!msrread (hook RDMSR instruction execution)
Description of the '!msrread' command in HyperDbg.
!msrread
!msrread [Msr (hex)] [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 an RDMSR instruction or, in other words, when Windows or a driver tries to read a Model-Specific Register (MSR).
[Msr (hex)]
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 RDMSR 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) and IA32_APERF (0x000000e8). If you explicitly specify these MSRs, you'll get the events for these MSRs like other regular MSRs but only use the '!msrread' on these MSRs when you know what you want to do.
[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 the rcxregister of when RDMSR is executed.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 RDMSR instruction, whereas the 'post' calling stage is triggered subsequent to running the RDMSR instruction; thus, you can read/modify the ECX register or ignore the event in the 'pre' stage, and view/modify the results (EDX:EAX) 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".
Imagine we want to break on all RDMSRs.
HyperDbg> !msrread
If we want to break on MSR 0xc0000082.
HyperDbg> !msrread 0xc0000082
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> !msrread 0xc0000082 script { HyperDbg Script Here }
The above command when messages don't need to be delivered immediately.
HyperDbg> !msrread 0xc0000082 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> !msrread 0xc0000082 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 execution of RDMSR for MSR 0xc0000082 and run 3 nops whenever the event is triggered. Take a look at Run Custom Code for more information.
HyperDbg> !msrread 0xc0000082 code {90 90 90}
Run Custom Code (Conditional)
Monitoring execution of RDMSR 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.
HyperDbg> !msrread 0xc0000082 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
RDMSR_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.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.
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.
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