Windows® Internals, Sixth Edition, Part 2

ASLR begins at the image level, with the executable for the process and its dependent DLLs. Any image file that has specified ASLR support in its PE header (IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE), typically specified by using the /DYNAMICBASE linker flag in Microsoft Visual Studio, and contains a relocation section will be processed by ASLR. When such an image is found, the system selects an image offset valid globally for the current boot. This offset is selected from a bucket of 256 values, all of which are 64-KB aligned.

Image Randomization

For executables, the load offset is calculated by computing a delta value each time an executable is loaded. This value is a pseudo-random 8-bit number from 0x10000 to 0xFE0000, calculated by taking the current processor’s time stamp counter (TSC), shifting it by four places, and then performing a division modulo 254 and adding 1. This number is then multiplied by the allocation granularity of 64 KB discussed earlier. By adding 1, the memory manager ensures that the value can never be 0, so executables will never load at the address in the PE header if ASLR is being used. This delta is then added to the executable’s preferred load address, creating one of 256 possible locations within 16 MB of the image address in the PE header.

For DLLs, computing the load offset begins with a per-boot, systemwide value called the image bias, which is computed by MiInitializeRelocations and stored in MiImageBias. This value corresponds to the time stamp counter (TSC) of the current CPU when this function was called during the boot cycle, shifted and masked into an 8-bit value, which provides 256 possible values. Unlike executables, this value is computed only once per boot and shared across the system to allow DLLs to remain shared in physical memory and relocated only once. If DLLs were remapped at different locations inside different processes, the code could not be shared. The loader would have to fix up address references differently for each process, thus turning what had been shareable read-only code into process-private data. Each process using a given DLL would have to have its own private copy of the DLL in physical memory.

Once the offset is computed, the memory manager initializes a bitmap called the MiImageBitMap. This bitmap is used to represent ranges from 0x50000000 to 0x78000000 (stored in MiImageBitMapHighVa), and each bit represents one unit of allocation (64 KB, as mentioned earlier). Whenever the memory manager loads a DLL, the appropriate bit is set to mark its location in the system; when the same DLL is loaded again, the memory manager shares its section object with the already relocated information.

As each DLL is loaded, the system scans the bitmap from top to bottom for free bits. The MiImageBias value computed earlier is used as a start index from the top to randomize the load across different boots as suggested. Because the bitmap will be entirely empty when the first DLL (which is always Ntdll.dll) is loaded, its load address can easily be calculated: 0x78000000 – MiImageBias * 0x10000. Each subsequent DLL will then load in a 64-KB chunk below. Because of this, if the address of Ntdll.dll is known, the addresses of other DLLs could easily be computed. To mitigate this possibility, the order in which known DLLs are mapped by the Session Manager during initialization is also randomized when Smss loads.

Finally, if no free space is available in the bitmap (which would mean that most of the region defined for ASLR is in use, the DLL relocation code defaults back to the executable case, loading the DLL at a 64-KB chunk within 16 MB of its preferred base address.

Stack Randomization

The next step in ASLR is to randomize the location of the initial thread’s stack (and, subsequently, of each new thread). This randomization is enabled unless the flag StackRandomizationDisabled was enabled for the process and consists of first selecting one of 32 possible stack locations separated by either 64 KB or 256 KB. This base address is selected by finding the first appropriate free memory region and then choosing the xth available region, where x is once again generated based on the current processor’s TSC shifted and masked into a 5-bit value (which allows for 32 possible locations).

Once this base address has been selected, a new TSC-derived value is calculated, this one 9 bits long. The value is then multiplied by 4 to maintain alignment, which means it can be as large as 2,048 bytes (half a page). It is added to the base address to obtain the final stack base.

Heap Randomization