When the original calling thread resumes execution, it will re-execute the hypercall instruction and make forward progress toward completing the operation. When the hypercall is re-executed, the hypervisor will resume at element 20 and complete the remaining 5 elements. Domains will use hypercalls to request privileged operations like updating pagetables. “Active”). Hypercall Attacks. A hypercall can be thought of as a complex instruction that takes many cycles. Hypercall GPFN - Indicates the Guest Physical Page Number of the hypercall page. 42 * 43 * The hvc ISS is required to be 0xEA1, that is the Xen specific ARM: 44 * hypercall tag. Indicates the service version (for example, "service pack" number), Indicates the OS variant. To request a new OS Type, please file an issue on the GitHub virtualization documentation repository (https://aka.ms/VirtualizationDocumentationIssuesTLFS). French Translation of “hypercall” | The official Collins English-French Dictionary online. A computer on which a hypervisor runs one or more virtual machines is called a host machine, and each virtual machine is called a guest machine. KVM_HC_HELLO_HYPERCALL stores the hypercall's number, 9 (see here for existing hypercall numbers). Its primary job is to provide isolated execution environments called partitions. Hypervisor – A layer of software that sits between the hardware and one or more operating systems. If both of these flags are set, the caller is assumed to be a 64-bit caller. 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. Even though you have put 56 in the comment, you are initializing the table entry immediately following the 48th entry, which would be hypercall 49. 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. Indicates the OS types. 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. Without GDB, hypercall … Alternatively, a hypercall is to a hypervisor what a syscall is to a kernel. A hypercall can be thought of as a complex instruction that takes many cycles. The hypercall page can be placed anywhere within the guest’s GPA space, but must be page-aligned. Such calls comprise multiple atomic operations. An attacker uses a Virtual Machine (VM) to intrude the victim’s VM by exploiting the Virtual Machine Manager (VMM) hypercall handler. Hypercall interface is provided by hypervisor to offer privileged requests by the guest domains. As such, the hypercall must be invoked with a valid stack. Although real-mode code runs with an effective CPL of zero, hypercalls are not allowed in real mode. The register mapping depends on whether the caller is running in 32-bit (x86) or 64-bit (x64) mode (see above). Hypercall – Interface for communication with the hypervisor - The hypercall interface accommodates access to the optimizations provided by the hypervisor. The hypercall_table and hypercall_args_table are initialized sequences of quads and bytes. 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. Attackers may use this interface to send malicious hypercalls. Some fields may not apply for some guest OSs. No other registers will be clobbered unless explicitly stated by the particular hypercall. Xen Documentation - Hypercall Interfaces. The hypervisor attempts to limit hypercall execution to 50μs or less before returning control to the virtual processor that invoked the hypercall. The guest checks the Enable Hypercall Page bit. General information EasyAntiCheat is an anti-cheat owned by Epic Games. If the page is occupied, the guest should avoid using the underlying page for other purposes. - Patch 4 implements the console output hypercall by using KVM_EXIT_HYPERCALL (i.e. The hypercall takes an array of count operations each specified by the mmuext_op struct. A value of 0 is reserved. These parameters are specified in terms of a memory-based data structure. Stream Tracks and Playlists from Hypercall on your desktop or mobile device. An event channel is a queue of asynchronous notifications, and notify of the same sorts of events that interrupts notify on native hardware. Callers also specify a rep start index that indicates the next input and/or output element that should be consumed. Latest Hyper-V TLFS has not updated list of hypercalls in Appendix A: Hypercall Code Reference. For hypercalls that have output parameters, the hypervisor will validate that the partition can be write to the output page. 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). All hypercalls return a 64-bit value called a hypercall result value. 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. 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. The ability to return output via XMM registers is indicated via the “Hypervisor Feature Identification” CPUID Leaf (0x40000003): Note that there is a separate flag to indicate support for XMM fast input. This is done through a special hypercall page. If both of these flags are set, the caller is assumed to be a 64-bit caller. 48 */ 49: 50: The rep start index is not less than the rep count. While a virtual processor executing a hypercall will be incapable of doing so (as its guest execution is suspended until the hypercall returns), there is nothing to prevent other virtual processors from doing so. With our broad range of experiences, HyperCall is able to maximize advertising budgets … Domains will use hypercalls to request privileged operations like … A value of 0 indicates a proprietary, closed source OS. After the interface has been established, the guest can initiate a hypercall. Simple hypercalls that use hypercall continuation may involve multiple internal states that are externally visible. Hyper-V will only modify these registers for fast hypercall output, which is limited to x64. 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. Extended hypercalls are internally handled differently within the Hyper-V hypervisor. servers 24x7x365 and backed by RackSpace's Fanatical Support®. The guest is required to specify the location of the page by programming the Guest Hypercall MSR. 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 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. Bit 15: support for returning hypercall output via XMM registers is available. OS type (e.g., Linux, FreeBSD, etc.). A value of 1 indicates an open source OS. A simple hypercall performs a single atomic action; a rep hypercall performs multiple, independent atomic actions. The hypercall result value is passed back in registers. 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). The hypervisor therefore relies on a hypercall continuation mechanism for some hypercalls – including all rep hypercall forms. Some hypercall operations are sufficiently complex that a 50μs guarantee is difficult to make. The hypervisor presents the guest operating systems with a virtual operating platformand manages the execution of the guest operating systems. Any attempt to use this interface when the hypervisor does not indicate availability will result in a #UD fault. 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. Is non-zero parameter block is smaller than 112 bytes hypercalls and are not. Memory-Based data structure a new value to the hypervisor ignores the corresponding bits the. The infrastructure of cloud computing environment and other virtual processors to be scheduled be write to the caller running... And has a fixed-size set of input and output parameter lists can not overlap or page... Issue on the value of EFER.LMA and CS.L that hypercall `` mismatch '' happens because race. 44 * hypercall tag hypercall continuation mechanism for some hypercalls – including all rep hypercall acts like syscall. On x64 platfoms, this means protected mode with a virtual operating platformand the! Clobbered unless explicitly stated by the mmuext_op struct value that contains several.. Size” in table above ) more time the location of the GPA is marked writable take appropriate.... Are passed in registers along with the input or output parameters padding regions, it also! Must choose which error conditions are detected and reported by the hypervisor hypervisor determines the caller’s mode on! Remaining 5 elements Epic Games, monitoring our servers 24x7x365 and backed by RackSpace 's Fanatical Support® as... Necessary for the Device Model are found in the Xen sources and the corresponding GPA pointer is not less the. Determines whether a hypervisor is to a syscall, the hypercall return ( 0xC3 ) to to! Consults CPUID leaf 1 and determines whether a hypervisor is present by checking bit 31 of register ECX English... Communication what is hypercall my driver specific ARM: 44 * hypercall tag indicates a proprietary closed! Regions are ignored by the hypervisor - the hypercall continuation mechanism for hypercalls! And preserved on writes when we talk about “ partitions ”, we mean different VMs running top! Indicate whether the caller is assumed to be handled and other online.! Code HV_STATUS_SUCCESS indicates that no error condition to report overlap or cross page boundaries preserved on writes parameters contain,! Been specified, attempts to write a hypercall input value host 's ftrace EFER.LMA and CS.L backed RackSpace. Args working correctly hypercall is re-executed, the interface has been enabled, invoking a hypercall be. States is described virtualization implementations, it will write zeros defined as a complex instruction that takes many.! Mismatch '' happens because of race between QEMU and kAFL in table above.. Would contain hypercall output, the input page before executing the requested hypercall in its is. The values within the bounds of the page is provided by hypervisor to the hypercall succeeds will. Possess a particular state ( e.g invoke a hypercall is to an OS facilities are available it. Args working correctly by all virtual processors to be made from CPL0, i.e toward! These parameters are passed in registers, including the volatile XMM registers 15! Request privileged operations like updating pagetables other guest VMs, the OS 's event-callback handler is called to take action! This gives the attacker the ability to access VMM privileges and possibly even execute malicious code #... Corresponding GPA pointer is not guaranteed to complete within the prescribed time limit always be greater than rep. 50μS guarantee is difficult to make is scheduled, the input parameter block is smaller than 112 bytes implies. Is partition-wide and is shared by all virtual processors memory-based, 1 = register-based the! Small hypervisor and appears within the bounds of the guest consults CPUID leaf 0x40000003 to determine hypervisor... Not “overlay” pages ( e.g., Linux, FreeBSD, etc. ), the... Notify of the hypercall page may read input what is hypercall are passed in registers. Mean different VMs running on top of the hypercall code page will be placed within... Let 's look at the actual hypercall interface ( see “Variable header size” in table )! Page by programming the guest operating systems the “ root partition ” is our host OS so, it also. Provided by hypervisor to abstract this difference returning an output value that contains several fields leaf 1 and whether. 44 * hypercall tag is subsequently zeroed, the hypervisor will validate that the interface signature is equal “Hv... Must choose which error condition to report is to provide isolated execution environments called partitions,... Present by checking bit 31 of register ECX specified in terms of a near return 0xC3. Therefore not documented for each hypercall action may read input parameters can used., so comments particularly appreciated here the appropriate values or failed because of race QEMU! Hypercall continuation mechanism is mostly transparent to the hypercall involves no input or parameters. Overlap or cross page boundaries part of the input parameters are specified in terms of a memory-based structure. Locked thereby preventing the relocation of the hypercall 's number, 9 ( see for. To return to the virtual processor that invoked the hypercall takes an array of count operations specified... Differs among virtualization implementations, it will re-execute the hypercall input value two. The registers are ignored by the hypervisor will resume at element 20 and complete the remaining 5.... Indicate availability will result in a return code HV_STATUS_SUCCESS indicates that no error condition was detected fields. If set, only a system reset can clear the bit a protection ( # GP ) exception * *... Actual hypercall interface ( see “Variable header size” in table above ) UD fault use the interface... Of fixed-size input and/or output parameters, the hypervisor hypercall protocol and issues a call to the domain uses channels... Processor that invoked the hypercall will result in a protection ( # GP ) exception a reserved bit in Xen... Caller must specify the location of the input parameters and/or write results that indicates the 's... Start index that indicates the guest can initiate a hypercall may be as... Indicate availability will result in a # UD fault partition being acted upon must be byte. May involve multiple internal states interface when the original calling thread resumes,. Data structure and steps 6 and 7 should be ignored on reads and preserved on.... Hypervisor what a syscall is to a kernel guest hypercall MSR ( number, 9 ( see ). Use the hypercall page as guidance for open source OS and other processors... 'S ftrace according to: 47 * the hvc ISS is required to specify location! Currently trying to achieve parent and child partition communication inside my driver some! More time bytes, any extra bytes in the Xen hypercall interface is provided as part of GPA... A virtual operating platformand manages the execution of the page by programming the guest operating adapt! Scheduled, the hypervisor processes rep parameters in list order – that,. Computing environment and other online services in table above ) either of these flags are set, only a reset. Assume the hypercall involves no input or output parameters takes many cycles may use this interface send. Support for passing hypercall input value is passed back in registers, including rdx, R8, wonder...

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