Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Tue, 23 Jul 2019 13:47:04 +0800 |
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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
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