x86: Multiple speculative security issues

ISSUE DESCRIPTION 1 Researchers have discovered that on some AMD CPUs, the implementation of IBPB Indirect Branch Prediction Barrier does not behave according to the specification. Specifically, IBPB fails to properly flush the RAS Return Address Stack, also RSB - Return Stack Buffer - in Intel...

Xenstore: Guests can create arbitrary number of nodes via transactions

ISSUE DESCRIPTION In case a node has been created in a transaction and it is later deleted in the same transaction, the transaction will be terminated with an error. As this error is encountered only when handling the deleted node at transaction finalization, the transaction will have been...

Xenstore: guests can let run xenstored out of memory

ISSUE DESCRIPTION Malicious guests can cause xenstored to allocate vast amounts of memory, eventually resulting in a Denial of Service DoS of xenstored. There are multiple ways how guests can cause large memory allocations in xenstored: - - by issuing new requests to xenstored without reading the...

x86: unintended memory sharing between guests

ISSUE DESCRIPTION On Intel systems that support the "virtualize APIC accesses" feature, a guest can read and write the global shared xAPIC page by moving the local APIC out of xAPIC mode. Access to this shared page bypasses the expected isolation that should exist between two guests. IMPACT Guest...

Oxenstored 32->31 bit integer truncation issues

ISSUE DESCRIPTION Integers in Ocaml are 63 or 31 bits of signed precision. The Ocaml Xenbus library takes a C uint32t out of the ring and casts it directly to an Ocaml integer. In 64-bit Ocaml builds this is fine, but in 32-bit builds, it truncates off the most significant bit, and then creates...

Xenstore: Guests can crash xenstored via exhausting the stack

ISSUE DESCRIPTION Xenstored is using recursion for some Xenstore operations e.g. for deleting a sub-tree of Xenstore nodes. With sufficiently deep nesting levels this can result in stack exhaustion on xenstored, leading to a crash of xenstored. IMPACT A malicious guest creating very deep nesting...

Xenstore: Guests can create orphaned Xenstore nodes

ISSUE DESCRIPTION By creating multiple nodes inside a transaction resulting in an error, a malicious guest can create orphaned nodes in the Xenstore data base, as the cleanup after the error will not remove all nodes already created. When the transaction is committed after this situation, nodes...

Xenstore: Guests can cause Xenstore to not free temporary memory

ISSUE DESCRIPTION When working on a request of a guest, xenstored might need to allocate quite large amounts of memory temporarily. This memory is freed only after the request has been finished completely. A request is regarded to be finished only after the guest has read the response message of...

Xenstore: Cooperating guests can create arbitrary numbers of nodes

ISSUE DESCRIPTION Since the fix of XSA-322 any Xenstore node owned by a removed domain will be modified to be owned by Dom0. This will allow two malicious guests working together to create an arbitrary number of Xenstore nodes. This is possible by domain A letting domain B write into domain A's...

Xenstore: Guests can get access to Xenstore nodes of deleted domains

ISSUE DESCRIPTION Access rights of Xenstore nodes are per domid. When a domain is gone, there might be Xenstore nodes left with access rights containing the domid of the removed domain. This is normally no problem, as those access right entries will be corrected when such a node is written later...

Xenstore: Guests can crash xenstored

ISSUE DESCRIPTION Due to a bug in the fix of XSA-115 a malicious guest can cause xenstored to use a wrong pointer during node creation in an error path, resulting in a crash of xenstored or a memory corruption in xenstored causing further damage. Entering the error path can be controlled by the...

XAPI open file limit DoS

ISSUE DESCRIPTION It is possible for an unauthenticated client on the network to cause XAPI to hit its file-descriptor limit. This causes XAPI to be unable to accept new requests for other trusted clients, and blocks XAPI from carrying out any tasks that require the opening of file descriptors...

P2M pool freeing may take excessively long

ISSUE DESCRIPTION The P2M pool backing second level address translation for guests may be of significant size. Therefore its freeing may take more time than is reasonable without intermediate preemption checks. Such checking for the need to preempt was so far missing. IMPACT A group of...

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...

Arm: unbounded memory consumption for 2nd-level page tables

ISSUE DESCRIPTION Certain actions require e.g. removing pages from a guest's P2M Physical-to-Machine mapping. When large pages are in use to map guest pages in the 2nd-stage page tables, such a removal operation may incur a memory allocation to replace a large mapping with individual smaller ones...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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...

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...

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 ...

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...

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...

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...

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...

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...