!msrwrite (hook WRMSR instruction execution)
Description of '!msrwrite' command in HyperDbg.

Command

!msrwrite

Syntax

!msrwrite [msr (hex value)] [pid (hex value)] [core (hex value)] [imm (yes|no)] [event options]

Description

Triggers when the debugging machine executes a WRMSR instruction or, in other words, when Windows or a driver tries to write on a Model-Specific Register (MSR).

Parameters

[msr (hex value)]
Trigger in the case of a special Model-Specific Register (MSR). If you don't specify this parameter, then it will be triggered for all WRMSR executions.
Generally, it's not a good practice to intercept all the MSR Reads (RDMSR) or MSR Writes (WRMSRs) because it makes your system substantially slower and undefined behavior in some cases. By the way, HyperDbg supports intercepting all the MSRs. If you don't specify any parameters to intercept all the MSRs, HyperDbg automatically IA32_KERNEL_GSBASE (0xC0000102), IA32_MPERF (0x000000e7), IA32_APERF (0x000000e8), IA32_SPEC_CTRL (0x00000048), and IA32_PRED_CMD (0x00000049).
If you explicitly specify these MSRs, you'll get the events for these MSRs like other regular MSRs but only use the '!msrwrite' on these MSRs when you know what you want to do.
[pid (hex value)]
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.
[core (hex value)]
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 (yes|no)]
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.
[event options]
Regular event parameters that are used in HyperDbg events. (For more information, read this topic)

Context

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 the rcxregister of when WRMSR is executed.

Debugger

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".

Break

Imagine we want to break on all WRMSRs.
1
HyperDbg> !msrwrite
Copied!
If we want to break on MSR 0xc0000082.
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HyperDbg> !msrwrite 0xc0000082
Copied!

Script

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.
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HyperDbg> !msrwrite 0xc0000082 script { HyperDbg Script Here }
Copied!
The above command when messages don't need to be delivered immediately.
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HyperDbg> !msrwrite 0xc0000082 script { HyperDbg Script Here } imm no
Copied!
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.
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HyperDbg> !msrwrite 0xc0000082 script {file:c:\users\sina\desktop\script.txt}
Copied!
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.

Custom Code

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 execution of WRMSR for MSR 0xc0000082 and run 3 nops whenever the event is triggered. Take a look at Run Custom Code for more information.
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HyperDbg> !msrwrite 0xc0000082 code {90 90 90}
Copied!
Run Custom Code (Conditional)
Monitoring execution of WRMSR for MSR 0xc0000082 and run 3 nops whenever the event condition is triggered and run 3 nops whenever the event is triggered. Take a look at Run Custom Code and how to create a condition for more information.
1
HyperDbg> !msrwrite 0xc0000082 code {90 90 90} condition {90 90 90}
Copied!
Keep in mind that a conditional event can be used in Breaking to Debugger and Running Script too.

IOCTL

This command uses the same method to send IOCTL for regular events.
As EventType use WRMSR_INSTRUCTION_EXECUTION and send the special MSR rcx (if any) if you want to monitor just a special MSR in OptionalParam1 in DEBUGGER_GENERAL_EVENT_DETAIL.

Design

Both !msrread and !msrwrite use the vm-exits caused by setting bits in the MSR Bitmap field of the hypervisor VMCS.
For !msrread vm-exit with (EXIT_REASON_MSR_READ) or exit-reason 31 is used.
For !msrwrite vm-exit with (EXIT_REASON_MSR_WRITE) or exit-reason 32 is used.

Remarks

When you enable this event, only your specific MSR will be hooked, so this command won't trigger on all MSRs thus won't make your computer slow.
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.

Requirements

None
Last modified 27d ago