Michael S. Tsirkin
2022-Apr-15 08:44 UTC
[RFC PATCH 0/6] virtio: Solution to restrict memory access under Xen using xen-virtio DMA ops layer
On Thu, Apr 14, 2022 at 10:19:27PM +0300, Oleksandr Tyshchenko wrote:> From: Oleksandr Tyshchenko <oleksandr_tyshchenko at 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!> 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 at 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