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

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Date:   Tue, 18 Jul 2023 13:00:29 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Roxana Bradescu <roxabee@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/vfio: ensure kvg instance stays around in
 kvm_vfio_group_add()

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(-)

This looks correct, I don't know of any lock cylces that could form
with kv->lock at least

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason