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ARM Exception Vector Info Leak
This is CVE-2013-0978.
This vulnerability is used by evasi0n in order to defeat KASLR. Since iOS6 the kernel base address is randomized at 29 possible locations. Actually it is not fully randomized due to the ARM vector table residing at a fixed address. The vector table is held at address zero and at runtime relocated to 0xffff0000 by setting the V-bit in CP15 c1. The following ARM vector table entries exist:
| Offset | Handler |
|---|---|
00
|
Reset |
04
|
Undefined Instruction |
08
|
Supervisor Call (SVC) |
0C
|
Prefetch Abort |
10
|
Data Abort |
14
|
Reserved |
18
|
Interrupt (IRQ) |
1C
|
Fast Interrupt (FIQ) |
evasi0n calls the Data Abort exception from a separate thread and catches the exception. In the exception handler, in_state->__pc is saved to global.exception_pc and leaks the base address because this exception was called from com.apple.iokit.IOUSBDeviceFamily.
In the exception handler it is also possible to get 4 bytes at a chosen address, so this is additionally an info leak for arbitrary memory.
Apple's description in the iOS 6.1.3 security fixes:
Kernel
Impact: A local user may be able to determine the address of structures in the kernel
Description: An information disclosure issue existed in the ARM prefetch abort handler. This issue was addressed by panicking if the prefetch abort handler is not being called from an abort context.
