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
Once the offset is computed, the memory manager initializes a bitmap called the
As each DLL is loaded, the system scans the bitmap from top to bottom for free bits. The
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
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