!vmcall (hook hypercalls)
Description of the '!vmcall' command in HyperDbg.
!vmcall
!vmcall [pid ProcessId (hex)] [core CoreId (hex)] [imm IsImmediate (yesno)] [sc EnableShortCircuiting (onoff)] [stage CallingStage (prepostall)] [buffer PreAllocatedBuffer (hex)] [script { Script (string) }] [condition { Condition (hex) }] [code { Code (hex) }]
Triggers when the debugging machine executes VMCALL instruction.
By using this command, you also receive HyperDbg's hypercalls. You shouldn't modify them.
[pid ProcessId (hex)] (optional)
Optional value to trigger the event in just a specific process. Add
pid xx to your command; thus, the command will be executed if the process id is equal to xx. If you don't specify this option, then by default, you receive events on all processes.Still, in the case of user-mode debugging, HyperDbg will apply it only to the current active debugging process (not all the processes). In that case, you can specify
pid all to intercept events from the entire system.[core CoreId (hex)] (optional)
Optional value to trigger the event in just a specific core. Add
core xx to your command thus command will be executed if core id is equal to xx. If you don't specify this option, then by default, you receive events on all cores.[imm IsImmediate (yesno)] (optional)
Optional value in which
yes means the results (printed texts in scripts) should be delivered immediately to the debugger. no means that the results can be accumulated and delivered as a couple of messages when the buffer is full; thus, it's substantially faster, but it's not real-time. By default, this value is set to yes.[sc EnableShortCircuiting (onoff)] (optional)
Optional value to ignore the emulation (skip execution) of the event. Add
sc on to your command thus whenever the event is triggered, the effects and the execution of the actual event will be ignored. For more information, please read this article. If you don't specify this option, then by default, all the events will be emulated (executed). By default, this value is set to off.[stage CallingStage (prepostall)] (optional)
Optional value to configure the calling stage of the event. To trigger the event before the emulation, include
stage pre in your command. Conversely, using stage post will cause the event to be triggered after the emulation. Additionally, using stage all will trigger the event both before and after the emulation. For more information, please read this article. By default, this value is set to pre.[buffer PreAllocatedBuffer (hex)] (optional)
[script { Script (string) }] (optional)
[condition { Condition (hex) }] (optional)
[code { Code (hex) }] (optional)
As the Context (
$context pseudo-register in the event's script, r8 in custom code, and rdx in condition code register) to the event trigger, HyperDbg sends NULL.This event supports 'event short-circuiting', which means that you can configure HyperDbg to ignore its execution and its effects. For additional details, please refer to the article provided here.
This event supports different calling stages. The 'pre' calling stage is triggered prior to running the VMCALL instruction, whereas the 'post' calling stage is triggered subsequent to running the VMCALL instruction; thus, you can read/modify the memory or registers or ignore the event in the 'pre' stage, and view/modify the results in the 'post' stage. In addition, the 'all' calling stage will trigger the event in both cases. For more information, please refer to the article provided here.
This event supports three debugging mechanisms.
- Break
- Script
- Custom Code
Please read "How to create a condition?" if you need a conditional event, a conditional event can be used in all "Break", "Script", and "Custom Code".
We want to break and get control over all hypercalls (VMCALLs) execution in our system.
HyperDbg> !vmcall
Imagine we want to break on all VMCALL executions of a process id 0x490 in core 2.
HyperDbg> !vmcall pid 490 core 2
Using the following command, you can use HyperDbg's Script Engine. You should replace the string between braces (
HyperDbg Script Here) with your script. You can find script examples here.HyperDbg> !vmcall script { HyperDbg Script Here }
The above command when messages don't need to be delivered immediately.
HyperDbg> !vmcall script { HyperDbg Script Here } imm no
Script (From File)
If you saved your script into a file then you can add
file: instead of a script and append the file path to it. For example, the following examples show how you can run a script from file:c:\users\sina\desktop\script.txt.HyperDbg> !vmcall script {file:c:\users\sina\desktop\script.txt}
You can use event forwarding to forward the event monitoring results from this event and other events to an external source, e.g., File, NamedPipe, or TCP Socket. This way, you can use HyperDbg as a monitoring tool and gather your target system's behavior and use it later or analyze it on other systems.
Please read "How to create an action?" to get an idea about how to run the custom buffer code in HyperDbg.
Your custom code will be executed in vmx-root mode. Take a look at this topic for more information. Running code in vmx-root is considered "unsafe".
Run Custom Code (Unconditional)
Monitoring process id 0x490 for VMCALL instruction execution and run 3 nops whenever the event is triggered. Take a look at Run Custom Code for more information.
HyperDbg> !vmcall pid 490 code {90 90 90}
Run Custom Code (Conditional)
Monitoring process id 0x490 for VMCALL instruction execution and run 3 nops whenever the event condition is triggered and runs 3 nops whenever the event is triggered. Take a look at Run Custom Code and how to create a condition for more information.
HyperDbg> !vmcall pid 490 code {90 90 90} condition {90 90 90}
Keep in mind that a conditional event can be used in Breaking to Debugger and Running Script too.
As EventType use
VMCALL_INSTRUCTION_EXECUTION in DEBUGGER_GENERAL_EVENT_DETAIL.This command uses VMCALL (EXIT_REASON_VMCALL - 18) vm-exit to implement hypercall hooks.
This is an event command, but in the current version of HyperDbg (in Debugger Mode), this command will continue the debuggee for some time; however, you can use this trick to make sure you won't lose any event.
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