Re: [PATCH v3 1/5] docs: IOMMU user API

[Alex Williamson]

On Tue, 23 Jun 2020 10:03:53 -0700
Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> wrote:

> IOMMU UAPI is newly introduced to support communications between guest
> virtual IOMMU and host IOMMU. There has been lots of discussions on how
> it should work with VFIO UAPI and userspace in general.
> 
> This document is indended to clarify the UAPI design and usage. The
> mechenics of how future extensions should be achieved are also covered
> in this documentation.
> 
> Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> ---
>  Documentation/userspace-api/iommu.rst | 244 ++++++++++++++++++++++++++++++++++
>  1 file changed, 244 insertions(+)
>  create mode 100644 Documentation/userspace-api/iommu.rst
> 
> diff --git a/Documentation/userspace-api/iommu.rst b/Documentation/userspace-api/iommu.rst
> new file mode 100644
> index 000000000000..f9e4ed90a413
> --- /dev/null
> +++ b/Documentation/userspace-api/iommu.rst
> @@ -0,0 +1,244 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +.. iommu:
> +
> +=====================================
> +IOMMU Userspace API
> +=====================================
> +
> +IOMMU UAPI is used for virtualization cases where communications are
> +needed between physical and virtual IOMMU drivers. For native
> +usage, IOMMU is a system device which does not need to communicate
> +with user space directly.
> +
> +The primary use cases are guest Shared Virtual Address (SVA) and
> +guest IO virtual address (IOVA), wherein a virtual IOMMU (vIOMMU) is
> +required to communicate with the physical IOMMU in the host.
> +
> +.. contents:: :local:
> +
> +Functionalities
> +===============
> +Communications of user and kernel involve both directions. The
> +supported user-kernel APIs are as follows:
> +
> +1. Alloc/Free PASID
> +2. Bind/unbind guest PASID (e.g. Intel VT-d)
> +3. Bind/unbind guest PASID table (e.g. ARM sMMU)
> +4. Invalidate IOMMU caches
> +5. Service page requests
> +
> +Requirements
> +============
> +The IOMMU UAPIs are generic and extensible to meet the following
> +requirements:
> +
> +1. Emulated and para-virtualised vIOMMUs
> +2. Multiple vendors (Intel VT-d, ARM sMMU, etc.)
> +3. Extensions to the UAPI shall not break existing user space
> +
> +Interfaces
> +==========
> +Although the data structures defined in IOMMU UAPI are self-contained,
> +there is no user API functions introduced. Instead, IOMMU UAPI is
> +designed to work with existing user driver frameworks such as VFIO.
> +
> +Extension Rules & Precautions
> +-----------------------------
> +When IOMMU UAPI gets extended, the data structures can *only* be
> +modified in two ways:
> +
> +1. Adding new fields by re-purposing the padding[] field. No size change.
> +2. Adding new union members at the end. May increase in size.
> +
> +No new fields can be added *after* the variable sized union in that it
> +will break backward compatibility when offset moves. In both cases, a
> +new flag must be accompanied with a new field such that the IOMMU
> +driver can process the data based on the new flag. Version field is
> +only reserved for the unlikely event of UAPI upgrade at its entirety.
> +
> +It's *always* the caller's responsibility to indicate the size of the
> +structure passed by setting argsz appropriately.
> +Though at the same time, argsz is user provided data which is not
> +trusted. The argsz field allows the user to indicate how much data
> +they're providing, it's still the kernel's responsibility to validate
> +whether it's correct and sufficient for the requested operation.
> +
> +Compatibility Checking
> +----------------------
> +When IOMMU UAPI extension results in size increase, user such as VFIO
> +has to handle the following cases:
> +
> +1. User and kernel has exact size match
> +2. An older user with older kernel header (smaller UAPI size) running on a
> +   newer kernel (larger UAPI size)
> +3. A newer user with newer kernel header (larger UAPI size) running
> +   on an older kernel.
> +4. A malicious/misbehaving user pass illegal/invalid size but within
> +   range. The data may contain garbage.

What exactly does vfio need to do to handle these?

> +
> +Feature Checking
> +----------------
> +While launching a guest with vIOMMU, it is important to ensure that host
> +can support the UAPI data structures to be used for vIOMMU-pIOMMU
> +communications. Without upfront compatibility checking, future faults
> +are difficult to report even in normal conditions. For example, TLB
> +invalidations should always succeed. There is no architectural way to
> +report back to the vIOMMU if the UAPI data is incompatible. If that
> +happens, in order to protect IOMMU iosolation guarantee, we have to
> +resort to not giving completion status in vIOMMU. This may result in
> +VM hang.
> +
> +For this reason the following IOMMU UAPIs cannot fail:
> +
> +1. Free PASID
> +2. Unbind guest PASID
> +3. Unbind guest PASID table (SMMU)
> +4. Cache invalidate
> +
> +User applications such as QEMU is expected to import kernel UAPI
> +headers. Backward compatibility is supported per feature flags.
> +For example, an older QEMU (with older kernel header) can run on newer
> +kernel. Newer QEMU (with new kernel header) may refuse to initialize
> +on an older kernel if new feature flags are not supported by older
> +kernel. Simply recompile existing code with newer kernel header should
> +not be an issue in that only existing flags are used.
> +
> +IOMMU vendor driver should report the below features to IOMMU UAPI
> +consumers (e.g. via VFIO).
> +
> +1. IOMMU_NESTING_FEAT_SYSWIDE_PASID
> +2. IOMMU_NESTING_FEAT_BIND_PGTBL
> +3. IOMMU_NESTING_FEAT_BIND_PASID_TABLE
> +4. IOMMU_NESTING_FEAT_CACHE_INVLD
> +5. IOMMU_NESTING_FEAT_PAGE_REQUEST
> +
> +Take VFIO as example, upon request from VFIO user space (e.g. QEMU),
> +VFIO kernel code shall query IOMMU vendor driver for the support of
> +the above features. Query result can then be reported back to the
> +user-space caller. Details can be found in
> +Documentation/driver-api/vfio.rst.
> +
> +
> +Data Passing Example with VFIO
> +------------------------------
> +As the ubiquitous userspace driver framework, VFIO is already IOMMU
> +aware and share many key concepts such as device model, group, and
> +protection domain. Other user driver frameworks can also be extended
> +to support IOMMU UAPI but it is outside the scope of this document.
> +
> +In this tight-knit VFIO-IOMMU interface, the ultimate consumer of the
> +IOMMU UAPI data is the host IOMMU driver. VFIO facilitates user-kernel
> +transport, capability checking, security, and life cycle management of
> +process address space ID (PASID).
> +
> +Unlike normal user data passed via VFIO UAPI IOTCL, IOMMU driver is the
> +ultimate consumer of its UAPI data. At VFIO layer, the IOMMU UAPI data
> +is wrapped in a VFIO UAPI data. It follows the
> +pattern below::
> +
> +   struct {
> +	__u32 argsz;
> +	__u32 flags;
> +	__u8  data[];
> +   };
> +
> +Here data[] contains the IOMMU UAPI data structures. VFIO has the
> +freedom to bundle the data as well as parse data size based on its own flags.
> +
> +In order to determine the size and feature set of the user data, argsz
> +and flags are also embedded in the IOMMU UAPI data structures.
> +A "__u32 argsz" field is *always* at the beginning of each structure.
> +
> +For example:
> +::
> +
> +   struct iommu_cache_invalidate_info {
> +	__u32	argsz;
> +	#define IOMMU_CACHE_INVALIDATE_INFO_VERSION_1 1
> +	__u32	version;
> +	/* IOMMU paging structure cache */
> +	#define IOMMU_CACHE_INV_TYPE_IOTLB	(1 << 0) /* IOMMU IOTLB */
> +	#define IOMMU_CACHE_INV_TYPE_DEV_IOTLB	(1 << 1) /* Device IOTLB */
> +	#define IOMMU_CACHE_INV_TYPE_PASID	(1 << 2) /* PASID cache */
> +	#define IOMMU_CACHE_INV_TYPE_NR		(3)
> +	__u8	cache;
> +	__u8	granularity;
> +	__u8	padding[2];
> +	union {
> +		struct iommu_inv_pasid_info pasid_info;
> +		struct iommu_inv_addr_info addr_info;
> +	} granu;
> +   };
> +
> +VFIO is responsible for checking its own argsz and flags then invokes
> +appropriate IOMMU UAPI functions. User pointer is passed to IOMMU
> +layer for further processing. The responsibilities are divided as
> +follows:
> +
> +- Generic IOMMU layer checks argsz range and override out-of-range
> +  value. If the exact argsz is based on generic flags, they are checked
> +  here as well.
> +
> +- Vendor IOMMU driver checks argsz based on vendor flags, UAPI data
> +  is consumed based on flags
> +
> +Once again, use guest TLB invalidation as an example, argsz is based
> +on generic flags in the invalidation information. IOMMU generic code
> +shall process the UAPI data as the following:
> +
> +::
> +
> + int iommu_cache_invalidate(struct iommu_domain *domain, struct device *dev,
> +			void __user *uinfo)
> + {
> +	/* Current kernel data size is the max to be copied from user */
> +	maxsz = sizeof(struct iommu_cache_invalidate_info);
> +	memset((void *)&inv_info, 0, maxsz);
> +
> +	/*
> +	 * No new spaces can be added before the variable sized union, the
> +	 * minimum size is the offset to the union.
> +	 */
> +	minsz = offsetof(struct iommu_cache_invalidate_info, granu);
> +
> +	/* Copy minsz from user to get flags and argsz */
> +	if (copy_from_user(&inv_info, uinfo, minsz))
> +		return -EFAULT;
> +
> +	/* Fields before variable size union is mandatory */
> +	if (inv_info.argsz < minsz)
> +		return -EINVAL;
> +	/*
> +	 * User might be using a newer UAPI header which has a larger data
> +	 * size, we shall support the existing flags within the current
> +	 * size.
> +	 */
> +	if (inv_info.argsz > maxsz)
> +		inv_info.argsz = maxsz;
> +
> +	/* Checking the exact argsz based on generic flags */
> +	if (inv_info.granularity == IOMMU_INV_GRANU_ADDR &&
> +		inv_info.argsz != offsetofend(struct iommu_cache_invalidate_info,
> +					granu.addr_info))

Is it really reasonable to expect the user to specify argsz to the
exact union element for the callback?  I'd certainly expect users to
simply use sizeof(struct iommu_cache_invalidate_info) and it should
therefore be sufficient to test >= here rather than jump through hoops
with an exact size.  We're already changing inv_info.argsz above to fit
our known structure, it's inconsistent to then expect it to be some
exact value.
 
> +		return -EINVAL;
> +
> +	if (inv_info.granularity == IOMMU_INV_GRANU_PASID &&
> +		inv_info.argsz != offsetofend(struct
> iommu_cache_invalidate_info,
> +					granu.pasid_info))
> +		return -EINVAL;
> +
> +	/* Copy the remaining user data _after_ minsz */
> +	if (copy_from_user((void *)&inv_info + minsz, uinfo + minsz,
> +				inv_info.argsz - minsz))
> +		return -EFAULT;
> +
> +	return domain->ops->cache_invalidate(domain, dev, &inv_info);
> + }
> + Add a wrapper
> +   __iommu_unbind_( kernel data, same user data, kernel copy)
> +
> +Notice that in this example, since union size is determined by
> generic +flags, all checking to argsz is validated in the generic
> IOMMU layer, +vendor driver does not need to check argsz. However, if
> union size is +based on vendor data, such as iommu_sva_bind_gpasid(),
> it will be +vendor driver's responsibility to validate the exact
> argsz.

struct iommu_cache_invalidate_info is a good example because it
explicitly states a table of type vs granularity validity.  When the
cache_invalidate() callback is used by an internal user we can consider
it a bug in the caller if its usage falls outside of these prescribed
valid combinations, ie. iommu_ops callbacks may assume a trusted caller
that isn't trying to exploit any loophole.  But here we've done nothing
more than validated the supplied size to pass it through to a non-user
hardened callback.  We didn't check the version, we didn't check that
any of the undefined bits in cache or granularity or padding were set,
we don't know what flags might be set in the union elements.  For
example, if a user is able to set a flag that gets ignored now, that
means we can never use that flag without potentially breaking that user
in the future.  If a user can pass in version 3141592654 now, then we
can never use version for validation.  I see that
intel_iommu_sva_invalidate() does test the version, but has no obvious
other hardening.  I'm afraid we're being far to lax about accepting a
data structure provided by a user, we should not assume good faith.
Thanks,

Alex