!syscall, !syscall2 (hook system-calls)

Description of '!syscall, !syscall2' commands in HyperDbg.

Command

!syscall

!syscall2

Syntax

!syscall [SyscallNumber (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) }] [output {OutputName (string)}]

!syscall2 [SyscallNumber (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) }] [output {OutputName (string)}]

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.

If this command produces weird behavior in your system, you can use the alternative method.

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

[SyscallNumber (hex)]

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 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)

Optional value which reserves a safe pre-allocated buffer to be accessed within the event codes.

[script { Script (string) }] (optional)

A HyperDbg script will be executed each time the event is triggered.

[condition { Condition (hex) }] (optional)

Optional hex assembly codes which check for conditions in assembly.

[code { Code (hex) }] (optional)

Optional hex assembly codes will be executed each time the event is triggered.

[output {OutputName (string)}] (optional)

Optional output resource name for forwarding events.

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 system-call number, which is rax register in Windows system-call calling convention.

Short-circuiting

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.

Calling Stages

This event supports different calling stages. The 'pre' calling stage is triggered prior to the kernel transfer, whereas the 'post' calling stage is triggered subsequent to the kernel transfer. In addition, the 'all' calling stage will trigger the event in both cases. You can ignore the event in the 'pre' stage. For more information, please refer to the article provided here.

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 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> !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 (e.g., attaching a kernel-mode WinDbg debugger at the start of the Windows) before using this command. Disabling Driver Signature Enforcement alone won't turn off the PatchGuard.

This command makes your computer substantially slower.

The difference between !syscall and !syscall2 is that we safely check the memory in the first command to see if the instruction that caused #UD is really an SYSRET or a SYSCALL. So, we access the memory in this command. However, we realized that older systems have problems with this way of memory access. In the second command, we just check for the RIP to see if it's a kernel address or a user address. Usually, this method works without error for several hours, but if one application generates a #UD, then a BSOD will happen. The second method is generally faster in speed, but we encourage you to use the first command and if your computer doesn't support the first command, then use the second command.

This command creates an event. Starting from HyperDbg v0.7, events are guaranteed to keep the debuggee in a halt state (in the Debugger Mode); thus, nothing will change during its execution and the context (registers and memory) remain untouched. You can visit instant events for more information.

Alternative Method For SYSCALL Interception

If the !syscall command triggered some weird behavior in your system, you could alternatively use the EPT hook on the SYSCALL handler. For example, you can use the following approach (script).

!epthook nt!KiSystemCall64+3 script {
    //
    // Your target script here
    //
}

The above code sets a breakpoint on the Windows System-call handler and +3 is added because normally the first instruction in the SYSCALL handler is SWAPGS, which we want to be changed because the interception is done in the kernel.

In case you don't have access to Windows symbols, you can read the address of nt!KiSystemCall64 from MSR 0xc0000082 using the 'rdmsr' command. After that, add +3 to the target MSR and run the above code without the function name.

Requirements

None

Related

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

!sysret (hook SYSRET instruction execution)