From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@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 13:47:04 +0800 [thread overview]
Message-ID: <b4696f2e-678a-bdb2-4b7c-fb4ce040ec2a@redhat.com> (raw)
In-Reply-To: <20190723010019-mutt-send-email-mst@kernel.org>
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? And in
fact, the synchronization is not even needed, does it help if I leave a
comment to explain?
>
>> 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
>
>
>> 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 5:47 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 [this message]
2019-07-23 7:23 ` Michael S. Tsirkin
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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