Update patch_hook_cred_label_update_execve.md

research/kernel_patch_jb/patch_hook_cred_label_update_execve.md

@@ -1,6 +1,6 @@
 # C23 `patch_hook_cred_label_update_execve`
 
-## How the patch works
+## 1) How the Patch Works
 - Source: `scripts/patchers/kernel_jb_patch_hook_cred_label.py`.
 - Locator strategy:
   1. Resolve `vnode_getattr` (symbol or string-near function).
@@ -8,25 +8,71 @@
   3. Pick cred-label execve hook entry from early ops indices by function-size heuristic.
 - Patch action:
   - Inject cave shellcode that:
-    - builds inline `vfs_context`,
+    - builds inline `vfs_context` via `mrs tpidr_el1` (current_thread),
     - calls `vnode_getattr`,
     - propagates uid/gid into new credential,
-    - updates csflags,
+    - updates csflags with CS_VALID,
     - jumps back to original hook.
   - Rewrite selected ops-table pointer entry to cave target (preserving auth-rebase upper bits).
 
-## Expected outcome
+## 2) Expected Outcome
 - Interpose sandbox cred-label execve hook with custom ownership/credential propagation logic.
 
-## Target
-- One `mac_policy_ops` function-pointer entry plus injected hook shellcode cave.
+## 3) Target
+- Ops table: `mac_policy_ops` at `0xFFFFFE0007A58488` (discovered via mac_policy_conf)
+- Hook index: 18 (largest function in ops[0:29], 4208 bytes)
+  - Original hook: `sub_FFFFFE00093BDB64` (Sandbox `hook..execve()` handler)
+  - Contains: sandbox profile evaluation, container assignment, entitlement processing
+- One `mac_policy_ops` function-pointer entry + injected hook shellcode cave.
 
-## IDA MCP evidence
-- `vnode_getattr` string-related hit:
-  - xref `0xfffffe00084c08ec` -> function start `0xfffffe00084c0718`
-- Seatbelt policy anchor:
-  - `Seatbelt sandbox policy` @ `0xfffffe00075fd493`
-  - data xref @ `0xfffffe0007a58430` (policy config region)
+## 4) IDA MCP Evidence
 
-## Risk
-- Table-entry rewrite + shellcode interposition is invasive; incorrect index/cave selection can break sandbox hook dispatch.
+### Ops table structure
+- `mac_policy_conf` at `0xFFFFFE0007A58428`:
+  - +0: `0xFE00075FF33D` → "Sandbox" (mpc_name)
+  - +8: `0xFE00075FD493` → "Seatbelt sandbox policy" (mpc_fullname)
+  - +32: `0xFE0007A58488` → mpc_ops (ops table pointer)
+- Ops table entries (non-null in first 30):
+  - [6]: `0xFE00093BDB58` (12 bytes)
+  - [7]: `0xFE00093B0C04` (36 bytes)
+  - [11]: `0xFE00093B0B68` (156 bytes)
+  - [13]: `0xFE00093B0B5C` (12 bytes)
+  - [18]: `0xFE00093BDB64` (4208 bytes) ← **selected by size heuristic**
+  - [19]: `0xFE00093B0AE8` (116 bytes)
+  - [29]: `0xFE00093B0830` (696 bytes)
+
+### vnode_getattr
+- String-related hit: xref `0xFE00084C08EC` → function start `0xFE00084C0718`
+
+### Chained fixup format
+- Ops table entries use auth rebase (bit63=1):
+  - Upper 32 bits: diversity(16) + addrDiv(1) + key(2) + next(12) + auth(1)
+  - Lower 32 bits: target file offset
+- The rewrite preserves upper bits and replaces lower 32 with cave file offset.
+- Kernel loader re-signs the new target with the same PAC key/diversity → valid PAC pointer.
+
+## 5) Previous Bug (PANIC root cause)
+The code cave was allocated in `__PRELINK_TEXT` segment. While marked R-X in the
+Mach-O, this segment is **non-executable at runtime** on ARM64e due to
+KTRR (Kernel Text Read-only Region) enforcement. The cave ended up at a low
+file offset (e.g. 0x5440) in __PRELINK_TEXT padding, which at runtime maps to a
+non-executable page.
+
+**Panic**: "Kernel instruction fetch abort at pc 0xfffffe004761d440"
+- The 0x47 in the upper nibble (instead of expected 0x07) indicates PAC poisoning:
+  the CPU attempted to branch through the ops table pointer, PAC auth succeeded,
+  but the target address was in a non-executable region → instruction fetch fault.
+
+## 6) Fix Applied
+- Modified `_find_code_cave()` in `kernel_jb_base.py` to only search `__TEXT_EXEC`
+  and `__TEXT_BOOT_EXEC` segments.
+- `__PRELINK_TEXT` is now explicitly excluded since KTRR makes it non-executable
+  at runtime despite the Mach-O R-X flags.
+- Verified __TEXT_EXEC has sufficient padding at segment end (0x1A10+ bytes of zeros
+  at `0xFE00095C25F0`) for the 180-byte shellcode.
+
+## 7) Risk Assessment
+- **Medium-High**: Table-entry rewrite + shellcode interposition is invasive.
+  Incorrect index selection or shellcode bugs can break sandbox hook dispatch.
+- Mitigated by: dynamic function-size heuristic for index detection, PAC-aware
+  chained fixup pointer rewrite, and __TEXT_EXEC-restricted code cave.