ASLR

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

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 indeed includes ASLR.

In reply to i0nic, 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.

• 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. For Example:

__text:80046F50 loc_80046F50                            ; CODE XREF: sub_80046F1C+C�j
__text:80046F50                 MOV             R0, 0x240BEA
__text:80046F58                 ADD             R0, PC  ; char *
__text:80046F5A                 BL              _panic
__text:80046F5E                 MOV             R0, R4
__text:80046F60                 POP             {R4,R7,PC}
__text:80046F60 ; End of function sub_80046F1C
And, as can be expected:

string:80287B24 aDf_memory_object_data_reclaimN DCB 0x22,"df_memory_object_data_reclaim\n",0x22,0

 (so sub_80046F1C is df_memory_object_data_reclaim)

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

• Template:Wp on Wikipedia
• ASLR in the dyld on the iPhone Dev Wiki