Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Tue, 23 Jul 2019 12:01:40 +0800 |
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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.
Btw, for kvm ioctl it still uses synchronize_rcu() in kvm_vcpu_ioctl(), (just a little bit more hard to trigger):
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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