gh-133767: Fix use-after-free in the unicode-escape decoder with an error handler

Include/internal/pycore_bytesobject.h

@@ -20,8 +20,9 @@ extern PyObject* _PyBytes_FromHex(
 
 // Helper for PyBytes_DecodeEscape that detects invalid escape chars.
 // Export for test_peg_generator.
-PyAPI_FUNC(PyObject*) _PyBytes_DecodeEscape(const char *, Py_ssize_t,
-                                            const char *, const char **);
+PyAPI_FUNC(PyObject*) _PyBytes_DecodeEscape2(const char *, Py_ssize_t,
+                                             const char *,
+                                             int *, const char **);
 
 
 // Substring Search.

Include/internal/pycore_unicodeobject.h

@@ -139,14 +139,18 @@ extern PyObject* _PyUnicode_DecodeUnicodeEscapeStateful(
 // Helper for PyUnicode_DecodeUnicodeEscape that detects invalid escape
 // chars.
 // Export for test_peg_generator.
-PyAPI_FUNC(PyObject*) _PyUnicode_DecodeUnicodeEscapeInternal(
+PyAPI_FUNC(PyObject*) _PyUnicode_DecodeUnicodeEscapeInternal2(
     const char *string,     /* Unicode-Escape encoded string */
     Py_ssize_t length,      /* size of string */
     const char *errors,     /* error handling */
     Py_ssize_t *consumed,   /* bytes consumed */
-    const char **first_invalid_escape); /* on return, points to first
-                                           invalid escaped char in
-                                           string. */
+    int *first_invalid_escape_char, /* on return, if not -1, contain the first
+                                       invalid escaped char (<= 0xff) or invalid
+                                       octal escape (> 0xff) in string. */
+    const char **first_invalid_escape_ptr); /* on return, if not NULL, may
+                                        point to the first invalid escaped
+                                        char in string.
+                                        May be NULL if errors is not NULL. */
 
 /* --- Raw-Unicode-Escape Codecs ---------------------------------------------- */
 

Lib/test/test_codeccallbacks.py

@@ -2,6 +2,7 @@
 import codecs
 import html.entities
 import itertools
+import re
 import sys
 import unicodedata
 import unittest
@@ -1125,7 +1126,7 @@ def test_bug828737(self):
             text = 'abc<def>ghi'*n
             text.translate(charmap)
 
-    def test_mutatingdecodehandler(self):
+    def test_mutating_decode_handler(self):
         baddata = [
             ("ascii", b"\xff"),
             ("utf-7", b"++"),
@@ -1160,6 +1161,42 @@ def mutating(exc):
         for (encoding, data) in baddata:
             self.assertEqual(data.decode(encoding, "test.mutating"), "\u4242")
 
+    def test_mutating_decode_handler_unicode_escape(self):
+        decode = codecs.unicode_escape_decode
+        def mutating(exc):
+            if isinstance(exc, UnicodeDecodeError):
+                r = data.get(exc.object[:exc.end])
+                if r is not None:
+                    exc.object = r[0] + exc.object[exc.end:]
+                    return ('\u0404', r[1])
+            raise AssertionError("don't know how to handle %r" % exc)
+
+        codecs.register_error('test.mutating2', mutating)
+        data = {
+            br'\x0': (b'\\', 0),
+            br'\x3': (b'xxx\\', 3),
+            br'\x5': (b'x\\', 1),
+        }
+        def check(input, expected, msg):
+            with self.assertWarns(DeprecationWarning) as cm:
+                self.assertEqual(decode(input, 'test.mutating2'), (expected, len(input)))
+            self.assertIn(msg, str(cm.warning))
+
+        check(br'\x0n\z', '\u0404\n\\z', r'"\z" is an invalid escape sequence')
+        check(br'\x0n\501', '\u0404\n\u0141', r'"\501" is an invalid octal escape sequence')
+        check(br'\x0z', '\u0404\\z', r'"\z" is an invalid escape sequence')
+
+        check(br'\x3n\zr', '\u0404\n\\zr', r'"\z" is an invalid escape sequence')
+        check(br'\x3zr', '\u0404\\zr', r'"\z" is an invalid escape sequence')
+        check(br'\x3z5', '\u0404\\z5', r'"\z" is an invalid escape sequence')
+        check(memoryview(br'\x3z5x')[:-1], '\u0404\\z5', r'"\z" is an invalid escape sequence')
+        check(memoryview(br'\x3z5xy')[:-2], '\u0404\\z5', r'"\z" is an invalid escape sequence')
+
+        check(br'\x5n\z', '\u0404\n\\z', r'"\z" is an invalid escape sequence')
+        check(br'\x5n\501', '\u0404\n\u0141', r'"\501" is an invalid octal escape sequence')
+        check(br'\x5z', '\u0404\\z', r'"\z" is an invalid escape sequence')
+        check(memoryview(br'\x5zy')[:-1], '\u0404\\z', r'"\z" is an invalid escape sequence')
+
     # issue32583
     def test_crashing_decode_handler(self):
         # better generating one more character to fill the extra space slot

Lib/test/test_codecs.py

@@ -1196,23 +1196,39 @@ def test_escape(self):
         check(br"[\1010]", b"[A0]")
         check(br"[\x41]", b"[A]")
         check(br"[\x410]", b"[A0]")
+
+    def test_warnings(self):
+        decode = codecs.escape_decode
+        check = coding_checker(self, decode)
         for i in range(97, 123):
             b = bytes([i])
             if b not in b'abfnrtvx':
-                with self.assertWarns(DeprecationWarning):
+                with self.assertWarnsRegex(DeprecationWarning,
+                        r'"\\%c" is an invalid escape sequence' % i):
                     check(b"\\" + b, b"\\" + b)
-            with self.assertWarns(DeprecationWarning):
+            with self.assertWarnsRegex(DeprecationWarning,
+                    r'"\\%c" is an invalid escape sequence' % (i-32)):
                 check(b"\\" + b.upper(), b"\\" + b.upper())
-        with self.assertWarns(DeprecationWarning):
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\8" is an invalid escape sequence'):
             check(br"\8", b"\\8")
         with self.assertWarns(DeprecationWarning):
             check(br"\9", b"\\9")
-        with self.assertWarns(DeprecationWarning):
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\\xfa" is an invalid escape sequence') as cm:
             check(b"\\\xfa", b"\\\xfa")
         for i in range(0o400, 0o1000):
-            with self.assertWarns(DeprecationWarning):
+            with self.assertWarnsRegex(DeprecationWarning,
+                    r'"\\%o" is an invalid octal escape sequence' % i):
                 check(rb'\%o' % i, bytes([i & 0o377]))
 
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\z" is an invalid escape sequence'):
+            self.assertEqual(decode(br'\x\z', 'ignore'), (b'\\z', 4))
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\501" is an invalid octal escape sequence'):
+            self.assertEqual(decode(br'\x\501', 'ignore'), (b'A', 6))
+
     def test_errors(self):
         decode = codecs.escape_decode
         self.assertRaises(ValueError, decode, br"\x")
@@ -2661,24 +2677,40 @@ def test_escape_decode(self):
         check(br"[\x410]", "[A0]")
         check(br"\u20ac", "\u20ac")
         check(br"\U0001d120", "\U0001d120")
+
+    def test_decode_warnings(self):
+        decode = codecs.unicode_escape_decode
+        check = coding_checker(self, decode)
         for i in range(97, 123):
             b = bytes([i])
             if b not in b'abfnrtuvx':
-                with self.assertWarns(DeprecationWarning):
+                with self.assertWarnsRegex(DeprecationWarning,
+                        r'"\\%c" is an invalid escape sequence' % i):
                     check(b"\\" + b, "\\" + chr(i))
             if b.upper() not in b'UN':
-                with self.assertWarns(DeprecationWarning):
+                with self.assertWarnsRegex(DeprecationWarning,
+                        r'"\\%c" is an invalid escape sequence' % (i-32)):
                     check(b"\\" + b.upper(), "\\" + chr(i-32))
-        with self.assertWarns(DeprecationWarning):
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\8" is an invalid escape sequence'):
             check(br"\8", "\\8")
         with self.assertWarns(DeprecationWarning):
             check(br"\9", "\\9")
-        with self.assertWarns(DeprecationWarning):
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\\xfa" is an invalid escape sequence') as cm:
             check(b"\\\xfa", "\\\xfa")
         for i in range(0o400, 0o1000):
-            with self.assertWarns(DeprecationWarning):
+            with self.assertWarnsRegex(DeprecationWarning,
+                    r'"\\%o" is an invalid octal escape sequence' % i):
                 check(rb'\%o' % i, chr(i))
 
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\z" is an invalid escape sequence'):
+            self.assertEqual(decode(br'\x\z', 'ignore'), ('\\z', 4))
+        with self.assertWarnsRegex(DeprecationWarning,
+                r'"\\501" is an invalid octal escape sequence'):
+            self.assertEqual(decode(br'\x\501', 'ignore'), ('\u0141', 6))
+
     def test_decode_errors(self):
         decode = codecs.unicode_escape_decode
         for c, d in (b'x', 2), (b'u', 4), (b'U', 4):

Misc/NEWS.d/next/Security/2025-05-09-20-22-54.gh-issue-133767.kN2i3Q.rst

@@ -0,0 +1,2 @@
+Fix use-after-free in the "unicode-escape" decoder with a non-"strict" error
+handler.

Objects/bytesobject.c

@@ -1075,10 +1075,11 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t format_len,
 }
 
 /* Unescape a backslash-escaped string. */
-PyObject *_PyBytes_DecodeEscape(const char *s,
+PyObject *_PyBytes_DecodeEscape2(const char *s,
                                 Py_ssize_t len,
                                 const char *errors,
-                                const char **first_invalid_escape)
+                                int *first_invalid_escape_char,
+                                const char **first_invalid_escape_ptr)
 {
     int c;
     char *p;
@@ -1092,7 +1093,8 @@ PyObject *_PyBytes_DecodeEscape(const char *s,
         return NULL;
     writer.overallocate = 1;
 
-    *first_invalid_escape = NULL;
+    *first_invalid_escape_char = -1;
+    *first_invalid_escape_ptr = NULL;
 
     end = s + len;
     while (s < end) {
@@ -1130,9 +1132,10 @@ PyObject *_PyBytes_DecodeEscape(const char *s,
                     c = (c<<3) + *s++ - '0';
             }
             if (c > 0377) {
-                if (*first_invalid_escape == NULL) {
-                    *first_invalid_escape = s-3; /* Back up 3 chars, since we've
-                                                    already incremented s. */
+                if (*first_invalid_escape_char == -1) {
+                    *first_invalid_escape_char = c;
+                    /* Back up 3 chars, since we've already incremented s. */
+                    *first_invalid_escape_ptr = s - 3;
                 }
             }
             *p++ = c;
@@ -1173,9 +1176,10 @@ PyObject *_PyBytes_DecodeEscape(const char *s,
             break;
 
         default:
-            if (*first_invalid_escape == NULL) {
-                *first_invalid_escape = s-1; /* Back up one char, since we've
-                                                already incremented s. */
+            if (*first_invalid_escape_char == -1) {
+                *first_invalid_escape_char = (unsigned char)s[-1];
+                /* Back up one char, since we've already incremented s. */
+                *first_invalid_escape_ptr = s - 1;
             }
             *p++ = '\\';
             s--;
@@ -1195,18 +1199,19 @@ PyObject *PyBytes_DecodeEscape(const char *s,
                                 Py_ssize_t Py_UNUSED(unicode),
                                 const char *Py_UNUSED(recode_encoding))
 {
-    const char* first_invalid_escape;
-    PyObject *result = _PyBytes_DecodeEscape(s, len, errors,
-                                             &first_invalid_escape);
+    int first_invalid_escape_char;
+    const char *first_invalid_escape_ptr;
+    PyObject *result = _PyBytes_DecodeEscape2(s, len, errors,
+                                             &first_invalid_escape_char,
+                                             &first_invalid_escape_ptr);
     if (result == NULL)
         return NULL;
-    if (first_invalid_escape != NULL) {
-        unsigned char c = *first_invalid_escape;
-        if ('4' <= c && c <= '7') {
+    if (first_invalid_escape_char != -1) {
+        if (first_invalid_escape_char > 0xff) {
             if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
-                                 "b\"\\%.3s\" is an invalid octal escape sequence. "
+                                 "b\"\\%o\" is an invalid octal escape sequence. "
                                  "Such sequences will not work in the future. ",
-                                 first_invalid_escape) < 0)
+                                 first_invalid_escape_char) < 0)
             {
                 Py_DECREF(result);
                 return NULL;
@@ -1216,7 +1221,7 @@ PyObject *PyBytes_DecodeEscape(const char *s,
             if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
                                  "b\"\\%c\" is an invalid escape sequence. "
                                  "Such sequences will not work in the future. ",
-                                 c) < 0)
+                                 first_invalid_escape_char) < 0)
             {
                 Py_DECREF(result);
                 return NULL;

Objects/unicodeobject.c

@@ -6621,21 +6621,24 @@ _PyUnicode_GetNameCAPI(void)
 /* --- Unicode Escape Codec ----------------------------------------------- */
 
 PyObject *
-_PyUnicode_DecodeUnicodeEscapeInternal(const char *s,
+_PyUnicode_DecodeUnicodeEscapeInternal2(const char *s,
                                Py_ssize_t size,
                                const char *errors,
                                Py_ssize_t *consumed,
-                               const char **first_invalid_escape)
+                               int *first_invalid_escape_char,
+                               const char **first_invalid_escape_ptr)
 {
     const char *starts = s;
+    const char *initial_starts = starts;
     _PyUnicodeWriter writer;
     const char *end;
     PyObject *errorHandler = NULL;
     PyObject *exc = NULL;
     _PyUnicode_Name_CAPI *ucnhash_capi;
 
     // so we can remember if we've seen an invalid escape char or not
-    *first_invalid_escape = NULL;
+    *first_invalid_escape_char = -1;
+    *first_invalid_escape_ptr = NULL;
 
     if (size == 0) {
         if (consumed) {
@@ -6723,9 +6726,12 @@ _PyUnicode_DecodeUnicodeEscapeInternal(const char *s,
                 }
             }
             if (ch > 0377) {
-                if (*first_invalid_escape == NULL) {
-                    *first_invalid_escape = s-3; /* Back up 3 chars, since we've
-                                                    already incremented s. */
+                if (*first_invalid_escape_char == -1) {
+                    *first_invalid_escape_char = ch;
+                    if (starts == initial_starts) {
+                        /* Back up 3 chars, since we've already incremented s. */
+                        *first_invalid_escape_ptr = s - 3;
+                    }
                 }
             }
             WRITE_CHAR(ch);
@@ -6820,9 +6826,12 @@ _PyUnicode_DecodeUnicodeEscapeInternal(const char *s,
             goto error;
 
         default:
-            if (*first_invalid_escape == NULL) {
-                *first_invalid_escape = s-1; /* Back up one char, since we've
-                                                already incremented s. */
+            if (*first_invalid_escape_char == -1) {
+                *first_invalid_escape_char = c;
+                if (starts == initial_starts) {
+                    /* Back up one char, since we've already incremented s. */
+                    *first_invalid_escape_ptr = s - 1;
+                }
             }
             WRITE_ASCII_CHAR('\\');
             WRITE_CHAR(c);
@@ -6867,19 +6876,20 @@ _PyUnicode_DecodeUnicodeEscapeStateful(const char *s,
                               const char *errors,
                               Py_ssize_t *consumed)
 {
-    const char *first_invalid_escape;
-    PyObject *result = _PyUnicode_DecodeUnicodeEscapeInternal(s, size, errors,
+    int first_invalid_escape_char;
+    const char *first_invalid_escape_ptr;
+    PyObject *result = _PyUnicode_DecodeUnicodeEscapeInternal2(s, size, errors,
                                                       consumed,
-                                                      &first_invalid_escape);
+                                                      &first_invalid_escape_char,
+                                                      &first_invalid_escape_ptr);
     if (result == NULL)
         return NULL;
-    if (first_invalid_escape != NULL) {
-        unsigned char c = *first_invalid_escape;
-        if ('4' <= c && c <= '7') {
+    if (first_invalid_escape_char != -1) {
+        if (first_invalid_escape_char > 0xff) {
             if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
-                                 "\"\\%.3s\" is an invalid octal escape sequence. "
+                                 "\"\\%o\" is an invalid octal escape sequence. "
                                  "Such sequences will not work in the future. ",
-                                 first_invalid_escape) < 0)
+                                 first_invalid_escape_char) < 0)
             {
                 Py_DECREF(result);
                 return NULL;
@@ -6889,7 +6899,7 @@ _PyUnicode_DecodeUnicodeEscapeStateful(const char *s,
             if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
                                  "\"\\%c\" is an invalid escape sequence. "
                                  "Such sequences will not work in the future. ",
-                                 c) < 0)
+                                 first_invalid_escape_char) < 0)
             {
                 Py_DECREF(result);
                 return NULL;