Deobfuscate Indirect Branches

Part 1: CSEL + BR Indirect Branch Pattern

Pattern Recognition

The obfuscator converts conditional branches into indirect jumps, where CSEL and BR instructions appear in pairs. CSEL selects one of two target addresses based on a condition, followed by some junk instructions, then BR jumps to the selected address.

Typical instruction sequence:

CMP             W10, W11
CSEL            W10, W13, W12, LS   ; select W13 or W12 based on LS condition
......                               ; junk code in between (address calculations, etc.)
ADD             X8, X8, X14
BR              X8                   ; indirect jump to the computed target

Key characteristics:

• CSEL selects one of two register values, representing two branch targets
• Code between CSEL and BR is address calculation or junk code
• BR performs the final indirect jump

Analysis Approach

Symbolic execution: when encountering a CSEL instruction, force different branch selections to obtain two different BR target addresses.

Steps:

1. Start two symbolic execution runs from the same basic block entry
2. First run: force CSEL to select the first register (condition-true branch)
3. Second run: force CSEL to select the second register (condition-false branch)
4. Each run reaches BR and yields a different target address (A and B)

Traversal Strategy

BFS traversal starting from the function entry block:

1. Add the function entry address to the work queue
2. Dequeue an address, run symbolic execution with both CSEL selections until BR
3. Record patch info (CSEL address, condition, two target addresses)
4. Add both target addresses to the work queue
5. Use a visited set to prevent revisiting
6. Mark blocks ending with RET as return blocks — do not continue from them

Patching Strategy

Code between CSEL and BR is junk — patch a conditional branch directly at the CSEL location.

For example, CSEL W11, W8, W9, CC: W8 is the target for the CC-true branch (A), W9 is the target for the CC-false branch (B).

Patch as:

BCC  A    ; if condition met, jump to A
B    B    ; otherwise jump to B

Two instructions = 8 bytes, overwriting the CSEL (4 bytes) and the next junk instruction (4 bytes).

Part 2: CSET + BR Indirect Branch Pattern (Jump Table Variant)

Pattern Recognition

Unlike the CSEL variant in Part 1, the CSET variant uses a 0/1 index to look up a jump table for computing the target address. The code between CSET and BR is NOT all junk — it contains useful instructions that subsequent basic blocks depend on.

Typical instruction sequence:

CMP             X27, X8
CSET            W8, EQ              ; W8 = 0 or 1 (index)
STR             W8, [SP, #offset]   ; store index (junk)
LDR             X9, [SP, #tbl_off]  ; load jump table pointer (junk)
LDR             X8, [X9, W8, UXTW#3] ; table[index] (junk)
ADRP            X9, #page           ; load encrypted constant (junk)
LDR             W9, [X9, #off]      ;   (junk)
MOV             W10, #imm           ; XOR key (junk)
MOVK            W10, #imm, LSL#16   ;   (junk)
EOR             W9, W9, W10         ; decrypt offset (junk)
NEG             W9, W9              ; negate (junk)
ADD             X8, X8, W9, SXTW    ; final target address (junk, boundary)
; --- useful code below ---
ADRP            X25, #0x100004000   ; register setup for successor blocks
ADD             X25, X25, #0x250
MOV             W28, #0xF065        ; constant init
MOVK            W28, #0x611A, LSL#16
LDR             X23, [SP, #0x50]    ; load state for successor
BR              X8                   ; indirect jump (junk)

Junk Code Identification

ADD Xn, Xn, Wm, SXTW is the boundary between junk and useful code. Everything from CSET to ADD (inclusive) is junk:

Analysis Approach

Same as the CSEL variant: symbolic execution forces CSET to take 1 and 0 respectively, runs until BR to obtain two target addresses.

One difference: a single basic block may contain multiple CSEL/CSET instructions (e.g., a data-selection CSEL followed by a branch-controlling CSET). The script forces selection on every CSEL/CSET encountered; the recorded csel_addr is the last one (the one that controls the BR target).

Patching Strategy

Cannot patch directly at the CSET location (would skip useful code). Correct approach:

1. Locate ADD Xn, Xn, Wm, SXTW (the boundary)
2. Extract useful code bytes between ADD+4 and BR
3. Move useful code up to the CSET location
4. Append Bcond A; B B immediately after

Before:  [CSET][junk...][ADD][useful code][BR]
After:   [useful code][Bcond A][B B][... dead code ...]

Prerequisite: useful code must be position-independent (SP-relative addressing, immediate assignments, or same-page ADRP). Moving ADRP within the same 4KB page requires no immediate adjustment.

Reference Implementation

Two script variants for different obfuscation sub-patterns:

script/deinbr-v3-csel.py — CSEL variant (ELF)

For binaries where CSEL directly selects between two target addresses and the code between CSEL and BR is pure address calculation junk. Patches 8 bytes at the CSEL location (Bcond + B), overwriting CSEL and the next junk instruction.

script/deinbr-v3-cset.py — CSET variant (Mach-O / ELF)

For binaries where CSET sets a 0/1 index and the code between CSET and BR contains useful side-effect instructions (register setup, stack stores for subsequent blocks) interleaved with address calculation junk.

Patching strategy: find ADD Xn, Xn, Wm, SXTW (last address calculation step), move the useful code (between ADD and BR) up to the CSET location, then append Bcond + B. This preserves register/memory state that successor blocks depend on. The moved instructions must be position-independent (SP-relative, immediates, or same-page ADRP).

Both scripts share the same core workflow:

1. analyze_br(proj, func_start) — BFS traversal, collects all patch points
2. run_until_br(proj, state, csel_selector) — execute from a given state until BR, forcing CSEL/CSET selection
3. do_patch(binary_path, save_path, proj, patch_list) — assemble patches with keystone and write to binary

Key angr options:

• CALLLESS — ignore function calls to prevent analysis divergence
• LAZY_SOLVES — defer constraint solving for performance
• ZERO_FILL_UNCONSTRAINED_MEMORY — fill unconstrained memory with zeros
• ZERO_FILL_UNCONSTRAINED_REGISTERS — fill unconstrained registers with zeros

Script Execution Rules

All non-trivial Python code (both analysis scripts and final patching scripts) MUST be written to a file and executed via python script.py. Never use python -c "..." inline in bash. This avoids shell quoting/escaping issues and makes scripts easier to debug and reuse.

Common Errors and Debugging

BR target is an invalid address

block: 41fe20, next_1: <SimState @ 0x2908f8c3>, next_2: <SimState @ 0x2908f8c3>
SimEngineError: No bytes in memory for block starting at 0x2908f8c3.

Cause: The user-provided address is not the function entry but an internal basic block. Stack-based jump table base addresses and offset constants have not been initialized (filled with zeros by ZERO_FILL), causing BR to compute an invalid target.

Solution: Confirm the address is the function start. Use the symbol table or IDA/Ghidra to find the function entry:

for sym in proj.loader.main_object.symbols:
    if sym.rebased_addr <= target_addr < sym.rebased_addr + sym.size:
        print(sym.name, hex(sym.rebased_addr))

Both branch targets are identical

block: XXXXX, next_1: <SimState @ 0xABCD>, next_2: <SimState @ 0xABCD>

Same cause as above — starting execution from a non-entry point means the two registers selected by CSEL hold the same value (both zero or the same uninitialized value), producing the same BR target after address calculation.

Solution: Use the correct function entry address.

Encountering non-CSEL conditional select instructions

block XXXXX: expected 1 successor, got 0

Or the script reaches BR without detecting CSEL, so csel_addr is missing from globals.

Cause: The basic block uses CSET instead of CSEL. CSET is a special form of CSEL, equivalent to CSEL Rd, WZR, WZR, invert(cond), selecting 1 or 0 as an index.

Handle the same way as CSEL — identify the condition and force both branch selections. The script must handle both csel and cset:

if insn.mnemonic == 'cset':
    # cset Wd, cond  is equivalent to  csel Wd, #1, #0, cond (condition met=1, not met=0)
    dst, cond = parse_cset(insn)
    val = 1 if csel_selector == 1 else 0
    setattr(state.regs, dst, val)
    ...