Re: [PATCH v5 00/14] Fix BUG_ON in vfio_iommu_group_notifier()

[Lu Baolu <baolu.lu@linux.intel.com>]

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@linux.intel.com/

Best regards,
baolu