The enable bit will remain zero even if a one is written to it. A hypercall is a software trap from a domain to the hypervisor, just as a syscall is a software trap from an application to the kernel. the first rep element must be 8 byte aligned. Unlike the other guest VMs, the “root partition” is our host OS. This is done through a special hypercall page. Vendor values are allocated by Microsoft. Xen hypercall interface documentation. Perth. The hypercall result value is passed back in registers. Bits should be ignored on reads and preserved on writes. The hypercall_table and hypercall_args_table are initialized sequences of quads and bytes. GPA pointers must by 8-byte aligned. Hypervisor – A layer of software that sits between the hardware and one or more operating systems. servers 24x7x365 and backed by RackSpace's Fanatical Support®. A hypercall is to a syscall what a hypervisor is to an OS. All hypercalls should be invoked through the architecturally-defined hypercall interface (see below). Several result codes are common to all hypercalls and are therefore not documented for each hypercall individually. The hypercall input value is passed in registers along with the input parameters. Parent Partition: A parent partition is an instance of partition within the Windows Hyper V virtualization environment that is responsible for running the virtualization stack and creating child partitions. However, some hypercalls require a variable amount of header data. No other registers will be clobbered unless explicitly stated by the particular hypercall. Latest Hyper-V TLFS has not updated list of hypercalls in Appendix A: Hypercall Code Reference. If it is set, the interface is already active, and steps 6 and 7 should be omitted. Indicates the service version (for example, "service pack" number), Indicates the OS variant. The hypervisor attempts to limit hypercall execution to 50μs or less before returning control to the virtual processor that invoked the hypercall. Hypercalls can be invoked only from the most privileged guest processor mode. The values within the padding regions are ignored by the hypervisor. In such cases, the operation involves two or more internal states. It is same as Windows Server 2016 hypercall list from previous TLFS. Although real-mode code runs with an effective CPL of zero, hypercalls are not allowed in real mode. * The hypercall number is passed in x16. The msdn documentation on hypercalls states that, in order to use the hypercall functions the header file should be included. Domains will use hypercalls to request privileged operations like updating pagetables. We can think about the r… The rep count is incorrect (for example, a non-zero rep count is passed to a non-rep call or a zero rep count is passed to a rep call). OS Type values are allocated by Microsoft. Domains will use hypercalls to request privileged operations like updating pagetables. For each hypercall that follows this pattern, the visible side effects of intermediate internal states is described. A hypercall is to a syscall what a hypervisor is to an OS. Virtualization is critical to the infrastructure of cloud computing environment and other online services. The hypervisor determines the caller’s mode based on the value of EFER.LMA and CS.L. However, a small number of simple hypercalls might require more time. The register mappings depend on whether the caller is running in 32-bit (x86) or 64-bit (x64) mode. A hypercall is a software trap from a domain to the hypervisor, just as a syscall is a software trap from an application to the kernel. Alternatively, a hypercall is to a hypervisor what a syscall is to a kernel. The rep start index is not less than the rep count. Hypercall Interfaces; ARM; x86_32; x86_64 In such a case the rep elements lie after the header in the usual fashion, except that the header's total size includes both the fixed and variable portions. Because this opcode differs among virtualization implementations, it is necessary for the hypervisor to abstract this difference. Its contents are readable and executable by the guest. A hypercall can be thought of as a complex instruction that takes many cycles. A status value field (of type HV_STATUS) is used to indicate whether the call succeeded or failed. We differentiate between three types of partitions: root partition (also known as a parent partition), enlightened guest partitions and unenlightened guest partitions. Microsoft operating systems are encoded as follows: 0=Undefined, 1=MS-DOS®, 2=Windows® 3.x, 3=Windows® 9x, 4=Windows® NT (and derivatives), 5=Windows® CE. This MSR is partition-wide and is shared among all virtual processors. Xen Documentation - Hypercall Interfaces. Hypercall interface is provided by hypervisor to offer privileged requests by the guest domains. After the interface has been established, the guest can initiate a hypercall. In other words, it is shared by all virtual processors in the partition. The hypervisor provides a calling mechanism for guests. Callers also specify a rep start index that indicates the next input and/or output element that should be consumed. If one virtual processor successfully writes to the MSR, another virtual processor will read the same value. An event channel is a queue of asynchronous notifications, and notify of the same sorts of events that interrupts notify on native hardware. I patched kAFL to run QEMU under GDB so I can set breakpoint on hypercall dispatching in kvm_cpu_exec, after second break I delete the breakpoint and fuzzing continues normally. The hypervisor determines the caller’s mode based on the value of EFER.LMA and CS.L. An attacker uses a Virtual Machine (VM) to intrude the victim’s VM by exploiting the Virtual Machine Manager (VMM) hypercall handler. Callers must specify the 64-bit guest physical address (GPA) of the input and/or output parameters. A hypercall is a software trap from a domain to the hypervisor, just as a syscall is a software trap from an application to the kernel. The hypervisor presents the guest operating systems with a virtual operating platformand manages the execution of the guest operating systems. S390: R2-R7 are used for parameters 1-6. Hypercall "There are no great limits to growth because there are no limits of human intelligence, imagination, and wonder." delegating the hypercall to userland). The amount of header data being passed from the guest to the hypervisor is therefore implicitly specified by the hypercall code and need not be specified separately. It is formatted as follows: For rep hypercalls, the rep count field indicates the total number of reps. Extended hypercalls use the same calling convention as normal hypercalls and appear identical from a guest VM’s perspective. Any attempt to use this interface when the hypervisor does not indicate availability will result in a #UD fault. It is possible that for a given invocation of a hypercall that does accept variable sized input headers that all the header input fits entirely within the fixed size header. The inputs to each action can be read at any granularity and at any time after the hypercall is made and before the action is executed. A value of 0 indicates a proprietary, closed source OS. Such calls are referred to as hypercalls. Unless explicitly stated otherwise, when a hypercall fails (that is, the result field of the hypercall result value contains a value other than HV_STATUS_SUCCESS), the content of all output parameters are indeterminate and should not be examined by the caller. If the guest attempts to move the hypercall page beyond the bounds of the GPA space, a #GP fault will result when the MSR is written. threats that hypercall interfaces pose, which will help to focus approaches for improving the security of hypervisors. For example, if a caller specifies a rep count of 25, and only 20 iterations are completed within the time constraints, the hypercall returns control back to the calling virtual processor after updating the rep start index to 20. 2 Information on hypercall vulnerabilities 2.1 Hypercall memory op The memory op hypercall is used for managing the memory of a guest VM, for example, altering The caller must specify how much data it is providing as input headers. For example, if the input parameter block is 20 bytes in size, the hypervisor would ignore the following 12 bytes. The calling partition must possess a particular privilege, The partition being acted upon must be in a particular state (e.g. RAX (x64) and EDX:EAX (x86) are always overwritten with the hypercall result value and output parameters, if any. See list of vendors below. When the original calling thread resumes execution, it will re-execute the hypercall instruction and make forward progress toward completing the operation. 42 * 43 * The hvc ISS is required to be 0xEA1, that is the Xen specific ARM: 44 * hypercall tag. If the input parameter block is smaller than 112 bytes, any extra bytes in the registers are ignored. The XMM fast hypercall interface uses six XMM registers to allow the caller to pass an input parameter block up to 112 bytes in size. If an error is encountered when processing an element, an appropriate status code is provided along with a reps completed count, indicating the number of elements that were successfully processed before the error was encountered. Some hypercall operations are sufficiently complex that a 50μs guarantee is difficult to make. Input and output data structures must both be placed in memory on an 8-byte boundary and padded to a multiple of 8 bytes in size. Programming Note: When running on implementations which implement the "embedded hypervisor" architecture, the guest or host may replace the guest hypercall instructions with the architecturally defined hypercall instruction at runtime. If both of these flags are set, the caller is assumed to be a 64-bit caller. It protects games through the use of hybrid anti-cheat mechanisms. footprint: In information technology, a footprint is the amount of space a particular unit of hardware or software occupies. All elements of the input and output data structures are padded to natural boundaries up to 8 bytes (that is, two-byte elements must be on two-byte boundaries and so on). It seems that hypercall "mismatch" happens because of race between QEMU and kAFL. The size of a variable header, in QWORDS. Without GDB, hypercall … There must be at least one parent partition in a hypervisor instance, running a supported version of Windows Server (2008 and later). If both of these flags are set, the caller is assumed to be a 64-bit caller. If this register is subsequently zeroed, the hypercall code page will be disabled. A hypercall is a software trap from a domain to the hypervisor, just as a syscall is a software trap from an application to the kernel. The guest should assume the hypercall page performs the equivalent of a near return (0xC3) to return to the caller. On x64 platforms, the hypervisor supports the use of XMM fast hypercalls, which allows some hypercalls to take advantage of the improved performance of the fast hypercall interface even though they require more than two input parameters. OSR_Community_User Member Posts: 110,217. The parent partition is the second layer of partition after the root partition. A value of 1 indicates an open source OS. Invoke the hypercall in the guest kernel to see its output on the host's ftrace. The guest must avoid the examination and/or manipulation of any input or output parameters related to an executing hypercall. When the hypercall is re-executed, the hypervisor will resume at element 20 and complete the remaining 5 elements. The following is the recommended encoding for this MSR. The specified input or output parameter lists spans pages. An attempt to invoke a hypercall by any other means (for example, copying the code from the hypercall code page to an alternate location and executing it from there) might result in an undefined operation (#UD) exception. The guest writes a new value to the Hypercall MSR (. As such, the hypercall must be invoked with a valid stack. In such cases the variable sized input header is zero-sized and the corresponding bits in the hypercall input should be set to zero. The order in which error conditions are detected and reported by the hypervisor is undefined. Sources for the Device Model are found in the ACRN Hypervisor GitHub repo. After the hypercall page has been enabled, invoking a hypercall simply involves a call to the start of the page. Simple hypercalls that use hypercall continuation may involve multiple internal states that are externally visible. You need to fill the entries from 49 to 55 in both tables with the appropriate values. An attempt to invoke a hypercall within an illegal processor mode will generate a #UD (undefined operation) exception. These hypercalls use hypercall continuation in a similar manner to rep hypercalls. The hypervisor therefore relies on a hypercall continuation mechanism for some hypercalls – including all rep hypercall forms. The guest creates an executable VA mapping to the hypercall page GPA. The return value is written to R2. Assuming the specified hypercall control word is valid (see the following) and the input / output parameter lists are accessible, the hypervisor is guaranteed to attempt at least one rep, but it is not required to process the entire list before returning control back to the caller. For output, the hypervisor is allowed to (but not guaranteed to) overwrite padding regions. The guest checks the Enable Hypercall Page bit. - Patch 4 implements the console output hypercall by using KVM_EXIT_HYPERCALL (i.e. This is only supported on x64 platforms. In arch/x86/kvm/x86.c, in the kvm_emulate_hypercall function, add the case where the the hypercall number matches KVM_HC_HELLO_HYPERCALL. This validation consists of two checks: the specified GPA is mapped and the GPA is marked writable. Attackers may use this interface to send malicious hypercalls. Its primary job is to provide isolated execution environments called partitions. This size is provided as part of the hypercall input value (see “Variable header size” in table above). “Active”). To request a new OS Type, please file an issue on the GitHub virtualization documentation repository (https://aka.ms/VirtualizationDocumentationIssuesTLFS). Once set, only a system reset can clear the bit. 16 Tracks. Availability of the XMM fast hypercall interface is indicated via the “Hypervisor Feature Identification” CPUID Leaf (0x40000003): Note that there is a separate flag to indicate support for XMM fast output. Hypercall interface is provided by hypervisor to offer privileged requests by the guest domains. The hypervisor is not guaranteed to deliver this exception. These include the following: The return code HV_STATUS_SUCCESS indicates that no error condition was detected. LIS Hypercalls In general, a hypercall may be defined as a software interface from the guest VM to the hypervisor. Priority should be given to those error codes offering greater security, the intent being to prevent the hypervisor from revealing information to callers lacking sufficient privilege. Hypercalls for a host machine and guest machines to a hypervisor are intercepted and routed to the hypervisor for execution on a hardware platform, responsive to the hypercall passing hypercall access rules. Hypercall APIs¶. 40 * 41 * The return value is in x0. The input or output GPA pointer is not within the bounds of the GPA space. Over 100,000 French translations of English words and phrases. RsvdP. Therefore, the rep count value must always be greater than the rep start index. This register’s value is initially zero. The specified input or output GPA pointer is not aligned to 8 bytes. OS type (e.g., Linux, FreeBSD, etc.). 48 */ 49: 50: The hypervisor will validate that the calling partition can read from the input page before executing the requested hypercall. The guest OS running within the partition must identify itself to the hypervisor by writing its signature and version to an MSR (HV_X64_MSR_GUEST_OS_ID) before it can invoke hypercalls. Extended hypercalls are internally handled differently within the Hyper-V hypervisor. Hypercalls will only modify the specified register values under the following conditions: Hypercalls may have restrictions associated with them for them to perform their intended function. It is formatted as follows: For rep hypercalls, the reps complete field is the total number of reps complete and not relative to the rep start index. When a caller initially invokes a rep hypercall, it specifies a rep count that indicates the number of elements in the input and/or output parameter list. HyperCall is a Company that provides high quality calls to clients looking to lower their ROI while aggressive adding to their customer base. A value of 0 is reserved. Hypercalls are invoked by using a special opcode. Like a syscall, the hypercall is synchronous, but the return path from the hypervisor to the domain uses event channels. Hypercall input and output pages are expected to be GPA pages and not “overlay” pages. The hypercall takes an array of count operations each specified by the mmuext_op struct. Hi, I am trying to achieve parent and child partition communication inside my driver. It … A rep hypercall acts like a series of simple hypercalls. HyperCall works with over 100 media, print, and telecommunications outlets to design, implement and deliver battle tested programs that produce the results our partners desire. Most hypercall input headers have fixed size. Register mapping for hypercall inputs when the Fast flag is zero: The hypercall input value is passed in registers along with a GPA that points to the input and output parameters. When using this calling convention, the input parameters are passed in registers, including the volatile XMM registers. https://aka.ms/VirtualizationDocumentationIssuesTLFS, Specifies whether the hypercall uses the register-based calling convention: 0 = memory-based, 1 = register-based. The results (that is, the output parameters) associated with each action may be written at any granularity and at any time after the action is executed and before the hypercall returns. On x64, the register mappings depend on whether the caller is running in 32-bit (x86) or 64-bit (x64) mode. In such a case the hypercall will result in a return code of HV_STATUS_INVALID_HYPERCALL_INPUT. The rep start index indicates the particular repetition relative to the start of the list (zero indicates that the first element in the list is to be processed). In addition to a fixed-size set of input and output parameters, rep hypercalls involve a list of fixed-size input and/or output elements. For example, the status code HV_STATUS_ACCESS_DENIED is the preferred status code over one that would reveal some context or state information purely based upon privilege. Indicates if the MSR is immutable. Stream Tracks and Playlists from Hypercall on your desktop or mobile device. For hypercalls that have output parameters, the hypervisor will validate that the partition can be write to the output page. A second hypercall calling convention can optionally be used for a subset of hypercalls – in particular, those that have two or fewer input parameters and no output parameters. Indicates the guest OS vendor. Register mapping for hypercall inputs when the Fast flag is one: The hypercall input value is passed in registers along with the input parameters. Each hypercall defines a set of input and/or output parameters. Indicates the OS types. 45 * 46 * Parameter structs passed to hypercalls are laid out according to: 47 * the ARM 64-bit EABI standard. If a hypercall is not able to complete within the prescribed time limit, control is returned back to the caller, but the instruction pointer is not advanced past the instruction that invoked the hypercall. Xen.org's servers are hosted with RackSpace, monitoring our Such calls comprise multiple atomic operations. In all other regards, hypercalls accepting variable sized input headers are otherwise similar to fixed size input header hypercalls with regards to calling conventions. Except where noted, the action performed by a hypercall is atomic both with respect to all other guest operations (for example, instructions executed within a guest) and all other hypercalls being executed on the system. Housey Business. group acrn_hypercall. The following is a detailed list of the steps involved in establishing the hypercall page: Hypercalls with call codes above 0x8000 are known as extended hypercalls. The following encoding is offered as guidance for open source operating system vendors intending to conform to this specification. To do so, it populates the registers per the hypercall protocol and issues a CALL to the beginning of the hypercall page. Its unclear if there is a more preferable approach to this, so comments particularly appreciated here. In other words, if the input parameter block is smaller than 112 bytes (rounded up to the nearest 16 byte aligned chunk), the remaining registers will return hypercall output. Hyper-V implements isolation of virtual machines in terms of a partition.A partition is a logical unit of isolation, supported by the hypervisor, in which each guest operating system executes. Locked. A simple hypercall performs a single atomic action; a rep hypercall performs multiple, independent atomic actions. In other words, if multiple errors exist, the hypervisor must choose which error condition to report. Callers specify a hypercall by a 64-bit value called a hypercall input value. The guest reads CPUID leaf 1 and determines whether a hypervisor is present by checking bit 31 of register ECX. The guest consults CPUID leaf 0x40000003 to determine which hypervisor facilities are available to it. In addition, R1 is used for hypercall number. The guest is required to specify the location of the page by programming the Guest Hypercall MSR. While it is a fully-fledged Windows VM, where we can run regular programs like a web browser, parts of the virtualization stack itself runs in the root partition kernel and userspace. The guest finds a page within its GPA space, preferably one that is not occupied by RAM, MMIO, and so on. Hyper-V will only modify these registers for fast hypercall output, which is limited to x64. January 2014 in NTFSD. Attackers may use this interface to send malicious hypercalls. It is suggested that open source operating systems adapt the following convention. Multiple instances of a variety of operating systems may share the virtualized hardw… A value of 1 indicates an open source OS. If the virtual processor writes the input parameters to an overlay page and specifies a GPA within this page, hypervisor access to the input parameter list is undefined. RDX, R8, and XMM0 through XMM5, when used for fast hypercall input, remain unmodified. This gives the attacker the ability to access VMM privileges and possibly even execute malicious code. This page was last edited on 8 November 2013, at 18:57. https://wiki.xenproject.org/index.php?title=Hypercall&oldid=10019. When using this calling convention, the input parameters are passed in general-purpose registers. The register mapping for hypercall outputs is as follows: Similar to how the hypervisor supports XMM fast hypercall inputs, the same registers can be shared to return output. This MSR is a partition-wide MSR. This signature implies that, The guest writes its OS identity into the MSR. Inside anti-cheat: EasyAntiCheat – Part 1. Hypercalls have to be made from CPL0, i.e. The hypercall number should be placed in rax and the return value will be placed in rax. These hypercalls typically have a fixed size input header and additional header input that is of variable size. 153 Followers. Attempts to write to the hypercall page will result in a protection (#GP) exception. All other rules remain the same, e.g. With our broad range of experiences, HyperCall is able to maximize advertising budgets … Hypercall – Interface for communication with the hypervisor - The hypercall interface accommodates access to the optimizations provided by the hypervisor. If the page is occupied, the guest should avoid using the underlying page for other purposes. A variable sized header is similar to a fixed hypercall input (aligned to 8 bytes and sized to a multiple of 8 bytes). This allows pending interrupts to be handled and other virtual processors to be scheduled. Bit 15: support for returning hypercall output via XMM registers is available. A simple hypercall performs a single operation and has a fixed-size set of input and output parameters. Bit 4: support for passing hypercall input via XMM registers is available. French Translation of “hypercall” | The official Collins English-French Dictionary online. These parameters are specified in terms of a memory-based data structure. A reserved bit in the specified hypercall input value is non-zero. The hypercall context switches from the child partition to the hypervisor to execute the hypercall code from a dispatch table, and a VMEXIT is then issued to return to the child partition from the hypervisor restoring state from the VMCS. The hypercall page appears as an “overlay” to the GPA space; that is, it covers whatever else is mapped to the GPA range. Alternatively, a hypercall is to a hypervisor what a syscall is to a kernel. Hypercall. Now let's look at the actual hypercall interface. Hypercall Attacks. This page is provided by the hypervisor and appears within the guest’s GPA space. If the hypercall involves no input or output parameters, the hypervisor ignores the corresponding GPA pointer. Extended hypercall capabilities can be queried with HvExtCallQueryCapabilities. Some fields may not apply for some guest OSs. The hypercall continuation mechanism is mostly transparent to the caller. Despite the scary name, it is not a security issue in and of itself, although there is always the possibility that one of the hypercall implementations enables some kind of security exploit. Before the hypercall page is enabled, the guest OS must report its identity by writing its version signature to a separate MSR (HV_X64_MSR_GUEST_OS_ID). Any attempt to use this interface when the hypervisor does not indicate availability will result in a #UD fault. If either of these tests fails, the hypervisor generates a memory intercept message. We are asking you to write a hypercall to become familiar with how they work and the codebase for KVM. The first invocation places the object (for example, the partition or virtual processor) into one state, and after repeated invocations, the state finally transitions to a terminal state. I'm currently trying to build a small hypervisor and kernel using kvm and I struggle to get hypercalls with multiple args working correctly. The remaining 80 bytes would contain hypercall output (if applicable). A hypervisor (or virtual machine monitor, VMM, virtualizer) is computer software, firmware or hardware that creates and runs virtual machines. Total number of reps (for rep call, must be zero otherwise), Starting index (for rep call, must be zero otherwise), Callers should ignore the value in these bits. See xen/include/public/xen.h in the Xen sources and the The hypercall instruction on legacy Book E implementations shall be the pattern 0x44000022 (SC with LEVEL=1). Hypercall GPFN - Indicates the Guest Physical Page Number of the hypercall page. Encoding is unique to the vendor. Cyber Security. Domains will use hypercalls to request privileged operations like … If all restrictions are not met, the hypercall will terminate with an appropriate error. It is also possible for a variable sized header hypercall to additionally support rep semantics. The hypercall page can be placed anywhere within the guest’s GPA space, but must be page-aligned. It verifies that the maximum leaf value is at least 0x40000005 and that the interface signature is equal to “Hv#1”. See list of known OS types below. For subsequent invocations of the rep hypercall, the rep start index indicates how many elements have been completed – and, in conjunction with the rep count value – how many elements are left. If no guest OS identity has been specified, attempts to enable the hypercall will fail. Domains will use hypercalls to request privileged operations like … On x64 platfoms, this means protected mode with a current privilege level (CPL) of zero.
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