linux-kernel - Re: [PATCH] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group

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Message-ID: <ZMferRPhhYjSno8B@google.com>
Date:   Mon, 31 Jul 2023 09:17:49 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Roxana Bradescu <roxabee@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] kvm/vfio: ensure kvg instance stays around in
 kvm_vfio_group_add()

On Mon, Jul 31, 2023 at 02:02:59PM +0200, Greg KH wrote:
> On Mon, Jul 10, 2023 at 03:20:31PM -0700, Dmitry Torokhov wrote:
> > kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
> > and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
> > dropping kv->lock. If we race group addition and deletion calls, kvg
> > instance may get freed by the time we get around to calling
> > kvm_vfio_file_set_kvm().
> > 
> > Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
> > of kv->lock. We already call it while holding the same lock when vfio
> > group is being deleted, so it should be safe here as well.
> > 
> > Fixes: ba70a89f3c2a ("vfio: Change vfio_group_set_kvm() to vfio_file_set_kvm()")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > ---
> >  virt/kvm/vfio.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> > index 9584eb57e0ed..cd46d7ef98d6 100644
> > --- a/virt/kvm/vfio.c
> > +++ b/virt/kvm/vfio.c
> > @@ -179,10 +179,10 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
> >  	list_add_tail(&kvg->node, &kv->group_list);
> >  
> >  	kvm_arch_start_assignment(dev->kvm);
> > +	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
> >  
> >  	mutex_unlock(&kv->lock);
> >  
> > -	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
> >  	kvm_vfio_update_coherency(dev);
> >  
> >  	return 0;
> > -- 
> > 2.41.0.255.g8b1d071c50-goog
> 
> What ever happened to this change?  Did it end up in a KVM tree
> somewhere?

It was posted as:

https://lore.kernel.org/all/20230714224538.404793-1-dmitry.torokhov@gmail.com/

and I believe Alex Williamson is planning to take it through VFIO tree.

Thanks.

-- 
Dmitry