linux-kernel - Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group

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Date:   Tue, 31 Jan 2023 11:12:47 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Anthony Krowiak <akrowiak@...ux.ibm.com>,
        Yi Liu <yi.l.liu@...el.com>, alex.williamson@...hat.com,
        pbonzini@...hat.com, kevin.tian@...el.com, cohuck@...hat.com,
        farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, frankja@...ux.ibm.com,
        imbrenda@...ux.ibm.com, david@...hat.com, jjherne@...ux.ibm.com,
        pasic@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        seanjc@...gle.com, linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock

On Tue, Jan 31, 2023 at 10:00:14AM -0500, Matthew Rosato wrote:
> On 1/31/23 9:48 AM, Jason Gunthorpe wrote:
> > On Tue, Jan 31, 2023 at 09:46:18AM -0500, Anthony Krowiak wrote:
> > 
> >>> Maybe you should split that lock and have a dedicated apcb lock?
> >>
> >> I don't think that would suffice for taking the vCPUs out of SIE.
> > 
> > Then I think we have to keep this patch and also do Matthew's patch to
> > keep kvm refs inside vfio as well.
> > 
> 
> I don't think keeping kvm refs inside vfio solves this issue though
> -- Even if we handle the kvm_put_kvm asynchronously within vfio as
> previously proposed, kvm_vfio_release will eventually get called and
> it gets called with the kvm->lock already held, then proceeds to
> call vfio_file_set_kvm which gets the group->lock.  That order
> conflicts with the hierarchy used by the driver during open_device
> of vfio->group_lock ... kvm->lock.

The group lock is held by vfio_file_set_kvm() only because we don't
have a refcount and we have to hold it across the open call to keep
the pointer alive.

With proper refcounting you'd split this to a spinlock and hold it
only while obtaining the get ref for the open thread.

Jason