> From: Jacob Pan > Sent: Tuesday, June 30, 2020 7:05 AM > > On Fri, 26 Jun 2020 16:19:23 -0600 > Alex Williamson <alex.william...@redhat.com> wrote: > > > On Tue, 23 Jun 2020 10:03:53 -0700 > > Jacob Pan <jacob.jun....@linux.intel.com> 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....@intel.com> > > > Signed-off-by: Jacob Pan <jacob.jun....@linux.intel.com> > > > --- > > > 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? > > > VFIO does nothing other than returning the status from IOMMU driver. > Based on the return status, users such as QEMU can cause fault > conditions within the vIOMMU.
But from above description, "user such as VFIO has to handle the following cases"... Thanks Kevin > > > > + > > > +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. > > > I was thinking argsz doesn't have to be the exact struct size. It should > be whatever the sufficient & correct size used by the user for a given > call. > > For example, current struct iommu_gpasid_bind_data {} only has VT-d > data. If it gets extended with SMMU data in the union, VT-d vIOMMU > emulation should only fill the union size of vt-d. > > > > + 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) > > > + > This should be removed. Sorry about the confusion. The patch does not > have two data pointers, just separate APIs for kernel and user. > > > > +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. > Separate APIs are proposed in the patchset to address UAPIs > with both kernel and user callers. Sorry about the last line in the > example above. Currently, only unbind_gpasid() and page_response() have > both kernel and userspace callers. e.g. > > /* userspace caller */ > int iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct > device *dev, > void __user *udata) > > /* in-kernel caller */ > int __iommu_sva_unbind_gpasid(struct iommu_domain *domain, struct > device *dev, > struct iommu_gpasid_bind_data *data) > > We don;t expect in-kernel caller for cache invalidate in that it is > implied in unmap, unbind operations. > > > 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, > Yes, I should move up the version check from vendor driver. > > > 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. > You are right, we should sanitize reserved bits. > > > 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. > Good point, all reserved/unused bits should be tested. > > > 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, > > > Agreed. will add checks in the IOMMU generic layer for reserved > bits. > For VT-d vendor driver, we do check all bits in cache types, i.e. in > intel/iommu.c > for_each_set_bit(cache_type, > (unsigned long *)&inv_info->cache, > IOMMU_CACHE_INV_TYPE_NR) { > > > one other hardening is to check vendor argsz. This is in the > bind_gpasid call. > > if (data->argsz != offsetofend(struct iommu_gpasid_bind_data, > vendor.vtd)) > return -EINVAL; > > > > > Alex > > > > [Jacob Pan] > > > > _______________________________________________ > iommu mailing list > iommu@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/iommu _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu