// Detect and defeat various kinds of XOR encryption
public void PostProcessImage <T>(FileFormatStream <T> stream, PluginPostProcessImageEventInfo data) where T : FileFormatStream <T>
{
if (stream is ElfReader32 stream32)
{
elf32 = stream32;
}
else if (stream is ElfReader64 stream64)
{
elf64 = stream64;
}
else
{
return;
}
PluginServices.For(this).StatusUpdate("Detecting encryption");
this.stream = stream;
sections = stream.GetSections().GroupBy(s => s.Name).ToDictionary(s => s.Key, s => s.First());
if (HasDynamicEntry(Elf.DT_INIT) && sections.ContainsKey(".rodata"))
{
// Use the data section to determine some possible keys
// If the data section uses striped encryption, bucketing the whole section will not give the correct key
var roDataBytes = stream.ReadBytes(sections[".rodata"].ImageStart, sections[".rodata"].ImageLength);
var xorKeyCandidateStriped = roDataBytes.Take(1024).GroupBy(b => b).OrderByDescending(f => f.Count()).First().Key;
var xorKeyCandidateFull = roDataBytes.GroupBy(b => b).OrderByDescending(f => f.Count()).First().Key;
// Select test nibbles and values for ARM instructions depending on architecture (ARMv7 / AArch64)
var testValues = new Dictionary <int, (int, int, int, int)> {
protected override (ulong, ulong) ConsiderCode(IFileFormatStream image, uint loc)
{
// Setup
var buffSize = 0x76; // minimum number of bytes to process the longest expected function
var leaSize = 7; // the length of an LEA instruction with a 64-bit register operand and a 32-bit memory operand
var xor64Size = 3; // the length of a XOR instruction of two 64-bit registers
var xor32Size = 2; // the length of a XOR instruction of two 32-bit registers
var pushSize = 2; // the length of a PUSH instruction with a 64-bit register
var offset = 0;
int RAX = 0, RBX = 3, RCX = 1, RDX = 2, RSI = 6, RDI = 7; // R8 = 8
ulong pCgr = 0; // the point to the code registration function
image.Position = loc;
var buff = image.ReadBytes(buffSize);
// We have seen two versions of the initializer:
// 1. Regular version
// 2. Inlined version with il2cpp::utils::RegisterRuntimeInitializeAndCleanup(CallbackFunction, CallbackFunction, order)
// Version 1 passes "this" in rcx and the arguments in rdx (our wanted pointer), r8d (always zero) and r9d (always zero)
// or "this" in rdi, and the arguments in rsi (our wanted pointer), edx (always zero) and ecx (always zero)
// Version 2 has a standard prologue and loads the wanted pointer into rax or rbp (lea rax/rbp)
(int reg, uint operand)? lea;
// Check for regular version
// Generalize it as follows:
// - each instruction must be lea r64, imm32 or xor r32, r32
// - xors must always have the same register for both operands
// - lea that can't be mapped into the file is the pointer to 'this', otherwise it's the pointer to the init function
// - the last instruction should always be jmp (not currently enforced)
// - function length should not be longer than 5 instructions (two leas, two xors and one jmp)
offset = 0;
for (var instructions = 0; instructions < 4; instructions++)
{
// All allowed instruction types
var xor32 = getXorR32R32(buff, offset);
var xor64 = getXorR64R64(buff, offset);
lea = getLea(buff, offset);
if (xor32 != null && xor32.Value.reg_op1 == xor32.Value.reg_op2)
{
offset += xor32Size;
}
else if (xor64 != null && xor64.Value.reg_op1 == xor64.Value.reg_op2)
{
offset += xor64Size;
}
else if (lea != null)
{
offset += leaSize;
if (pCgr == 0)
{
try {
// We may have found Il2CppCodegenRegistration(void)
pCgr = image.GlobalOffset + loc + (ulong)offset + lea.Value.operand;
var newLoc = image.MapVATR(pCgr);
}
catch (InvalidOperationException) {
// this pointer
pCgr = 0;
}
}
}
else
{
// not lea or xor
pCgr = 0;
break;
}
}
// Check for inlined version
if (pCgr == 0)
{
// Check for prologue
// - A sequence of 0 or more mov [rsp+argX], rXX followed by 1 or more push rXX
offset = 0;
while (isMovRM64R64(buff, offset))
{
offset += 5;
}
if (isPushR64(buff, offset))
{
// Linear sweep for LEA
var leaInlined = findLea(buff, pushSize, buffSize - pushSize);
if (leaInlined != null)
{
pCgr = image.GlobalOffset + loc + (uint)leaInlined.Value.foundOffset + (uint)leaSize + leaInlined.Value.operand;
}
}
}
// Assume we've found the pointer to Il2CppCodegenRegistration(void) and jump there
if (pCgr != 0)
{
try {
Image.Position = Image.MapVATR(pCgr);
}
// Couldn't map virtual address to data in file, so it's not this function
catch (InvalidOperationException) {
pCgr = 0;
}
}
// Find the first 2 LEAs which we'll hope contain pointers to CodeRegistration and MetadataRegistration
// There are two options here:
// 1. il2cpp::vm::MetadataCache::Register is called directly with arguments in rcx, rdx, r8 or rdi, rsi, rdx (lea, lea, lea, jmp)
// 2. The two functions being inlined. The arguments are loaded sequentially into rax after the prologue
if (pCgr != 0)
{
var buff2Size = 0x50;
var buff2 = image.ReadBytes(buffSize);
offset = 0;
var leas = new Dictionary <(int index, ulong address), int>();
// Find the first three LEAs in the function
while (offset + leaSize < buff2Size && leas.Count < 3)
{
var nextLea = findLea(buff2, offset, buff2Size - (offset + leaSize));
// Use the original pointer found, not the file location + GlobalOffset because the data may be in a different section
if (nextLea != null)
{
leas.Add((leas.Count, pCgr + (uint)nextLea.Value.foundOffset + (uint)leaSize + nextLea.Value.operand), nextLea.Value.reg);
}
offset = nextLea?.foundOffset + leaSize ?? buff2Size;
}
if ((image.Version < 21 && leas.Count == 2) || (image.Version >= 21 && leas.Count == 3))
{
// Register-based argument passing?
var leaRSI = leas.FirstOrDefault(l => l.Value == RSI).Key.address;
var leaRDI = leas.FirstOrDefault(l => l.Value == RDI).Key.address;
if (leaRSI != 0 && leaRDI != 0)
{
return(leaRDI, leaRSI);
}
var leaRCX = leas.FirstOrDefault(l => l.Value == RCX).Key.address;
var leaRDX = leas.FirstOrDefault(l => l.Value == RDX).Key.address;
if (leaRCX != 0 && leaRDX != 0)
{
return(leaRCX, leaRDX);
}
// RAX sequential loading? If so, take the first two arguments
var leasRAX = leas.Where(l => l.Value == RAX).OrderBy(l => l.Key.index).Select(l => l.Key.address).ToArray();
if (leasRAX.Length > 1)
{
return(leasRAX[0], leasRAX[1]);
}
}
}
// If no initializer is found, we may be looking at a DT_INIT function which calls its own function table manually
// In the sample we have seen (PlayStation 4), this function runs through two function tables:
// 1. Start address of table loaded into rbx, pointer past end of table in r12 (lea rbx; lea r12)
// 2. Pointer to final address of 2nd table loaded into rbx (lea rbx), runs backwards (8 bytes per entry) until finding 0xFFFFFFFF_FFFFFFFF
// The strategy: find these LEAs, acquire and merge the two function tables, then call ourselves in a loop to check each function address
// Expect function prologue and at least 3 64-bit register pushes (there are probably more)
if (!isPrologue(buff) || !isPushR64(buff, 4) || !isPushR64(buff, 6) || !isPushR64(buff, 8))
{
return(0, 0);
}
// Find the start and end addresses of the first function table
var leaOfStart = findLea(buff, 10, buffSize - 10);
if (leaOfStart == null || leaOfStart.Value.reg != RBX) // Most be lea rbx
{
return(0, 0);
}
var leaOfEnd = findLea(buff, leaOfStart.Value.foundOffset + leaSize, buffSize - (leaOfStart.Value.foundOffset + leaSize));
if (leaOfEnd == null || leaOfEnd.Value.reg == RBX) // Must be lea with any register besides rbx
{
return(0, 0);
}
var ptrStart1 = leaOfStart.Value.foundOffset + leaSize + leaOfStart.Value.operand;
var ptrEnd1 = leaOfEnd.Value.foundOffset + leaSize + leaOfEnd.Value.operand;
// Find the address of the last item in the second function table
var leaOfLastItem = findLea(buff, leaOfEnd.Value.foundOffset + leaSize, buffSize - (leaOfEnd.Value.foundOffset + leaSize));
if (leaOfLastItem == null || leaOfLastItem.Value.reg != 0b11) // Must be lea rbx
{
return(0, 0);
}
var entrySize = 8; // 64-bit array entries
var ptrEnd2 = leaOfLastItem.Value.foundOffset + leaSize + leaOfLastItem.Value.operand + entrySize;
// Work backwards to find the address of the first item in the second function table
var ptrStart2 = ptrEnd2;
while (image.ReadUInt64(image.MapVATR((ulong)ptrStart2)) != 0xFFFF_FFFF_FFFF_FFFF)
{
ptrStart2 -= entrySize;
}
ptrStart2 += entrySize;
// Acquire both function tables
var funcs1 = image.ReadMappedWordArray((ulong)ptrStart1, (int)(ptrEnd1 - ptrStart1) / entrySize);
var funcs2 = image.ReadMappedWordArray((ulong)ptrStart2, (int)(ptrEnd2 - ptrStart2) / entrySize);
// Check every function
var funcs = funcs1.Concat(funcs2);
foreach (var pFunc in funcs)
{
var result = ConsiderCode(image, image.MapVATR((ulong)pFunc));
if (result != (0, 0))
{
return(result);
}
}
return(0, 0);
}
protected override (ulong, ulong) ConsiderCode(IFileFormatStream image, uint loc)
{
ulong metadata, code;
long pCgr;
// x86
// Assembly bytes to search for at start of each function
var bytes = new byte[] { 0x6A, 0x00, 0x6A, 0x00, 0x68 };
image.Position = loc;
var buff = image.ReadBytes(5);
if (bytes.SequenceEqual(buff))
{
// Next 4 bytes are the function pointer being pushed onto the stack
pCgr = image.ReadUInt32();
// Start of next instruction
if (image.ReadByte() != 0xB9)
{
return(0, 0);
}
// Jump to Il2CppCodegenRegistration
if (image.Version < 21)
{
image.Position = image.MapVATR((ulong)pCgr + 1);
metadata = image.ReadUInt32();
image.Position = image.MapVATR((ulong)pCgr + 6);
code = image.ReadUInt32();
}
else
{
image.Position = image.MapVATR((ulong)pCgr + 6);
metadata = image.ReadUInt32();
image.Position = image.MapVATR((ulong)pCgr + 11);
code = image.ReadUInt32();
}
return(code, metadata);
}
// x86 based on ELF PLT
if (image is IElfReader elf)
{
var plt = elf.GetPLTAddress();
// push ebp; mov ebp, esp; push ebx; and esp, 0FFFFFFF0h; sub esp, 20h; call $+5; pop ebx
bytes = new byte[]
{ 0x55, 0x89, 0xE5, 0x53, 0x83, 0xE4, 0xF0, 0x83, 0xEC, 0x20, 0xE8, 0x00, 0x00, 0x00, 0x00, 0x5B };
image.Position = loc;
buff = image.ReadBytes(16);
if (!bytes.SequenceEqual(buff))
{
return(0, 0);
}
// lea eax, (pCgr - offset)[ebx] (Position + 6 is the opcode lea eax; Position + 8 is the operand)
image.Position += 6;
// Ensure it's lea eax, #address
if (image.ReadUInt16() != 0x838D)
{
return(0, 0);
}
try {
pCgr = image.MapVATR(image.ReadUInt32() + plt);
}
// Could not find a mapping in the section table
catch (InvalidOperationException) {
return(0, 0);
}
// Extract Metadata pointer
// An 0x838D opcode indicates LEA (no indirection)
image.Position = pCgr + 0x20;
var opcode = image.ReadUInt16();
metadata = image.ReadUInt32() + plt;
// An 8x838B opcode indicates MOV (pointer indirection)
if (opcode == 0x838B)
{
image.Position = image.MapVATR(metadata);
metadata = image.ReadUInt32();
}
if (opcode != 0x838B && opcode != 0x838D)
{
return(0, 0);
}
// Repeat the same logic for extracting the Code pointer
image.Position = pCgr + 0x2A;
opcode = image.ReadUInt16();
code = image.ReadUInt32() + plt;
if (opcode == 0x838B)
{
image.Position = image.MapVATR(code);
code = image.ReadUInt32();
}
if (opcode != 0x838B && opcode != 0x838D)
{
return(0, 0);
}
return(code, metadata);
}
return(0, 0);
}