Didier Stevens

Thursday 29 September 2011

Add Bottom Up Randomization To (Your Own) Source Code

Filed under: Vulnerabilities,Windows 7,Windows Vista — Didier Stevens @ 19:14

EMET’s new Bottom Up Randomization spectacularly increased the entropy of DLL’s base addresses loaded into my test program. Instead of 15 different addresses, I had more than 200.

Matt Miller told me how he implemented Bottom Up Randomization:

“It works by reserving a random number (between [0,256]) of 64K regions via VirtualAlloc. This has the effect of consuming a small portion of the bottom part of the address space. Since the Windows kernel assigns base addresses for collided DLLs by searching for a free region starting at the bottom of the address space, bottom up randomization ensures that a random base address will be assigned. Without bottom up randomization the bottom part of the address space remains fairly static (with some exceptions, such as due to heap, stack, and EXE randomization).”

So I decided to add this algorithm at the start of my test program:

int iIter;
int iRand;

srand(time(NULL));
iRand = rand() % 256 + 1;
for (iIter = 0; iIter < iRand; iIter++)
 VirtualAlloc(NULL, 64*1024, MEM_COMMIT | MEM_RESERVE, PAGE_NOACCESS);

Again, the result is spectacular. In stead of 15 base addresses, with the most frequent address being using 30% of the time, my Bottom Up Randomization implementation gives me more than 300 addresses after 150.000 runs. And there’s no single address being used more than 0,5% of the time.

From now on, I’m going to include this in my programs, and I advise you to do the same with your programs. Or to open source programs you use.

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