Re: [PATCH] vfio/type1: Add proper error unwind for vfio_iommu_replay()

[Cornelia Huck]

On Fri, 07 Aug 2020 14:14:48 -0600
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> The vfio_iommu_replay() function does not currently unwind on error,
> yet it does pin pages, perform IOMMU mapping, and modify the vfio_dma
> structure to indicate IOMMU mapping.  The IOMMU mappings are torn down
> when the domain is destroyed, but the other actions go on to cause
> trouble later.  For example, the iommu->domain_list can be empty if we
> only have a non-IOMMU backed mdev attached.  We don't currently check
> if the list is empty before getting the first entry in the list, which
> leads to a bogus domain pointer.  If a vfio_dma entry is erroneously
> marked as iommu_mapped, we'll attempt to use that bogus pointer to
> retrieve the existing physical page addresses.
> 
> This is the scenario that uncovered this issue, attempting to hot-add
> a vfio-pci device to a container with an existing mdev device and DMA
> mappings, one of which could not be pinned, causing a failure adding
> the new group to the existing container and setting the conditions
> for a subsequent attempt to explode.
> 
> To resolve this, we can first check if the domain_list is empty so
> that we can reject replay of a bogus domain, should we ever encounter
> this inconsistent state again in the future.  The real fix though is
> to add the necessary unwind support, which means cleaning up the
> current pinning if an IOMMU mapping fails, then walking back through
> the r-b tree of DMA entries, reading from the IOMMU which ranges are
> mapped, and unmapping and unpinning those ranges.  To be able to do
> this, we also defer marking the DMA entry as IOMMU mapped until all
> entries are processed, in order to allow the unwind to know the
> disposition of each entry.
> 
> Fixes: a54eb55045ae ("vfio iommu type1: Add support for mediated devices")
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
>  drivers/vfio/vfio_iommu_type1.c |   71 ++++++++++++++++++++++++++++++++++++---
>  1 file changed, 66 insertions(+), 5 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>