Apple iOS/macOS - NSKeyedArchiver Heap Corruption Due to Rounding Error in 'TIKeyboardLayout initWithCoder:'

EDB-ID: 42051
Author: Google Security Research
Published: 2017-05-23
CVE: CVE-2017-2524
Type: Dos
Platform: Multiple
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

Using lldb inside a simple hello_world app for iOS we can see that there are over 600 classes which we could get deserialized
(for persistance for example.)

The TextInput framework which is loaded has a class TIKeyboardLayout. The initWithCoder: implementation has this code:

(this is the x86 code, the framework is there on both platforms.)

mov r15, cs:selRef_decodeBytesForKey_returnedLength_
lea rdx, cfstr_Frames ; "frames"
lea rcx, [rbp+var_40] <-- length of serialized binary data
mov rdi, r14
mov rsi, r15
call r13 ; _objc_msgSend
mov r12, rax <-- pointer to serialized binary data
mov rdx, [rbp+var_40]
shr rdx, 3 <-- divide length by 8
mov rax, cs:_OBJC_IVAR_$_TIKeyboardLayout__count ; uint64_t _count;
mov [rbx+rax], rdx
mov rsi, cs:selRef_ensureFrameCapacity_
mov rdi, rbx
call r13 ; _objc_msgSend <-- will calloc(len/8, 8) and assign to _frames
mov rax, cs:_OBJC_IVAR_$_TIKeyboardLayout__frames ; struct _ShortRect *_frames;
mov rdi, [rbx+rax] ; void *
mov rdx, [rbp+var_40] ; size_t <-- original length not divided by 8
mov rsi, r12 ; void *
call _memcpy <-- can memcpy up to 7 bytes more than was allocated

This method reads binary data from the NSCoder, it divides the length by 8 and passes that to ensureFrameCapacity
which passes it to calloc with an item size of 8. This has the effect of mallocing the original size rounded down
to the nearest multiple of 8.

The memcpy then uses the original length (not rounded down) causing a controlled heap buffer overflow.

I've created a serialized TIKeyboardLayout with a frames value which is "A"*0x107.

Use ASAN to see the crash clearly (see provided compiler invokation.)

tested on MacOS 10.12.3 (16D32)

Proof of Concept:

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