On Mittwoch, 17. März 2021 11:05:32 CET Stefan Hajnoczi wrote:
> On Fri, Dec 18, 2020 at 05:39:34PM +0800, Jiachen Zhang wrote:
> > Thanks for the suggestions. Actually, we choose to save all state
> > information to QEMU because a virtiofsd has the same lifecycle as its
> > QEMU master. However, saving things to a file do avoid communication with
> > QEMU, and we no longer need to increase the complexity of vhost-user
> > protocol. The suggestion to save fds to the systemd is also very
> > reasonable
> > if we don't consider the lifecycle issues, we will try it.
>
> Hi,
> We recently discussed crash recovery in the virtio-fs bi-weekly call and
> I read some of this email thread because it's a topic I'm interested in.
I just had a quick fly over the patches so far. Shouldn't there be some kind
of constraint for an automatic reconnection feature after a crash to prevent
this being exploited by ROP brute force attacks?
E.g. adding some (maybe continuously increasing) delay and/or limiting the
amount of reconnects within a certain time frame would come to my mind.
Best regards,
Christian Schoenebeck
Thanks, Christian. I am still trying to figure out the details of the ROP attacks.
However, QEMU's vhost-user reconnection is based on chardev socket reconnection. The socket reconnection can be enabled by the "--chardev socket,...,reconnect=N" in QEMU command options, in which N means QEMU will try to connect the disconnected socket every N seconds. We can increase N to increase the reconnect delay. If we want to change the reconnect delay dynamically, I think we should change the chardev socket reconnection code. It is a more generic mechanism than vhost-user-fs and vhost-user backend.
By the way, I also considered the socket reconnection delay time in the performance aspect. As the reconnection delay increase, if an application in the guest is doing I/Os, it will suffer larger tail latency. And for now, the smallest delay is 1 second, which is rather large for high-performance virtual I/O devices today. I think maybe a more performant and safer reconnect delay adjustment mechanism should be considered in the future. What are your thoughts?
Jiachen