linux-kernel - Re: [RFC] vfio: remove useless judgement

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Date:   Fri, 24 Jun 2022 21:13:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     lizhe.67@...edance.com
Cc:     alex.williamson@...hat.com, cohuck@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lizefan.x@...edance.com
Subject: Re: [RFC] vfio: remove useless judgement

On Thu, Jun 23, 2022 at 07:56:03PM +0800, lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
> 
> In function vfio_dma_do_unmap(), we currently prevent process to unmap
> vfio dma region whose mm_struct is different from the vfio_dma->task.
> In our virtual machine scenario which is using kvm and qemu, this
> judgement stops us from liveupgrading our qemu, which uses fork() &&
> exec() to load the new binary but the new process cannot do the
> VFIO_IOMMU_UNMAP_DMA action during vm exit because of this judgement.
> 
> This judgement is added in commit 8f0d5bb95f76 ("vfio iommu type1: Add
> task structure to vfio_dma") for the security reason. But it seems that
> no other task who has no family relationship with old and new process
> can get the same vfio_dma struct here for the reason of resource
> isolation. So this patch delete it.
> 
> Signed-off-by: Li Zhe <lizhe.67@...edance.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 6 ------
>  1 file changed, 6 deletions(-)

I'm inclined to agree with this reasoning and deliberately did not
include a similar check in iommufd. Any process with access to the
container FD should be able to manipulate the IOVA space.

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

Jason