arm: Host crash when preempting a multicall

ISSUE DESCRIPTION Early versions of Xen on ARM did not support "multicall" functionality the ability to perform multiple operations via a single hypercall and therefore stubbed out the functionality needed to support preemption of multicalls in a manner which crashed the host. When multicall...

x86: populate-on-demand balloon size inaccuracy can crash guests

ISSUE DESCRIPTION The design of the memory populate-on-demand PoD system requires that a guest's memory ballooning driver reach its memory reduction target. The target is not entirely well-defined in terms of the information visible to the appropriate parts of the system, so some unknown set of...

x86: some pmu and profiling hypercalls log without rate limiting

ISSUE DESCRIPTION HYPERCALLxenoprofop and HYPERVISORxenpmuop log some errors and attempts at invalid operations. These log messages are not rate-limited, even though they can be triggered by guests. IMPACT A malicious guest could cause repeated logging to the hypervisor console, leading to a Deni...

x86: leak of per-domain profiling-related vcpu pointer array

ISSUE DESCRIPTION A domain's xenoprofile state contains an array of per-vcpu information, which is allocated once in the lifetime of a domain in response to that domain using the XENOPROFgetbuffer hypercall on itself or by a domain with the privilege to profile a target domain using the...

leak of main per-domain vcpu pointer array

ISSUE DESCRIPTION A domain's primary array of vcpu pointers can be allocated by a toolstack exactly once in the lifetime of a domain via the XENDOMCTLmaxvcpus hypercall. This array is leaked on domain teardown. This memory leak could -- over time -- exhaust the host's memory. IMPACT A domain give...

arm: Race between domain destruction and memory allocation decrease

ISSUE DESCRIPTION While freeing the memory associated with a domain during domain destruction Xen could race with a toolstack domain reducing the amount of memory associated with that same domain via the XENMEMdecreasereservation. In the case where this race is hit the host will crash. The race i...

x86: Uncontrolled creation of large page mappings by PV guests

ISSUE DESCRIPTION The code to validate level 2 page table entries is bypassed when certain conditions are satisfied. This means that a PV guest can create writeable mappings using super page mappings. Such writeable mappings can violate Xen intended invariants for pages which Xen is supposed to...

libxl fails to honour readonly flag on disks with qemu-xen

ISSUE DESCRIPTION Callers of libxl can specify that a disk should be read-only to the guest. However, there is no code in libxl to pass this information to qemu-xen the upstream-based qemu; and indeed there is no way in qemu to make a disk read-only. The vulnerability is exploitable only via...

printk is not rate-limited in xenmem_add_to_physmap_one

ISSUE DESCRIPTION XENMAPSPACEgmfnforeign dumps the p2m, on ARM, when it fails to get a reference on the foreign page. However, dumpp2mlookup does not use rate-limited printk. A malicious infrastructure domain, which is allowed to map memory of a foreign guest, would be able to flood the Xen...

Use after free in QEMU/Xen block unplug protocol

ISSUE DESCRIPTION When unplugging an emulated block device the device was not fully unplugged, meaning a second unplug attempt would attempt to unplug the device a second time using a previously freed pointer. IMPACT An HVM guest which has access to an emulated IDE disk device may be able to...

QEMU leak of uninitialized heap memory in rtl8139 device model

ISSUE DESCRIPTION The QEMU model of the RTL8139 network card did not sufficiently validate inputs in the C+ mode offload emulation. This results in uninitialised memory from the QEMU process's heap being leaked to the domain as well as to the network. IMPACT A guest may be able to read sensitive...

QEMU heap overflow flaw while processing certain ATAPI commands.

ISSUE DESCRIPTION The QEMU security team has predisclosed the following advisory: A heap overflow flaw was found in the way QEMU's IDE subsystem handled I/O buffer access while processing certain ATAPI commands. A privileged guest user in a guest with CDROM drive enabled could potentially use thi...

xl command line config handling stack overflow

ISSUE DESCRIPTION The xl command line utility mishandles long configuration values when passed as command line arguments, with a buffer overrun. VULNERABLE SYSTEMS Systems built on top of xl which pass laundered or checked but otherwise untrusted configuration values onto xl's command line, witho...

GNTTABOP_swap_grant_ref operation misbehavior

ISSUE DESCRIPTION With the introduction of version 2 grant table operations, a version check became necessary for most grant table related hypercalls. The GNTTABOPswapgrantref call was lacking such a check. As a result, the subsequent code behaved as if version 2 was in use, when a guest issued...

vulnerability in the iret hypercall handler

ISSUE DESCRIPTION A buggy loop in Xen's compatiret function iterates the wrong way around a 32-bit index. Any 32-bit PV guest kernel can trigger this vulnerability by attempting a hypercalliret with EFLAGS.VM set. Given the use of get/putuser, and that the virtual addresses in question are...

Heap overflow in QEMU PCNET controller, allowing guest->host escape

ISSUE DESCRIPTION The QEMU security team has predisclosed the following advisory: pcnettransmit loads a transmit-frame descriptor from the guest into the /tmd/ local variable to recover a length field, a status field and a guest-physical location of the associated frame buffer. If the status fiel...

PCI MSI mask bits inadvertently exposed to guests

ISSUE DESCRIPTION The mask bits optionally available in the PCI MSI capability structure are used by the hypervisor to occasionally suppress interrupt delivery. Unprivileged guests were, however, nevertheless allowed direct control of these bits. IMPACT Interrupts may be observed by Xen at...

Guest triggerable qemu MSI-X pass-through error messages

ISSUE DESCRIPTION Device model code dealing with guest PCI MSI-X interrupt management activities logs messages on certain supposedly invalid guest operations. IMPACT A buggy or malicious guest repeatedly invoking such operations may result in the host disk to fill up, possibly leading to a Denial...

Potential unintended writes to host MSI message data field via qemu

ISSUE DESCRIPTION Logic is in place to avoid writes to certain host config space fields when the guest must nevertheless be able to access their virtual counterparts. A bug in how this logic deals with accesses spanning multiple fields allows the guest to write to the host MSI message data field...

Unmediated PCI register access in qemu

ISSUE DESCRIPTION Qemu allows guests to not only read, but also write all parts of the PCI config space but not extended config space of passed through PCI devices not explicitly dealt with for partial emulation purposes. IMPACT Since the effect depends on the specific purpose of the the config...

Privilege escalation via emulated floppy disk drive

ISSUE DESCRIPTION The code in qemu which emulates a floppy disk controller did not correctly bounds check accesses to an array and therefore was vulnerable to a buffer overflow attack. IMPACT A guest which has access to an emulated floppy device can exploit this vulnerability to take over the qem...

Information leak through XEN_DOMCTL_gettscinfo

ISSUE DESCRIPTION The handler for XENDOMCTLgettscinfo failed to initialize a padding field subsequently copied to guest memory. A similar bug existed in XENSYSCTLgetdomaininfolist, which is addressed by the patches provided here even though that operation was declared by XSA-77 not to provide...

Certain domctl operations may be abused to lock up the host

ISSUE DESCRIPTION XSA-77 put the majority of the domctl operations on a list excepting them from having security advisories issued for them if any effects their use might have could hamper security. Subsequently some of them got declared disaggregation safe, but for a small subset this was not...

Long latency MMIO mapping operations are not preemptible

ISSUE DESCRIPTION The XENDOMCTLmemorymapping hypercall allows long running operations without implementing preemption. This hypercall is used by the device model as part of the emulation associated with configuration of PCI devices passed through to HVM guests and is therefore indirectly exposed ...

Unmediated PCI command register access in qemu

ISSUE DESCRIPTION HVM guests are currently permitted to modify the memory and I/O decode bits in the PCI command register of devices passed through to them. Unless the device is an SR-IOV virtual function, after disabling one or both of these bits subsequent accesses to the MMIO or I/O port range...

HVM qemu unexpectedly enabling emulated VGA graphics backends

ISSUE DESCRIPTION When instantiating an emulated VGA device for an x86 HVM guest qemu will by default enable a backend to expose that device, either SDL or VNC depending on the version of qemu and the build time configuration. The libxl toolstack library does not explicitly disable these default...

Non-standard PCI device functionality may render pass-through insecure

ISSUE DESCRIPTION Devices with capabilities or defects that are undocumented or that virtualization software is unaware of may allow guests to control parts of the host that they shouldn't be in control of. Here are some examples of the kind of problem: While XSA-120 deals with standard PCI confi...

Hypervisor memory corruption due to x86 emulator flaw

ISSUE DESCRIPTION Instructions with register operands ignore eventual segment overrides encoded for them. Due to an insufficiently conditional assignment such a bogus segment override can, however, corrupt a pointer used subsequently to store the result of the instruction. IMPACT A malicious gues...

Non-maskable interrupts triggerable by guests

ISSUE DESCRIPTION Guests are currently permitted to modify all of the writable bits in the PCI command register of devices passed through to them. This in particular allows them to disable memory and I/O decoding on the device unless the device is an SR-IOV virtual function, in which case...

Information leak via internal x86 system device emulation

ISSUE DESCRIPTION Emulation routines in the hypervisor dealing with certain system devices check whether the access size by the guest is a supported one. When the access size is unsupported these routines failed to set the data to be returned to the guest for read accesses, so that hypervisor sta...

Information leak through version information hypercall

ISSUE DESCRIPTION The code handling certain sub-operations of the HYPERVISORxenversion hypercall fails to fully initialize all fields of structures subsequently copied back to guest memory. Due to this hypervisor stack contents are copied into the destination of the operation, thus becoming visib...

arm: vgic-v2: GICD_SGIR is not properly emulated

ISSUE DESCRIPTION When decoding a guest write to a specific register in the virtual interrupt controller Xen would treat an invalid value as a critical error and crash the host. IMPACT By writing an invalid value to the GICD.SGIR register a guest can crash the host, resulting in a Denial of Servi...

arm: vgic: incorrect rate limiting of guest triggered logging

ISSUE DESCRIPTION On ARM systems the code which deals with virtualising the GIC distributor would, under various circumstances, log messages on a guest accessible code path without appropriate rate limiting. IMPACT A malicious guest could cause repeated logging to the hypervisor console, leading ...

xen crash due to use after free on hvm guest teardown

ISSUE DESCRIPTION Certain data accessible via hypercalls by the domain controlling the execution of a HVM domain is being freed prematurely, leading to the respective memory regions to possibly be read from and written to in ways unexpected by their new owners. IMPACT Malicious or buggy stub doma...

p2m lock starvation

ISSUE DESCRIPTION The current read/write lock implementation is read-biased, which allows a consistent stream of readers to starve writers indefinitely. There are certain rwlocks where guests are capable of applying arbitrary read pressure. IMPACT A malicious guest administrator can deny service ...

Insufficient bounding of "REP MOVS" to MMIO emulated inside the hypervisor

ISSUE DESCRIPTION Acceleration support for the "REP MOVS" instruction, when the first iteration accesses memory mapped I/O emulated internally in the hypervisor, incorrectly assumes that the whole range accessed is handled by the same hypervisor sub-component. IMPACT A buggy or malicious HVM gues...

Excessive checking in compatibility mode hypercall argument translation

ISSUE DESCRIPTION The hypercall argument translation needed for 32-bit guests running on 64-bit hypervisors performs checks on the final register state. These checks cover all registers potentially holding hypercall arguments, not just the ones actually doing so for the hypercall being processed,...

Guest effectable page reference leak in MMU_MACHPHYS_UPDATE handling

ISSUE DESCRIPTION An error handling path in the processing of MMUMACHPHYSUPDATE failed to drop a page reference which was acquired in an earlier processing step. IMPACT Malicious or buggy stub domain kernels or tool stacks otherwise living outside of Domain0 can mount a denial of service attack...

Missing privilege level checks in x86 emulation of far branches

ISSUE DESCRIPTION The emulation of far branch instructions CALL, JMP, and RETF in Intel assembly syntax, LCALL, LJMP, and LRET in AT&T assembly syntax incompletely performs privilege checks. However these instructions are not usually handled by the emulator. Exceptions to this are - when a memory...

Insufficient restrictions on certain MMU update hypercalls

ISSUE DESCRIPTION MMU update operations targeting page tables are intended to be used on PV guests only. The lack of a respective check made it possible for such operations to access certain function pointers which remain NULL when the target guest is using Hardware Assisted Paging HAP. IMPACT...

Improper MSR range used for x2APIC emulation

ISSUE DESCRIPTION The MSR range specified for APIC use in the x2APIC access model spans 256 MSRs. Hypervisor code emulating read and write accesses to these MSRs erroneously covered 1024 MSRs. While the write emulation path is written such that accesses to the extra MSRs would not have any bad...

Missing privilege level checks in x86 emulation of software interrupts

ISSUE DESCRIPTION The emulation of instructions which generate software interrupts fails to perform supervisor mode permission checks. However these instructions are not usually handled by the emulator. Exceptions to this are - when a memory operand implicit for the affected instructions lives in...

Missing privilege level checks in x86 HLT, LGDT, LIDT, and LMSW emulation

ISSUE DESCRIPTION The emulation of the instructions HLT, LGDT, LIDT, and LMSW fails to perform supervisor mode permission checks. However these instructions are not usually handled by the emulator. Exceptions to this are - when the instruction's memory operand if any lives in emulated or passed...

Race condition in HVMOP_track_dirty_vram

ISSUE DESCRIPTION The routine controlling the setup of dirty video RAM tracking latches the value of a pointer before taking the respective guarding lock, thus making it possible for a stale pointer to be used by the time the lock got acquired and the pointer gets dereferenced. The hypercall...

Mishandling of uninitialised FIFO-based event channel control blocks

ISSUE DESCRIPTION When using the FIFO-based event channels, there are no checks for the existence of a control block when binding an event or moving it to a different VCPU. This is because events may be bound when the ABI is in 2-level mode e.g., by the toolstack before the domain is started. The...

Flaw in handling unknown system register access from 64-bit userspace on ARM

ISSUE DESCRIPTION When handling an unknown system register access from 64-bit userspace Xen would incorrectly return to the second instruction of the trap handler for faults in kernel space rather than the first instruction of the trap handler for faults in 64-bit userspace. Any user in a guest...

Flaws in handling traps from 32-bit userspace on 64-bit ARM

ISSUE DESCRIPTION When handling a trap from guest mode on ARM, Xen asserts that the current guest mode must match the domain address width. This assertion is false when a guest takes a trap from a 32-bit userspace running on a 64-bit kernel in a 64-bit domain. IMPACT Any user in a guest which is...

Long latency virtual-mmu operations are not preemptible

ISSUE DESCRIPTION Some MMU virtualization operations on HVM guests must process every page assigned to a guest. For larger guests, this can tie up a vcpu for a significant amount of time, as the operations are not preemptible. For guests using Hardware Assisted Paging HAP, see below this is...

information leak via gnttab_setup_table on ARM

ISSUE DESCRIPTION When initialising an internal data structure on ARM platform Xen was not correctly initialising the memory containing the list of a domain's grant table pages. This list is returned by the GNTTABOPsetuptable subhypercall, leading to an information leak. IMPACT Malicious guest...

unexpected pitfall in xenaccess API

ISSUE DESCRIPTION A test/example program, for exercising the Xen memaccess API, does not take all necessary precautions against hostile guest behaviour. As a result, software developers using it as an example or template might have written and deployed vulnerable code. See the patch for technical...