bpo-40958: Avoid buffer overflow in the parser when indexing the current line (GH-20875)

Misc/NEWS.d/next/Core and Builtins/2020-06-15-01-20-44.bpo-40958.7O2Wh1.rst

@@ -0,0 +1,2 @@
+Fix a possible buffer overflow in the PEG parser when gathering information
+for emitting syntax errors. Patch by Pablo Galindo.

Parser/pegen.c

@@ -140,21 +140,18 @@ _create_dummy_identifier(Parser *p)
 }
 
 static inline Py_ssize_t
-byte_offset_to_character_offset(PyObject *line, int col_offset)
+byte_offset_to_character_offset(PyObject *line, Py_ssize_t col_offset)
 {
     const char *str = PyUnicode_AsUTF8(line);
     if (!str) {
         return 0;
     }
+    assert(col_offset >= 0 && (unsigned long)col_offset <= strlen(str));
     PyObject *text = PyUnicode_DecodeUTF8(str, col_offset, "replace");
     if (!text) {
         return 0;
     }
     Py_ssize_t size = PyUnicode_GET_LENGTH(text);
-    str = PyUnicode_AsUTF8(text);
-    if (str != NULL && (int)strlen(str) == col_offset) {
-        size = strlen(str);
-    }
     Py_DECREF(text);
     return size;
 }
@@ -366,7 +363,7 @@ void *
 _PyPegen_raise_error(Parser *p, PyObject *errtype, const char *errmsg, ...)
 {
     Token *t = p->known_err_token != NULL ? p->known_err_token : p->tokens[p->fill - 1];
-    int col_offset;
+    Py_ssize_t col_offset;
     if (t->col_offset == -1) {
         col_offset = Py_SAFE_DOWNCAST(p->tok->cur - p->tok->buf,
                                       intptr_t, int);
@@ -386,7 +383,7 @@ _PyPegen_raise_error(Parser *p, PyObject *errtype, const char *errmsg, ...)
 
 void *
 _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
-                                    int lineno, int col_offset,
+                                    Py_ssize_t lineno, Py_ssize_t col_offset,
                                     const char *errmsg, va_list va)
 {
     PyObject *value = NULL;
@@ -406,16 +403,17 @@ _PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
 
     if (!error_line) {
         Py_ssize_t size = p->tok->inp - p->tok->buf;
-        if (size && p->tok->buf[size-1] == '\n') {
-            size--;
-        }
         error_line = PyUnicode_DecodeUTF8(p->tok->buf, size, "replace");
         if (!error_line) {
             goto error;
         }
     }
 
-    Py_ssize_t col_number = byte_offset_to_character_offset(error_line, col_offset);
+    Py_ssize_t col_number = col_offset;
+
+    if (p->tok->encoding != NULL) {
+        col_number = byte_offset_to_character_offset(error_line, col_offset);
+    }
 
     tmp = Py_BuildValue("(OiiN)", p->tok->filename, lineno, col_number, error_line);
     if (!tmp) {

Parser/pegen.h

@@ -34,7 +34,7 @@ typedef struct _memo {
 typedef struct {
     int type;
     PyObject *bytes;
-    int lineno, col_offset, end_lineno, end_col_offset;
+    Py_ssize_t lineno, col_offset, end_lineno, end_col_offset;
     Memo *memo;
 } Token;
 
@@ -132,7 +132,7 @@ void *_PyPegen_string_token(Parser *p);
 const char *_PyPegen_get_expr_name(expr_ty);
 void *_PyPegen_raise_error(Parser *p, PyObject *errtype, const char *errmsg, ...);
 void *_PyPegen_raise_error_known_location(Parser *p, PyObject *errtype,
-                                          int lineno, int col_offset,
+                                          Py_ssize_t lineno, Py_ssize_t col_offset,
                                           const char *errmsg, va_list va);
 void *_PyPegen_dummy_name(Parser *p, ...);