Re: [RFC PATCH 0/6] virtio: Solution to restrict memory access under Xen using xen-virtio DMA ops layer

[Oleksandr <olekstysh@gmail.com>]

On 15.04.22 11:44, Michael S. Tsirkin wrote:


Hello Michael



> On Thu, Apr 14, 2022 at 10:19:27PM +0300, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> Hello all.
>>
>> The purpose of this RFC patch series is to add support for restricting memory access under Xen using specific
>> grant table based DMA ops layer. Patch series is based on Juergen Gross’ initial work [1] which implies using
>> grant references instead of raw guest physical addresses (GPA) for the virtio communications (some kind of
>> the software IOMMU).
>>
>> The high level idea is to create new Xen’s grant table based DMA ops layer for the guest Linux whose main
>> purpose is to provide a special 64-bit DMA address which is formed by using the grant reference (for a page
>> to be shared with the backend) with offset and setting the highest address bit (this is for the backend to
>> be able to distinguish grant ref based DMA address from normal GPA). For this to work we need the ability
>> to allocate contiguous (consecutive) grant references for multi-page allocations. And the backend then needs
>> to offer VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 feature bits (it must support virtio-mmio modern
>> transport for 64-bit addresses in the virtqueue).
> I'm not enough of a xen expert to review this, and I didn't get
> all patches, but I'm very happy to see that approach being
> taken. VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 are
> exactly the way to declare not all of memory is accessible.
> Thanks!

I am happy to hear that! Thank you.


Regarding the "all patches" I have already redirect missing ones, I hope 
you and Christoph will get them.

Sorry for the inconvenience.


>
>> Xen's grant mapping mechanism is the secure and safe solution to share pages between domains which proven
>> to work and works for years (in the context of traditional Xen PV drivers for example). So far, the foreign
>> mapping is used for the virtio backend to map and access guest memory. With the foreign mapping, the backend
>> is able to map arbitrary pages from the guest memory (or even from Dom0 memory). And as the result, the malicious
>> backend which runs in a non-trusted domain can take advantage of this. Instead, with the grant mapping
>> the backend is only allowed to map pages which were explicitly granted by the guest before and nothing else.
>> According to the discussions in various mainline threads this solution would likely be welcome because it
>> perfectly fits in the security model Xen provides.
>>
>> What is more, the grant table based solution requires zero changes to the Xen hypervisor itself at least
>> with virtio-mmio and DT (in comparison, for example, with "foreign mapping + virtio-iommu" solution which would
>> require the whole new complex emulator in hypervisor in addition to new functionality/hypercall to pass IOVA
>> from the virtio backend running elsewhere to the hypervisor and translate it to the GPA before mapping into
>> P2M or denying the foreign mapping request if no corresponding IOVA-GPA mapping present in the IOMMU page table
>> for that particular device). We only need to update toolstack to insert a new "xen,dev-domid" property to
>> the virtio-mmio device node when creating a guest device-tree (this is an indicator for the guest to use grants
>> and the ID of Xen domain where the corresponding backend resides, it is used as an argument to the grant mapping
>> APIs). It worth mentioning that toolstack patch is based on non  upstreamed yet “Virtio support for toolstack
>> on Arm” series which is on review now [2].
>>
>> Please note the following:
>> - Patch series only covers Arm and virtio-mmio (device-tree) for now. To enable the restricted memory access
>>    feature on Arm the following options should be set:
>>    CONFIG_XEN_VIRTIO = y
>>    CONFIG_XEN_HVM_VIRTIO_GRANT = y
>> - Some callbacks in xen-virtio DMA ops layer (map_sg/unmap_sg, etc) are not implemented yet as they are not
>>    needed/used in the first prototype
>>
>> Patch series is rebased on Linux 5.18-rc2 tag and tested on Renesas Salvator-X board + H3 ES3.0 SoC (Arm64)
>> with standalone userspace (non-Qemu) virtio-mmio based virtio-disk backend running in Driver domain and Linux
>> guest running on existing virtio-blk driver (frontend). No issues were observed. Guest domain 'reboot/destroy'
>> use-cases work properly. I have also tested other use-cases such as assigning several virtio block devices
>> or a mix of virtio and Xen PV block devices to the guest.
>>
>> 1. Xen changes located at (last patch):
>> https://github.com/otyshchenko1/xen/commits/libxl_virtio_next
>> 2. Linux changes located at:
>> https://github.com/otyshchenko1/linux/commits/virtio_grant5
>> 3. virtio-disk changes located at:
>> https://github.com/otyshchenko1/virtio-disk/commits/virtio_grant
>>
>> Any feedback/help would be highly appreciated.
>>
>> [1] https://www.youtube.com/watch?v=IrlEdaIUDPk
>> [2] https://lore.kernel.org/xen-devel/1649442065-8332-1-git-send-email-olekstysh@gmail.com/
>>
>> Juergen Gross (2):
>>    xen/grants: support allocating consecutive grants
>>    virtio: add option to restrict memory access under Xen
>>
>> Oleksandr Tyshchenko (4):
>>    dt-bindings: xen: Add xen,dev-domid property description for
>>      xen-virtio layer
>>    virtio: Various updates to xen-virtio DMA ops layer
>>    arm/xen: Introduce xen_setup_dma_ops()
>>    arm/xen: Assign xen-virtio DMA ops for virtio devices in Xen guests
>>
>>   .../devicetree/bindings/virtio/xen,dev-domid.yaml  |  39 +++
>>   arch/arm/include/asm/xen/xen-ops.h                 |   1 +
>>   arch/arm/mm/dma-mapping.c                          |   5 +-
>>   arch/arm/xen/enlighten.c                           |  11 +
>>   arch/arm64/include/asm/xen/xen-ops.h               |   1 +
>>   arch/arm64/mm/dma-mapping.c                        |   5 +-
>>   arch/x86/mm/init.c                                 |  15 +
>>   arch/x86/mm/mem_encrypt.c                          |   5 -
>>   arch/x86/xen/Kconfig                               |   9 +
>>   drivers/xen/Kconfig                                |  20 ++
>>   drivers/xen/Makefile                               |   1 +
>>   drivers/xen/grant-table.c                          | 238 +++++++++++++--
>>   drivers/xen/xen-virtio.c                           | 335 +++++++++++++++++++++
>>   include/xen/arm/xen-ops.h                          |  20 ++
>>   include/xen/grant_table.h                          |   4 +
>>   include/xen/xen-ops.h                              |  13 +
>>   16 files changed, 679 insertions(+), 43 deletions(-)
>>   create mode 100644 Documentation/devicetree/bindings/virtio/xen,dev-domid.yaml
>>   create mode 100644 arch/arm/include/asm/xen/xen-ops.h
>>   create mode 100644 arch/arm64/include/asm/xen/xen-ops.h
>>   create mode 100644 drivers/xen/xen-virtio.c
>>   create mode 100644 include/xen/arm/xen-ops.h
>>
>> -- 
>> 2.7.4

-- 
Regards,

Oleksandr Tyshchenko


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