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Date:   Thu, 7 May 2020 17:59:08 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        cohuck@redhat.com, jgg@ziepe.ca
Subject: Re: [PATCH v2 0/3] vfio-pci: Block user access to disabled device
 MMIO
Message-ID: <20200507215908.GQ228260@xz-x1>
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On Tue, May 05, 2020 at 03:54:36PM -0600, Alex Williamson wrote:
> v2:
> 
> Locking in 3/ is substantially changed to avoid the retry scenario
> within the fault handler, therefore a caller who does not allow retry
> will no longer receive a SIGBUS on contention.  IOMMU invalidations
> are still not included here, I expect that will be a future follow-on
> change as we're not fundamentally changing that issue in this series.
> The 'add to vma list only on fault' behavior is also still included
> here, per the discussion I think it's still a valid approach and has
> some advantages, particularly in a VM scenario where we potentially
> defer the mapping until the MMIO BAR is actually DMA mapped into the
> VM address space (or the guest driver actually accesses the device
> if that DMA mapping is eliminated at some point).  Further discussion
> and review appreciated.  Thanks,

Hi, Alex,

I have a general question on the series.

IIUC this series tries to protect illegal vfio userspace writes to device MMIO
regions which may cause platform-level issues.  That makes perfect sense to me.
However what if the write comes from the devices' side?  E.g.:

  - Device A maps MMIO region X

  - Device B do VFIO_IOMMU_DMA_MAP on Device A's MMIO region X
    (so X's MMIO PFNs are mapped in device B's IOMMU page table)

  - Device A clears PCI_COMMAND_MEMORY (reset, etc.)
    - this should zap all existing vmas that mapping region X, however device
      B's IOMMU page table is not aware of this?

  - Device B writes to MMIO region X of device A even if PCI_COMMAND_MEMORY
    cleared on device A's PCI_COMMAND register

Could this happen?

Thanks,

-- 
Peter Xu