lock order inversion in transitive grant copy handling

ISSUE DESCRIPTION As part of XSA-226 a missing cleanup call was inserted on an error handling path. While doing so, locking requirements were not paid attention to. As a result two cooperating guests granting each other transitive grants can cause locks to be acquired nested within one another, b...

insufficient TLB flush for x86 PV guests in shadow mode

ISSUE DESCRIPTION For migration as well as to work around kernels unaware of L1TF see XSA-273, PV guests may be run in shadow paging mode. To address XSA-401, code was moved inside a function in Xen. This code movement missed a variable changing meaning / value between old and new code positions...

Retbleed - arbitrary speculative code execution with return instructions

ISSUE DESCRIPTION Researchers at ETH Zurich have discovered Retbleed, allowing for arbitrary speculative execution in a victim context. For more details, see: https://comsec.ethz.ch/retbleed ETH Zurich have allocated CVE-2022-29900 for AMD and CVE-2022-29901 for Intel. Despite the similar...

Arm guests can cause Dom0 DoS via PV devices

ISSUE DESCRIPTION When mapping pages of guests on Arm, dom0 is using an rbtree to keep track of the foreign mappings. Updating of that rbtree is not always done completely with the related lock held, resulting in a small race window, which can be used by unprivileged guests via PV devices to caus...

Linux disk/nic frontends data leaks

ISSUE DESCRIPTION Linux Block and Network PV device frontends don't zero memory regions before sharing them with the backend CVE-2022-26365, CVE-2022-33740. Additionally the granularity of the grant table doesn't allow sharing less than a 4K page, leading to unrelated data residing in the same 4K...

network backend may cause Linux netfront to use freed SKBs

ISSUE DESCRIPTION While adding logic to support XDP eXpress Data Path, a code label was moved in a way allowing for SKBs having references pointers retained for further processing to nevertheless be freed. IMPACT A misbehaving or malicious backend may cause a Denial of Service DoS in the guest...

x86: MMIO Stale Data vulnerabilities

ISSUE DESCRIPTION This issue is related to the SRBDS, TAA and MDS vulnerabilities. Please see: https://xenbits.xen.org/xsa/advisory-320.html SRBDS https://xenbits.xen.org/xsa/advisory-305.html TAA https://xenbits.xen.org/xsa/advisory-297.html MDS Please see Intel's whitepaper:...

x86 pv: Insufficient care with non-coherent mappings

ISSUE DESCRIPTION Xen maintains a type reference count for pages, in addition to a regular reference count. This scheme is used to maintain invariants required for Xen's safety, e.g. PV guests may not have direct writeable access to pagetables; updates need auditing by Xen. Unfortunately, Xen's...

x86 pv: Race condition in typeref acquisition

ISSUE DESCRIPTION Xen maintains a type reference count for pages, in addition to a regular reference count. This scheme is used to maintain invariants required for Xen's safety, e.g. PV guests may not have direct writeable access to pagetables; updates need auditing by Xen. Unfortunately, the log...

race in VT-d domain ID cleanup

ISSUE DESCRIPTION Xen domain IDs are up to 15 bits wide. VT-d hardware may allow for only less than 15 bits to hold a domain ID associating a physical device with a particular domain. Therefore internally Xen domain IDs are mapped to the smaller value range. The cleaning up of the housekeeping...

IOMMU: RMRR (VT-d) and unity map (AMD-Vi) handling issues

ISSUE DESCRIPTION Certain PCI devices in a system might be assigned Reserved Memory Regions specified via Reserved Memory Region Reporting, "RMRR" for Intel VT-d or Unity Mapping ranges for AMD-Vi. These are typically used for platform tasks such as legacy USB emulation. Since the precise purpose...

Racy interactions between dirty vram tracking and paging log dirty hypercalls

ISSUE DESCRIPTION Activation of log dirty mode done by XENDMOPtrackdirtyvram was named HVMOPtrackdirtyvram before Xen 4.9 is racy with ongoing log dirty hypercalls. A suitably timed call to XENDMOPtrackdirtyvram can enable log dirty while another CPU is still in the process of tearing down the...

Linux PV device frontends vulnerable to attacks by backends

ISSUE DESCRIPTION Several Linux PV device frontends are using the grant table interfaces for removing access rights of the backends in ways being subject to race conditions, resulting in potential data leaks, data corruption by malicious backends, and denial of service triggered by malicious...

Multiple speculative security issues

ISSUE DESCRIPTION Note: Multiple issues are contained in this XSA due to their interactions. 1 Researchers at VU Amsterdam have discovered Spectre-BHB, pertaining to the use of Branch History between privilege levels. ARM have assigned CVE-2022-23960. Intel have assigned CVE-2022-0001 Branch...

Insufficient cleanup of passed-through device IRQs

ISSUE DESCRIPTION The management of IRQs associated with physical devices exposed to x86 HVM guests involves an iterative operation in particular when cleaning up after the guest's use of the device. In the case where an interrupt is not quiescent yet at the time this cleanup gets invoked, the...

A PV guest could DoS Xen while unmapping a grant

ISSUE DESCRIPTION To address XSA-380, reference counting was introduced for grant mappings for the case where a PV guest would have the IOMMU enabled. PV guests can request two forms of mappings. When both are in use for any individual mapping, unmapping of such a mapping can be requested in two...

arm: guest_physmap_remove_page not removing the p2m mappings

ISSUE DESCRIPTION The functions to remove one or more entries from a guest p2m pagetable on Arm p2mremovemapping, guestphysmapremovepage, and p2msetentry with mfn set to INVALIDMFN do not actually clear the pagetable entry if the entry doesn't have the valid bit set. It is possible to have a vali...

frontends vulnerable to backends

ISSUE DESCRIPTION Xen offers the ability to run PV backends in regular unprivileged guests, typically referred to as "driver domains". Running PV backends in driver domains has one primary security advantage: if a driver domain gets compromised, it doesn't have the privileges to take over the...

Guest can force Linux netback driver to hog large amounts of kernel memory

ISSUE DESCRIPTION Incoming data packets for a guest in the Linux kernel's netback driver are buffered until the guest is ready to process them. There are some measures taken for avoiding to pile up too much data, but those can be bypassed by the guest: There is a timeout how long the client side ...

Rogue backends can cause DoS of guests via high frequency events

ISSUE DESCRIPTION Xen offers the ability to run PV backends in regular unprivileged guests, typically referred to as "driver domains". Running PV backends in driver domains has one primary security advantage: if a driver domain gets compromised, it doesn't have the privileges to take over the...

PoD operations on misaligned GFNs

ISSUE DESCRIPTION x86 HVM and PVH guests may be started in populate-on-demand PoD mode, to provide a way for them to later easily have more memory assigned. Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specifie...

grant table v2 status pages may remain accessible after de-allocation (take two)

ISSUE DESCRIPTION Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched back from v2 to v1. The freeing of such...

issues with partially successful P2M updates on x86

ISSUE DESCRIPTION x86 HVM and PVH guests may be started in populate-on-demand PoD mode, to provide a way for them to later easily have more memory assigned. Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specifie...

guests may exceed their designated memory limit

ISSUE DESCRIPTION When a guest is permitted to have close to 16TiB of memory, it may be able to issue hypercalls to increase its memory allocation beyond the administrator established limit. This is a result of a calculation done with 32-bit precision, which may overflow. It would then only be th...

certain VT-d IOMMUs may not work in shared page table mode

ISSUE DESCRIPTION For efficiency reasons, address translation control structures page tables may and, on suitable hardware, by default will be shared between CPUs, for second-level translation EPT, and IOMMUs. These page tables are presently set up to always be 4 levels deep. However, an IOMMU ma...

PCI devices with RMRRs not deassigned correctly

ISSUE DESCRIPTION Certain PCI devices in a system might be assigned Reserved Memory Regions specified via Reserved Memory Region Reporting, "RMRR". These are typically used for platform tasks such as legacy USB emulation. If such a device is passed through to a guest, then on guest shutdown the...

Another race in XENMAPSPACE_grant_table handling

ISSUE DESCRIPTION Guests are permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, are de-allocated when a guest switches back from v2 to v1. Freeing such pages...

IOMMU page mapping issues on x86

ISSUE DESCRIPTION Both AMD and Intel allow ACPI tables to specify regions of memory which should be left untranslated, which typically means these addresses should pass the translation phase unaltered. While these are typically device specific ACPI properties, they can also be specified to apply ...

inadequate grant-v2 status frames array bounds check

ISSUE DESCRIPTION The v2 grant table interface separates grant attributes from grant status. That is, when operating in this mode, a guest has two tables. As a result, guests also need to be able to retrieve the addresses that the new status tracking table can be accessed through. For 32-bit gues...

grant table v2 status pages may remain accessible after de-allocation

ISSUE DESCRIPTION Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched back from v2 to v1. The freeing of such...

long running loops in grant table handling

ISSUE DESCRIPTION In order to properly monitor resource use, Xen maintains information on the grant mappings a domain may create to map grants offered by other domains. In the process of carrying out certain actions, Xen would iterate over all such entries, including ones which aren't in use...

xen/arm: No memory limit for dom0less domUs

ISSUE DESCRIPTION The dom0less feature allows an administrator to create multiple unprivileged domains directly from Xen. Unfortunately, the memory limit from them is not set. This allow a domain to allocate memory beyond what an administrator originally configured. IMPACT Malicious dom0less gues...

Guest triggered use-after-free in Linux xen-netback

ISSUE DESCRIPTION A malicious or buggy network PV frontend can force Linux netback to disable the interface and terminate the receive kernel thread associated with queue 0 in response to the frontend sending a malformed packet. Such kernel thread termination will lead to a use-after-free in Linux...

Speculative Code Store Bypass

ISSUE DESCRIPTION Modern superscalar processors may employ sophisticated decoding and caching of the instruction stream to improve performance. However, a consequence is that self-modifying code updates may not take effect instantly. Whatever the architectural guarantees, some CPUs have...

xen/arm: Boot modules are not scrubbed

ISSUE DESCRIPTION The bootloader will load boot modules e.g. kernel, initramfs... in a temporary area before they are copied by Xen to each domain memory. To ensure sensitive data is not leaked from the modules, Xen must "scrub" them before handing the page over to the allocator. Unfortunately, i...

x86: TSX Async Abort protections not restored after S3

ISSUE DESCRIPTION This issue relates to the TSX Async Abort speculative security vulnerability. Please see https://xenbits.xen.org/xsa/advisory-305.html for details. Mitigating TAA by disabling TSX the default and preferred option requires selecting a non-default setting in MSRTSXCTRL. This setti...

inappropriate x86 IOMMU timeout detection / handling

ISSUE DESCRIPTION IOMMUs process commands issued to them in parallel with the operation of the CPUs issuing such commands. In the current implementation in Xen, asynchronous notification of the completion of such commands is not used. Instead, the issuing CPU spin-waits for the completion of the...

x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests

ISSUE DESCRIPTION 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different...

Linux: blkback driver may leak persistent grants

ISSUE DESCRIPTION The fix for XSA-365 includes initialization of pointers such that subsequent cleanup code wouldn't use uninitialized or stale values. This initialization went too far and may under certain conditions also overwrite pointers which are in need of cleaning up. The lack of cleanup...

HVM soft-reset crashes toolstack

ISSUE DESCRIPTION libxl requires all data structures passed across its public interface to be initialized before use and disposed of afterwards by calling a specific set of functions. Many internal data structures also require this initialize / dispose discipline, but not all of them. When the...

Linux: special config may crash when trying to map foreign pages

ISSUE DESCRIPTION With CONFIGXENBALLOONMEMORYHOTPLUG disabled and CONFIGXENUNPOPULATEDALLOC enabled the Linux kernel will use guest physical addresses allocated via the ZONEDEVICE functionality for mapping foreign guest's pages. This will result in problems, as the p2m list will only cover the...

Linux: netback fails to honor grant mapping errors

ISSUE DESCRIPTION XSA-362 tried to address issues here, but in the case of the netback driver the changes were insufficient: It left the relevant function invocation with, effectively, no error handling at all. As a result, memory allocation failures there could still lead to frontend-induced...

missed flush in XSA-321 backport

ISSUE DESCRIPTION An oversight was made when backporting XSA-321, leading entries in the IOMMU not being properly updated under certain circumstances. IMPACT A malicious guest may be able to retain read/write DMA access to frames returned to Xen's free pool, and later reused for another purpose...

Linux: backends treating grant mapping errors as bugs

ISSUE DESCRIPTION Block, net, and SCSI backends consider certain errors a plain bug, deliberately causing a kernel crash. For errors potentially being at least under the influence of guests, like out of memory conditions, it isn't correct to assume so. Memory allocations potentially causing such...

Linux: grant mapping error handling issues

ISSUE DESCRIPTION Grant mapping operations often occur in batch hypercalls, where a number of operations are done in a single hypercall, the success or failure of each one reported to the backend driver, and the backend driver then loops over the results, performing follow-up actions based on the...

Linux: error handling issues in blkback's grant mapping

ISSUE DESCRIPTION To service requests, the driver maps grant references provided by the frontend. In this process, errors may be encountered. In one case an error encountered earlier might be discarded by later processing, resulting in the caller assuming successful mapping, and hence subsequent...

Linux: display frontend "be-alloc" mode is unsupported

ISSUE DESCRIPTION The backend allocation mode of Linux'es drmxenfront drivers was not meant to be a supported configuration, but this wasn't stated accordingly in its support status entry. IMPACT Use of the feature may have unknown effects. VULNERABLE SYSTEMS Linux versions from 4.18 onwards are...

arm: The cache may not be cleaned for newly allocated scrubbed pages

ISSUE DESCRIPTION On Arm, a guest is allowed to control whether memory access bypass the cache. This means that Xen needs to ensure that all writes such as the ones during scrubbing have reached memory before handing over the page to a guest. Unfortunately the operation to clean the cache happens...

IRQ vector leak on x86

ISSUE DESCRIPTION An x86 HVM guest with PCI pass through devices can force the allocation of all IDT vectors on the system by rebooting itself with MSI or MSI-X capabilities enabled and entries setup. Such reboots will leak any vectors used by the MSI-X entries that the guest might had enabled, a...

oxenstored: node ownership can be changed by unprivileged clients

ISSUE DESCRIPTION Nodes in xenstore have an ownership. In oxenstored, a owner could give a node away. But node ownership has quota implications. Any guest can run another guest out of quota, or create an unbounded number of nodes owned by dom0, thus running xenstored out of memory IMPACT A...