code, the
> > > synchronize_rcu() in ioctl() could be removed, since it was serialized with
> > > memory accessor.
> >
> > Really let's just use kfree_rcu. It's way cleaner: fire and forget.
>
>
> Looks not, you need rate limit the fire as you've figured out?
See the discussion that followed. Basically no, it's good enough
already and is only going to be better.
> And in fact,
> the synchronization is not even needed, does it help if I leave a comment to
> explain?
Let's try to figure it out in the mail first. I'm pretty sure the
current logic is wrong.
>
> >
> > > Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(),
> > > (just a little bit more hard to trigger):
> >
> > AFAIK these never run in response to guest events.
> > So they can take very long and guests still won't crash.
>
>
> What if guest manages to escape to qemu?
>
> Thanks
Then it's going to be slow. Why do we care?
What we do not want is synchronize_rcu that guest is blocked on.
>
> >
> >
> > > case KVM_RUN: {
> > > ...
> > > if (unlikely(oldpid != task_pid(current))) {
> > > /* The thread running this VCPU changed. */
> > > struct pid *newpid;
> > >
> > > r = kvm_arch_vcpu_run_pid_change(vcpu);
> > > if (r)
> > > break;
> > >
> > > newpid = get_task_pid(current, PIDTYPE_PID);
> > > rcu_assign_pointer(vcpu->pid, newpid);
> > > if (oldpid)
> > > synchronize_rcu();
> > > put_pid(oldpid);
> > > }
> > > ...
> > > break;
> > >
> > >
> > > > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > > Let me try to figure out the root cause then decide whether or not to go for
> > > > > this way.
> > > > >
> > > > > Thanks
> > > > The root cause of the crash is relevant, but we still need
> > > > to fix issues 1-4.
> > > >
> > > > More issues (my patch tries to fix them too):
> > > >
> > > > 5. page not dirtied when mappings are torn down outside
> > > > of invalidate callback
> > >
> > > Yes.
> > >
> > >
> > > > 6. potential cross-VM DOS by one guest keeping system busy
> > > > and increasing synchronize_rcu latency to the point where
> > > > another guest stars timing out and crashes
> > > >
> > > >
> > > >
> > > This will be addressed after I remove the synchronize_rcu() from ioctl path.
> > >
> > > Thanks
next prev parent reply other threads:[~2019-07-23 7:23 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000008dd6bb058e006938@google.com>
2019-07-20 10:08 ` WARNING in __mmdrop syzbot
2019-07-21 10:02 ` Michael S. Tsirkin
2019-07-21 12:18 ` Michael S. Tsirkin
2019-07-22 5:24 ` Jason Wang
2019-07-22 8:08 ` Michael S. Tsirkin
2019-07-23 4:01 ` Jason Wang
2019-07-23 5:01 ` Michael S. Tsirkin
2019-07-23 5:47 ` Jason Wang
2019-07-23 7:23 ` Michael S. Tsirkin [this message]
2019-07-23 7:53 ` Jason Wang
2019-07-23 8:10 ` Michael S. Tsirkin
2019-07-23 8:49 ` Jason Wang
2019-07-23 9:26 ` Michael S. Tsirkin
2019-07-23 13:31 ` Jason Wang
2019-07-25 5:52 ` Michael S. Tsirkin
2019-07-25 7:43 ` Jason Wang
2019-07-25 8:28 ` Michael S. Tsirkin
2019-07-25 13:21 ` Jason Wang
2019-07-25 13:26 ` Michael S. Tsirkin
2019-07-25 14:25 ` Jason Wang
2019-07-26 11:49 ` Michael S. Tsirkin
2019-07-26 12:00 ` Jason Wang
2019-07-26 12:38 ` Michael S. Tsirkin
2019-07-26 12:53 ` Jason Wang
2019-07-26 13:36 ` Jason Wang
2019-07-26 13:49 ` Michael S. Tsirkin
2019-07-29 5:54 ` Jason Wang
2019-07-29 8:59 ` Michael S. Tsirkin
2019-07-29 14:24 ` Jason Wang
2019-07-29 14:44 ` Michael S. Tsirkin
2019-07-30 7:44 ` Jason Wang
2019-07-30 8:03 ` Jason Wang
2019-07-30 15:08 ` Michael S. Tsirkin
2019-07-31 8:49 ` Jason Wang
2019-07-31 23:00 ` Jason Gunthorpe
2019-07-26 13:47 ` Michael S. Tsirkin
2019-07-26 14:00 ` Jason Wang
2019-07-26 14:10 ` Michael S. Tsirkin
2019-07-26 15:03 ` Jason Gunthorpe
2019-07-29 5:56 ` Jason Wang
2019-07-21 12:28 ` RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop) Michael S. Tsirkin
2019-07-21 13:17 ` Paul E. McKenney
2019-07-21 17:53 ` Michael S. Tsirkin
2019-07-21 19:28 ` Paul E. McKenney
2019-07-22 7:56 ` Michael S. Tsirkin
2019-07-22 11:57 ` Paul E. McKenney
2019-07-21 21:08 ` Matthew Wilcox
2019-07-21 23:31 ` Paul E. McKenney
2019-07-22 7:52 ` Michael S. Tsirkin
2019-07-22 11:51 ` Paul E. McKenney
2019-07-22 13:41 ` Jason Gunthorpe
2019-07-22 15:52 ` Paul E. McKenney
2019-07-22 16:04 ` Jason Gunthorpe
2019-07-22 16:15 ` Michael S. Tsirkin
2019-07-22 16:15 ` Paul E. McKenney
2019-07-22 15:14 ` Joel Fernandes
2019-07-22 15:47 ` Michael S. Tsirkin
2019-07-22 15:55 ` Paul E. McKenney
2019-07-22 16:13 ` Michael S. Tsirkin
2019-07-22 16:25 ` Paul E. McKenney
2019-07-22 16:32 ` Michael S. Tsirkin
2019-07-22 18:58 ` Paul E. McKenney
2019-07-22 5:21 ` WARNING in __mmdrop Jason Wang
2019-07-22 8:02 ` Michael S. Tsirkin
2019-07-23 3:55 ` Jason Wang
2019-07-23 5:02 ` Michael S. Tsirkin
2019-07-23 5:48 ` Jason Wang
2019-07-23 7:25 ` Michael S. Tsirkin
2019-07-23 7:55 ` Jason Wang
2019-07-23 7:56 ` Michael S. Tsirkin
2019-07-23 8:42 ` Jason Wang
2019-07-23 10:27 ` Michael S. Tsirkin
2019-07-23 13:34 ` Jason Wang
2019-07-23 15:02 ` Michael S. Tsirkin
2019-07-24 2:17 ` Jason Wang
2019-07-24 8:05 ` Michael S. Tsirkin
2019-07-24 10:08 ` Jason Wang
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-25 3:44 ` Jason Wang
2019-07-25 5:09 ` Michael S. Tsirkin
2019-07-24 16:53 ` Jason Gunthorpe
2019-07-24 18:25 ` Michael S. Tsirkin
2019-07-23 10:42 ` Michael S. Tsirkin
2019-07-23 13:37 ` Jason Wang
2019-07-22 14:11 ` Jason Gunthorpe
2019-07-25 6:02 ` Michael S. Tsirkin
2019-07-25 7:44 ` Jason Wang
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