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@xxxxxxxxxx> > > > > > > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > 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@xxxxxxxxxx> > > > > > 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
