Code Injection

Code Injection

Sandboxie employs a particularly low level approach of injecting its code into processes during creation.

Trigger

The driver registers a PsSetCreateProcessNotifyRoutine callback and when this is triggered inspects if the process should be sandboxed, when it decides so it blocks and requests the SbieSvc service to inject a loader into the process image. Alternatively a suspended process can be created and the driver triggered to put it into a sandbox by using API_START_PROCESS and resuming the process once the driver has finished.

The injection mechanism itself can be adapted to be utilized without the driver. As of version 5.44 the loader code has been moved from the SbieSvc.exe to SbieDll.dll and a cli based tool SbieLdr.exe to utilize it is provided.

Overview

The Code Injection mechanism is made up of 3 components, the injector itself, a low-level shell code (LowLevel.dll), and the to be injected payload (SbieDll.dll). Note that the LowLevel.dll is embedded into the loader as a resource.

Remote Injection

The injection is done calling _FX ULONG SbieDll_InjectLow(HANDLE hProcess, BOOLEAN is_wow64, BOOLEAN bHostInject, BOOLEAN dup_drv_handle) and providing the required arguments, the function than:

  • Starts with preparing a data block lowdata of type SBIELOW_DATA, and filling in variouse values like is_wow64, bHostInject and others...
  • Than it uses SbieDll_InjectLow_CopyCode to allocate sizeof(shell_code) + sizeof(SBIELOW_J_TABLE) + 0x400 bytes of Memory in the target process and write the shell code to it.
    This function also, in an unrelated last step, copies 48 bytes from the begin of ntdll!LdrInitializeThunk into lowdata.LdrInitializeThunk_tramp.
  • Than if dup_drv_handle was set SbieDll_InjectLow_SendHandle is used to open a handle to the driver and duplicate it into the process, saving its value to lowdata.api_device_handle.
  • Than duplicates of a couple of required NTDLL functions are saved to the lowdata data block, and the address of the SBIELOW_J_TABLE section is stored to lowdata.Sbie64bitJumpTable.
  • Than the actual trampoline is build by SbieDll_InjectLow_BuildTramp in lowdata.LdrInitializeThunk_tramp.
  • Now the function uses SbieDll_InjectLow_CopySyscalls to allocate and fill in another memory segment syscall_data.
    This block is made up of 2 sections one containing information from the driver that are used to hook all system calls,
    this is optionally done by the shell code when bHostInject == 0, that is followed by the SBIELOW_EXTRA_DATA that points to values stored behind it in the memory block.
    The data stored there a couple of offsets, as well as the full paths to the SbieDll.dll that is to be injected later on.
  • The address of that auxiliary memory is saved to lowdata.syscall_data and the lowdata block is written with SbieDll_InjectLow_CopyData directly into the shell code memory.
  • Finally the ntdll!LdrInitializeThunk in the target process gets overwritten using SbieDll_InjectLow_WriteJump with a jump instruction into the shell code's entry point.

Now the process can be resumeed and the injected code will do its thing.

An important note to make here is that this function does the same for native 64 bit and wow64 emulated 32 bit processes,
in fact, on a 64-bit system the injected shell code is always 64 bit. Only much later in the initialization of the process running under wow64 it switches to 32-bit.

Shell Code (LowLevel.dll) operation

The LowLevel.dll is written partially in assembler and partially in C, its base address is set to 0 to gain position independence.
The initial entry point _Start retrieves the current address and calculates the addresses of the data block data of type SBIELOW_DATA and those of a couple of helper functions written in assembler, with those values as parameter it calls the EntrypointC function handing off the operation to the C portion.

The EntrypointC function ensures that it will be executed only once, using a spinlock, and then checks if the data->bHostInject field is set to 0 it first hooks all the ntdll sys call functions using InitSyscalls than it prepares the later loading of the SbieDll.dll using InitInject and , on 64 bit systems only, it calls InitConsole to modify the ConsoleHandle. If bHostInject != 0 the function only calls InitInject. Last the trampoline to the original function data->LdrInitializeThunk_tramp is called.

InitInject

The InitInject function checks if the process is running natively (i.e. 32-bit on a x86 system or 64-bit in a x64 system), or if its running under wow64 (that is a 32-bit process on a 64-bit system) and selects eider the native ntdll base address or the one of the wow64 ntdll. On windows prior to 8 that address was located in KUSER_SHARED_DATA::Wow64SharedInformation structure, but not on later windows. Sandboxie used the driver to record the address of the wow64 ntdll during image loading and InitInject queried the driver for it. Since version 5.44 however its driver independent, the loader code uses NtQueryVirtualMemory to find the image base address and saves it into the ntdll_wow64_base field of the data block.

At this point the top portion of the data->syscall_data before the SBIELOW_EXTRA_DATA region is no longer required and is repurposed to store temporary data of the type INJECT_DATA.

The function than finds the addresses of LdrLoadDll, LdrGetProcedureAddress, NtRaiseHardError and RtlFindActivationContextSectionString using a custom FindDllExport lookup function by parsing through the previously selected ntdll image, these addresses are stored into the INJECT_DATA region, then a couple values from the SBIELOW_EXTRA_DATA are also copied into that region, containing paths to the SbieDll.dll (booth 32 and 64 bit paths), as well as the name of kernel32.dll.

On 64-bit systems the function distinguishes between the native and the wow64 execution, in the latter case branching of to InitInjectWow64.
In the native case it continues with hooking the RtlFindActivationContextSectionString function in the ntdll.dll.

  • An original copy of the functions begin is first saved to the INJECT_DATA structure
  • The address of the structure is written into the detour function which is implemented in assembler.
  • Than the RtlFindActivationContextSectionString begin is overwritten with a jump instruction to the detour function.
  • Last a pointer to the SBIELOW_DATA region is saved into the very top of the INJECT_DATA region, and the function exits.

In the wow64 case InitInjectWow64 sets up the RtlFindActivationContextSectionString hook on the 32-bit version of the function in the wow64 ntdll.dll in a similar way.

RtlFindActivationContextSectionString Detour

In contrary to the above operations which are always executed natively, the RtlFindActivationContextSectionString Detour function is executed in the mode matching the bit-ness of the started process.

  • The function first restores the original RtlFindActivationContextSectionString begin.
  • Than it loads the kernel32.dll followed by loading the SbieDll.dll and retrieving the address of Ordinal 1.
  • Than it saves value of the first argument to the INJECT_DATA structure and replaces it with a pointer to said structure
  • Finally, it jumps to address of Ordinal 1, it uses a jump rather than call to invoke it so that when it returns it will return directly to the current caller.

Payload (SbieDll.dll) operation

The SbieDll.dll hook entry point Dll_Ordinal1 function starts of by obtaining a few required values from the INJECT_DATA structure that was passed as first argument, like the address of SBIELOW_DATA data block, and the original value of the first argument. Having copied the required values, it can free the no longer needed INJECT_DATA, formally syscall_data region.
The function now checks if bHostInject is set to 0 in which case it Calls SbieDll!Dll_InitInjected this function hooks pretty much everything, …, last but not least it calls SbieDll!Ldr_Init which sets up callbacks for dll loading and calls SbieDll!Ldr_Inject_Init. If bHostInject != 0 however SbieDll!Ldr_Inject_Init is called directly from Dll_Ordinal1. Once the initialization is completed Dll_Ordinal1 runs the real RtlFindActivationContextSectionString with its original arguments and returns.

As if all this hooking wouldn’t be enough SbieDll!Ldr_Inject_Init sets up yet an other hook, this time targeting the actual entry point of the starting process. The function saves the initial bytes of the entry point, and overwrites it with a jump to SbieDll!Ldr_Inject_Entry64 or to SbieDll!Ldr_Inject_Entry32 respectively.
Those are implemented in assembler, they pass a pointer to the return address location as argument to SbieDll!Ldr_Inject_Entry and clean up the stack, then they return to the begin of the entry point.

Ldr_Inject_Entry

This function first restores the original entry point function from SbieDll!Ldr_Inject_SaveBytes and changes its caller’s return address to point to the begin of the entry point. This way once the caller returns the real entry point will be invoked. Then the function checks if bHostInject is set to 0 in which case it first calls SbieDll!Ldr_LoadInjectDlls and then SbieDll!Dll_InitExeEntry which performs the last initialization steps. If bHostInject != 0 it calls only SbieDll!Ldr_LoadInjectDlls this function checks the Sandboxie.ini for the InjectDll or the InjectDll64 respectively, and loads the additional dll’s if any are configured.

Open Source sandbox-based isolation software