Thursday, December 24, 2020

Experiment in extracting runtime drivers on Windows

This post explains the concept of UEFI runtime drivers, how they interact with OS, and an experimental attempt to extract them.

Here is a quick takeaway from this article. 

  • UEFI runtime drivers are part of firmware that run with the ring-0 privilege before OS starts.
  • They provide interfaces to some firmware-dependent features, called runtime services, to OS. 
  • Windows saves the addresses of those runtime services into HalEfiRuntimeServicesBlock
  • The base addresses of runtime drivers can be located from the contents of  HalEfiRuntimeServicesBlock, but it is difficult to safely find HalEfiRuntimeServicesBlock and base addresses.
  • Dumping the runtime drivers are useful for diagnosing issues with them, but the HalEfiRuntimeServicesBlock-based approach is fundamentally limited to drivers that implement runtime services.
Happy holidays?

What are UEFI runtime drivers?

Any retail x86_64 PCs have UEFI-based system firmware those days, and such firmware must implement some modules that reside in memory from system start up to shutdown. Those modules are called UEFI "runtime drivers" and meant to provide interfaces to certain firmware-implemented services to an operating system (OS), as specified by the UEFI specification

For example, UEFI defines ResetSystem() as one of such interfaces and requires OEM to implement firmware containing the runtime driver that implements it. Taking my ASUS laptop as an example, ResetSystem() is implemented in a runtime driver called SBRun. 

Those interfaces are called "runtime services" and defined for 14 services.  One can find the C-representation of their definitions in EDK2.
/// EFI Runtime Services Table.
typedef struct {
  /// The table header for the EFI Runtime Services Table.
  EFI_TABLE_HEADER                Hdr;

  // Time Services
  EFI_GET_TIME                    GetTime;
  EFI_SET_TIME                    SetTime;
  EFI_GET_WAKEUP_TIME             GetWakeupTime;
  EFI_SET_WAKEUP_TIME             SetWakeupTime;

  // Virtual Memory Services
  EFI_SET_VIRTUAL_ADDRESS_MAP     SetVirtualAddressMap;
  EFI_CONVERT_POINTER             ConvertPointer;

  // Variable Services
  EFI_GET_VARIABLE                GetVariable;
  EFI_GET_NEXT_VARIABLE_NAME      GetNextVariableName;
  EFI_SET_VARIABLE                SetVariable;

  // Miscellaneous Services
  EFI_GET_NEXT_HIGH_MONO_COUNT    GetNextHighMonotonicCount;
  EFI_RESET_SYSTEM                ResetSystem;

  // UEFI 2.0 Capsule Services
  EFI_UPDATE_CAPSULE              UpdateCapsule;

  // Miscellaneous UEFI 2.0 Service
  EFI_QUERY_VARIABLE_INFO         QueryVariableInfo;

Why we care? 

Runtime drivers have some interesting characteristics:
  • They start before the OS is loaded and can influence the boot process
  • They run with the ring-0 privilege
  • They are called during normal OS execution through the runtime services
  • They are not listed by any widely known monitoring tools or debuggers (unlike device drivers) on Windows
  • They can be developed by anyone and be loaded as long as Secure Boot is disabled
  • They may not exist as files in storage that is accessible from OS 
Because of the lack of visibility into them and access to files, diagnosing issues with them can be more challenging than the issues with OS-based kernel modules. For example, to reverse engineer runtime driver code, you first have to find the runtime drivers in memory and extract them to a file, instead of grabbing a file on the disk.

The other reason is that runtime drivers have gained popularity in a reverse engineering community and are used more and more widely in a way that breaks the standard OS/system integrity. For example, the owner of the system may install a 3rd party runtime driver that overrides the runtime services provided  by OEM firmware, to have a "backdoor" for reverse engineering. EfiGuard and efi-memory are examples of those. While that is the owner's very intention, some software may still want to detect this and be aware of the fact that system integrity might be tampered.  

How can we find runtime drivers on Windows?

There is no documented interface to locate any runtime drivers in the Windows kernel, unfortunately. However, there are few implementation details that can be abused for it, for example,
  • The addresses of some runtime services are stored in HalEfiRuntimeServicesBlock
  • There is an EFI_RUNTIME_SERVICES global variable, which contains pointers to the runtime services as seen above, has a distinctive RUNTSERV signature, and is memory resident
  • Runtime drivers are also memory resident, mapped in a certain contiguous physical and virtual memory range, and have the DOS header at 4KB aligned addresses
  • Physical memory addresses backing runtime drivers are outside the ranges of Windows manages
Based on those facts, one may scan the DOS header in physical or virtual memory and attempt to find all runtime drivers. 

The other limited (details later) but arguably easier and safer way is to refer to HalEfiRuntimeServicesBlock. HalEfiRuntimeServicesBlock is a Windows defined structure made up of copies of a handful of runtime service addresses [ref], as shown below. 


Windows initializes this structure on its startup and uses it to invoke runtime services (as opposed to directly using the EFI_RUNTIME_SERVICES structure). By locating HalEfiRuntimeServicesBlock, one can find runtime services' addresses and runtime drivers implementing them. 

PoC and challenges 

I authored the tool implementing this idea, named kraft_dinner. Greater details can be found in source code, and here are some notable discoveries with this approach.
  • HalEfiRuntimeServicesBlock can be found with HalQuerySystemInformation() up until only 19H2. One has to get creative for newer versions.
  • The physical memory address range backing runtime drivers is not known to Windows and not reported by MmGetPhysicalMemoryRanges(). This can be used to test a probable runtime driver address. 
  • MmCopyMemory() never succeeds in reading memory that backs runtime drivers, regardless of whether virtual or physical memory is specified. This makes implementing a safe search operation harder.


While the HalEfiRuntimeServicesBlock approach works reasonably well, it has some fundamental limitations.

Firstly, runtime drivers that do not implement runtime services are not found. Such runtime drivers do not have any formally defined way to directly influence Windows and system integrity, but may still hook (patch) other code to implement backdoor instead. umap and voyager are examples of such hacking drivers. A memory scanning-based approach would address this issue.  

Secondly, runtime drivers can hide from this approach easily by writing trampoline code at the beginning of the original runtime service, instead of replacing the pointer in EFI_RUNTIME_SERVICES, or by nullifying the PE header. Those may be mitigated with more intelligent analysis and dumping, but is an easy countermeasure against scanning. 

Thirdly, as a general challenge with memory analysis, classifying memory dumped files is not straightforward. Because memory contents can slightly vary between boots because of relocation (code patches), hash values change each time. Fuzzy hashing such as ssdeep is required to classify dump files and build a useful database. Also, a dump file does not contain driver's name and GUID as found in the actual firmware.

Finally, this is Windows-specific and hacky. It depends heavily on Windows implementation details which may break soon. While PoC worked fined with multiple devices, I would not be comfortable deploying this logic for millions of systems.  

So why did I do this?

I encountered a bug in one of OEM runtime drivers and thought I would tool something quick, but in Rust :)