ASLR

ASLR (Address Space Layout Randomization) is a form of data security used to randomize data on the RAM to help prevent exploits from taking control of the system. It first appeared in iOS 4.3.

Program and dyld

• On program load, the address space offset of the program is randomized between 0x0 and 0x100000
• It always falls on a 0x1000 page boundary
• dyld is included in this sliding section

Kernel space ASLR

Mountain Lion boasts a xnu 2150 kernel, which includes, for the first time, ASLR in kernel space. Because OS X and iOS are so closely tied together, as previously surmised here, iOS 6.0b1 (XNU 2107.1.78) indeed includes ASLR.

This is the lowdown of ASLR:

• When the kernel boots, i386_vm_init (iOS: arm_vm_init) initializes the value of vm_kernel_slide
• The kernel supports a new system call (#439 on Mountain Lion, likely #440 on iOS 6), called kas_info. This will return the value of vm_kernel_slide, but only for a privileged process. Update for b3: Apple probably realized how stupid this is, and so the syscall (at 0x8021C9E8 on 6.0b3, n81ap) returns 0x2D (ENOTSUP).
• kld is updated to reflect the slide in symbols. Likewise OSKext::LoadExecutable and friends
• stackshot and other kernel functions take the vm_kernel_slide into consideration and subtract it from the actual positions of functions/symbols.
• Disassembly is somewhat harder. Rather than store addresses of symbols and strings at the end of each function (DCD), the symbols/string offsets with respect to PC are now hardcoded in the instructions themselves, and need to be calculated.

An upcoming book on OS X/iOS internals discusses this in detail.

dyld_shared_cache

• The system libraries are now stored in a big cache file, see
• This address randomized at boot time, in many possible places, higher in the address space than the program
• The functions retain a fixed offset to each other.

External Links

• ASLR on Wikipedia
• ASLR in the dyld on the iPhone Dev Wiki