Apple macOS/iOS – ‘TIKeyboardLayout initWithCoder:’ NSKeyedArchiver Heap Corruption Due to Rounding Error
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1172 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: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/42051.zip |