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Re: NULL scheduler DoS

  • To: "julien@xxxxxxx" <julien@xxxxxxx>, "ahmeddan@xxxxxxxxxxxx" <ahmeddan@xxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Dario Faggioli <dfaggioli@xxxxxxxx>
  • Date: Wed, 11 Aug 2021 10:09:25 +0000
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  • Cc: "sstabellini@xxxxxxxxxx" <sstabellini@xxxxxxxxxx>, "mpohlack@xxxxxxxxx" <mpohlack@xxxxxxxxx>, "jgrall@xxxxxxxxxxxx" <jgrall@xxxxxxxxxxxx>, "doebel@xxxxxxxxx" <doebel@xxxxxxxxx>
  • Delivery-date: Wed, 11 Aug 2021 10:09:33 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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  • Thread-topic: NULL scheduler DoS

On Mon, 2021-08-09 at 21:38 +0100, Julien Grall wrote:
> On 09/08/2021 18:35, Julien Grall wrote:
> > 
> > This implies that a pCPU may temporarily be assigned to two vCPUs
> > and we 
> > expect to be fixed up afterwards. However, a domain may be
> > destroyed 
> > before this is happening.
> > 
> > So it looks like that unit_deassign() is not able to cope with this
> > case. From a brief look, I think we may want to check if the pCPU
> > is in 
> > the wait list. If it is, then we should bail out.
> Actually, I was wrong. It looks like null_unit_remove() is already 
> checking this condition. Also, the vCPU should be offline (and the
> unit 
> as well) because they haven't come online yet:
That's what is currently puzzling me.

I mean, the crash happens in unit_deassign(), called by
null_unit_remove(), called by sched_destroy_vcpu().

And I agree that the unit should be offline, but null_unit_remove()
calls unit_deassign() only if the unit *is* online, so... What's going
on? :-/

Dario Faggioli, Ph.D
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
<<This happens because _I_ choose it to happen!>> (Raistlin Majere)

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