MacOS kernel memory disclosure due to lack of bounds checking in AppleIntelCapriController::getDisplayPipeCapability(CVE-2017-2489)

MacOS kernel memory disclosure due to lack of bounds checking in AppleIntelCapriController::getDisplayPipeCapability

Selector 0x710 of IntelFBClientControl ends up in AppleIntelCapriController::getDisplayPipeCapability.

This method takes a structure input and output buffer. It reads an attacker controlled dword from the input buffer which it uses to index an array of pointers with no bounds checking:

AppleIntelCapriController::getDisplayPipeCapability(AGDCFBGetDisplayCapability_t *, AGDCFBGetDisplayCapability_t *) __text:000000000002A3AB mov r14, rdx ; output buffer, readable from userspace __text:000000000002A3AE mov rbx, rsi ; input buffer, controlled from userspace ... __text:000000000002A3B8 mov eax, [rbx] ; read dword __text:000000000002A3BA mov rsi, [rdi+rax*8+0E40h] ; use as index for small inline buffer in this object __text:000000000002A3C2 cmp byte ptr [rsi+1DCh], 0 ; fail if the byte at +0x1dc is 0 __text:000000000002A3C9 jz short ___fail __text:000000000002A3CB add rsi, 1E0Dh ; otherwise, the memcpy from that pointer +0x1e0dh __text:000000000002A3D2 mov edx, 1D8h ; 0x1d8 bytes __text:000000000002A3D7 mov rdi, r14 ; to the buffer which will be sent back to userspace __text:000000000002A3DA call _memcpy

For this PoC we try to read the pointers at 0x2000 byte boundaries after this allocation; with luck there will be a vtable pointer there which will allow us to read back vtable contents and defeat kASLR.

With a bit more effort this could be turned into an (almost) arbitrary read by for example spraying the kernel heap with the desired read target then using a larger offset hoping to land in one of the sprayed buffers. A kernel arbitrary read would, for example, allow you to read the sandbox. kext The HMAC key and forge sandbox extensions if it still works like that.

tested on MacOS Sierra 10.12.2 (16C67)

Attachment: capri_mem. c