Re: [Qemu-devel] [PATCH] vfio/common: Work around kernel overflow bug in DMA unmap

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 09 Jan 2019 16:10:51 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> A kernel bug was introduced in v4.15 via commit 71a7d3d78e3c which
> adds a test for address space wrap-around in the vfio DMA unmap path.
> Unfortunately due to overflow, the kernel detects an unmap of the last
> page in the 64-bit address space as a wrap-around.  In QEMU, a Q35
> guest with VT-d emulation and guest IOMMU enabled will attempt to make
> such an unmap request during VM system reset, triggering an error:
> 
>   qemu-kvm: VFIO_UNMAP_DMA: -22
>   qemu-kvm: vfio_dma_unmap(0x561f059948f0, 0xfef00000, 0xffffffff01100000) = -22 (Invalid argument)
> 
> Here the IOVA start address (0xfef00000) and the size parameter
> (0xffffffff01100000) add to exactly 2^64, triggering the bug.  A
> kernel fix is queued for the Linux v5.0 release to address this.
> 
> This patch implements a workaround to retry the unmap, excluding the
> final page of the range when we detect an unmap failing which matches
> the requirements for this issue.  This is expected to be a safe and
> complete workaround as the VT-d address space does not extend to the
> full 64-bit space and therefore the last page should never be mapped.
> 
> This workaround can be removed once all kernels with this bug are
> sufficiently deprecated.
> 
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=1662291
> Reported-by: Pei Zhang <pezhang@redhat.com>
> Debugged-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  hw/vfio/common.c     |   20 +++++++++++++++++++-
>  hw/vfio/trace-events |    1 +
>  2 files changed, 20 insertions(+), 1 deletion(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>