!syscall (hook system-calls)

Description of '!syscall' command in HyperDbg.

Command

!syscall

Syntax

!syscall [syscall-number (hex value)] [pid (hex value)] [core (hex value)] [imm (yes|no)] [event options]

Description

Triggers when the debugging machine executes a syscall instruction or, in other words, when Windows tries to run a system call, this event will be triggered.

When you enable this event, all syscall instructions from all processes will be monitored, and due to the limitation in hardware, you can't expect it to trigger for just one process. Still, you can configure the debugger to trigger the event for you in the case of a special process by adding pid xxto the command.

Parameters

[syscall-number (hex value)]

Trigger in the case of a special system-call (system-call number). If you don't specify this parameter, then it will be triggered for all system-calls.

[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 (r8 in custom code and rdx in condition code register) to the event trigger, HyperDbg sends the system-call number, which is rax register in Windows system-call calling convention.

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

You can use the condition if you want to check the syscall parameters. For example, Windows uses an x64 fast-call calling convention on its system-call entry; thus, you can check whether parameters of syscall rcx,rdx,r8, etc. match to your debugging logic or not and have a conditional syscall hooker, and also, you can change the parameters of a syscall.

Imagine we want to break on all system-calls of a process id 0x490.

HyperDbg> !syscall pid 490

Suppose we want to break on system-call 0x55 in process id 0x490. Take a look at the Syscall Tables.

HyperDbg> !syscall 55 pid 490

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.

HyperDbg> !syscall 55 script { HyperDbg Script Here }

The above command when messages don't need to be delivered immediately.

HyperDbg> !syscall 55 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> !syscall 55 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.

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 process id 0x490 for syscall-number 0x55 and run 3 nops whenever the event is triggered. Take a look at Run Custom Code for more information.

HyperDbg> !syscall 55 pid 490 code {90 90 90}

Run Custom Code (Conditional)

Monitoring process id 0x490 for syscall-number 0x55 and runs 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> !syscall 55 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.

IOCTL

This command uses the same method to send IOCTL for regular events.

As EventType use SYSCALL_HOOK_EFER_SYSCALLand send the start syscall-number (if any) if you want just a special system-call in OptionalParam1 in DEBUGGER_GENERAL_EVENT_DETAIL.

Design

Take a look at "Design of !syscall & !sysret" to see how it works.

Remarks

This command is not PatchGurad compatible, which means that PatchGuard detects this command and will cause BSOD; thus, make sure to turn it off (by disabling driver signature enforcement or attaching a Windbg debugger to the Windows) before using this command.

This command makes your computer substantially slower.

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

Post-Nehalem Processor (EPT)

Windows X86-64 System Call Table (XP/2003/Vista/2008/7/2012/8/10)

!sysret (hook SYSRET instruction execution)