gh-148178: Validate remote debug offset tables on load

Treat the debug offset tables read from a target process as untrusted input
and validate them before the unwinder uses any reported sizes or offsets.

Add a shared validator in debug_offsets_validation.h and run it once when
_Py_DebugOffsets is loaded and once when AsyncioDebug is loaded. The checks
cover section sizes used for fixed local buffers and every offset that is
later dereferenced against a local buffer or local object view. This keeps
the bounds checks out of the sampling hot path while rejecting malformed
tables up front.

Author: pablogsal

Misc/NEWS.d/next/Security/2026-04-06-13-55-00.gh-issue-148178.Rs7kLm.rst

@@ -0,0 +1,2 @@
+Hardened :mod:`!_remote_debugging` by validating remote debug offset tables
+before using them to size memory reads or interpret remote layouts.

Modules/_remote_debugging/_remote_debugging.h

@@ -417,6 +417,7 @@ extern void cached_code_metadata_destroy(void *ptr);
 /* Validation */
 extern int is_prerelease_version(uint64_t version);
 extern int validate_debug_offsets(struct _Py_DebugOffsets *debug_offsets);
+#define PY_REMOTE_DEBUG_INVALID_ASYNC_DEBUG_OFFSETS (-2)
 
 /* ============================================================================
  * MEMORY READING FUNCTION DECLARATIONS

Modules/_remote_debugging/asyncio.c

@@ -6,6 +6,7 @@
  ******************************************************************************/
 
 #include "_remote_debugging.h"
+#include "debug_offsets_validation.h"
 
 /* ============================================================================
  * ASYNCIO DEBUG ADDRESS FUNCTIONS
@@ -71,8 +72,13 @@ read_async_debug(RemoteUnwinderObject *unwinder)
     int result = _Py_RemoteDebug_PagedReadRemoteMemory(&unwinder->handle, async_debug_addr, size, &unwinder->async_debug_offsets);
     if (result < 0) {
         set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to read AsyncioDebug offsets");
+        return result;
     }
-    return result;
+    if (_PyRemoteDebug_ValidateAsyncDebugOffsets(&unwinder->async_debug_offsets) < 0) {
+        set_exception_cause(unwinder, PyExc_RuntimeError, "Invalid AsyncioDebug offsets");
+        return PY_REMOTE_DEBUG_INVALID_ASYNC_DEBUG_OFFSETS;
+    }
+    return 0;
 }
 
 int
@@ -85,7 +91,11 @@ ensure_async_debug_offsets(RemoteUnwinderObject *unwinder)
 
     // Try to load async debug offsets (the target process may have
     // loaded asyncio since we last checked)
-    if (read_async_debug(unwinder) < 0) {
+    int result = read_async_debug(unwinder);
+    if (result == PY_REMOTE_DEBUG_INVALID_ASYNC_DEBUG_OFFSETS) {
+        return -1;
+    }
+    if (result < 0) {
         PyErr_Clear();
         PyErr_SetString(PyExc_RuntimeError, "AsyncioDebug section not available");
         set_exception_cause(unwinder, PyExc_RuntimeError,