On 1/4/22 9:56 AM, Lu Baolu wrote:
Hi folks,
The iommu group is the minimal isolation boundary for DMA. Devices in
a group can access each other's MMIO registers via peer to peer DMA
and also need share the same I/O address space.
Once the I/O address space is assigned to user control it is no longer
available to the dma_map* API, which effectively makes the DMA API
non-working.
Second, userspace can use DMA initiated by a device that it controls
to access the MMIO spaces of other devices in the group. This allows
userspace to indirectly attack any kernel owned device and it's driver.
Therefore groups must either be entirely under kernel control or
userspace control, never a mixture. Unfortunately some systems have
problems with the granularity of groups and there are a couple of
important exceptions:
- pci_stub allows the admin to block driver binding on a device and
make it permanently shared with userspace. Since PCI stub does not
do DMA it is safe, however the admin must understand that using
pci_stub allows userspace to attack whatever device it was bound
it.
- PCI bridges are sometimes included in groups. Typically PCI bridges
do not use DMA, and generally do not have MMIO regions.
Generally any device that does not have any MMIO registers is a
possible candidate for an exception.
Currently vfio adopts a workaround to detect violations of the above
restrictions by monitoring the driver core BOUND event, and hardwiring
the above exceptions. Since there is no way for vfio to reject driver
binding at this point, BUG_ON() is triggered if a violation is
captured (kernel driver BOUND event on a group which already has some
devices assigned to userspace). Aside from the bad user experience
this opens a way for root userspace to crash the kernel, even in high
integrity configurations, by manipulating the module binding and
triggering the BUG_ON.
This series solves this problem by making the user/kernel ownership a
core concept at the IOMMU layer. The driver core enforces kernel
ownership while drivers are bound and violations now result in a error
codes during probe, not BUG_ON failures.
Patch partitions:
[PATCH 1-7]: Detect DMA ownership conflicts during driver binding;
[PATCH 8-10]: Add security context management for assigned devices;
[PATCH 11-14]: Various cleanups.
This is also part one of three initial series for IOMMUFD:
* Move IOMMU Group security into the iommu layer
- Generic IOMMUFD implementation
- VFIO ability to consume IOMMUFD
Thank you very much for your comments. A new version of this series has
been posted here:
https://lore.kernel.org/linux-iommu/20220218005521.172832-1-baolu.lu@xxxxxxxxxxxxxxx/
Best regards,
baolu
