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Difference between revisions of "ASLR"
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* stackshot and other kernel functions take the vm_kernel_slide into consideration and subtract it from the actual positions of functions/symbols. |
* stackshot and other kernel functions take the vm_kernel_slide into consideration and subtract it from the actual positions of functions/symbols. |
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| − | * 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. |
+ | * 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. |
| − | |||
| − | <pre> |
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| − | __text:80046F50 loc_80046F50 ; CODE XREF: sub_80046F1C+C�j |
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| − | __text:80046F50 MOV R0, 0x240BEA |
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| − | __text:80046F58 ADD R0, PC ; char * |
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| − | __text:80046F5A BL _panic |
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| − | __text:80046F5E MOV R0, R4 |
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| − | __text:80046F60 POP {R4,R7,PC} |
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| − | __text:80046F60 ; End of function sub_80046F1C |
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| − | And, as can be expected: |
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| − | |||
| − | string:80287B24 aDf_memory_object_data_reclaimN DCB 0x22,"df_memory_object_data_reclaim\n",0x22,0 |
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| − | |||
| − | (so sub_80046F1C is df_memory_object_data_reclaim) |
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| − | </pre> |
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Revision as of 11:44, 12 June 2012
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.
An upcoming book on OS X/iOS internals discusses this in detail.
- 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
