gh-139283: correctly handle size limit in cursor.fetchmany() (#139296)

Passing a negative or zero size to `cursor.fetchmany()` made it fetch all rows
instead of none.

While this could be considered a security vulnerability, it was decided to treat
this issue as a regular bug as passing a non-sanitized *size* value in the first
place is not recommended.

Author: picnixz

Doc/library/sqlite3.rst

@@ -1611,6 +1611,9 @@ Cursor objects
       If the *size* parameter is used, then it is best for it to retain the same
       value from one :meth:`fetchmany` call to the next.
 
+      .. versionchanged:: next
+         Negative *size* values are rejected by raising :exc:`ValueError`.
+
    .. method:: fetchall()
 
       Return all (remaining) rows of a query result as a :class:`list`.
@@ -1638,6 +1641,9 @@ Cursor objects
       Read/write attribute that controls the number of rows returned by :meth:`fetchmany`.
       The default value is 1 which means a single row would be fetched per call.
 
+      .. versionchanged:: next
+         Negative values are rejected by raising :exc:`ValueError`.
+
    .. attribute:: connection
 
       Read-only attribute that provides the SQLite database :class:`Connection`

Lib/test/test_sqlite3/test_dbapi.py

@@ -21,6 +21,7 @@
 # 3. This notice may not be removed or altered from any source distribution.
 
 import contextlib
+import functools
 import os
 import sqlite3 as sqlite
 import subprocess
@@ -1060,7 +1061,7 @@ def test_array_size(self):
         # now set to 2
         self.cu.arraysize = 2
 
-        # now make the query return 3 rows
+        # now make the query return 2 rows from a table of 3 rows
         self.cu.execute("delete from test")
         self.cu.execute("insert into test(name) values ('A')")
         self.cu.execute("insert into test(name) values ('B')")
@@ -1070,13 +1071,50 @@ def test_array_size(self):
 
         self.assertEqual(len(res), 2)
 
+    def test_invalid_array_size(self):
+        UINT32_MAX = (1 << 32) - 1
+        setter = functools.partial(setattr, self.cu, 'arraysize')
+
+        self.assertRaises(TypeError, setter, 1.0)
+        self.assertRaises(ValueError, setter, -3)
+        self.assertRaises(OverflowError, setter, UINT32_MAX + 1)
+
     def test_fetchmany(self):
+        # no active SQL statement
+        res = self.cu.fetchmany()
+        self.assertEqual(res, [])
+        res = self.cu.fetchmany(1000)
+        self.assertEqual(res, [])
+
+        # test default parameter
+        self.cu.execute("select name from test")
+        res = self.cu.fetchmany()
+        self.assertEqual(len(res), 1)
+
+        # test when the number of requested rows exceeds the actual count
         self.cu.execute("select name from test")
         res = self.cu.fetchmany(100)
         self.assertEqual(len(res), 1)
         res = self.cu.fetchmany(100)
         self.assertEqual(res, [])
 
+        # test when size = 0
+        self.cu.execute("select name from test")
+        res = self.cu.fetchmany(0)
+        self.assertEqual(res, [])
+        res = self.cu.fetchmany(100)
+        self.assertEqual(len(res), 1)
+        res = self.cu.fetchmany(100)
+        self.assertEqual(res, [])
+
+    def test_invalid_fetchmany(self):
+        UINT32_MAX = (1 << 32) - 1
+        fetchmany = self.cu.fetchmany
+
+        self.assertRaises(TypeError, fetchmany, 1.0)
+        self.assertRaises(ValueError, fetchmany, -3)
+        self.assertRaises(OverflowError, fetchmany, UINT32_MAX + 1)
+
     def test_fetchmany_kw_arg(self):
         """Checks if fetchmany works with keyword arguments"""
         self.cu.execute("select name from test")

Misc/NEWS.d/next/Security/2025-09-24-13-39-56.gh-issue-139283.jODz_q.rst

@@ -0,0 +1,4 @@
+:mod:`sqlite3`: correctly handle maximum number of rows to fetch in
+:meth:`Cursor.fetchmany <sqlite3.Cursor.fetchmany>` and reject negative
+values for :attr:`Cursor.arraysize <sqlite3.Cursor.arraysize>`. Patch by
+Bénédikt Tran.

Modules/_sqlite/clinic/cursor.c.h

@@ -1159,35 +1159,31 @@ pysqlite_cursor_fetchone_impl(pysqlite_Cursor *self)
 /*[clinic input]
 _sqlite3.Cursor.fetchmany as pysqlite_cursor_fetchmany
 
-    size as maxrows: int(c_default='((pysqlite_Cursor *)self)->arraysize') = 1
+    size as maxrows: uint32(c_default='((pysqlite_Cursor *)self)->arraysize') = 1
         The default value is set by the Cursor.arraysize attribute.
 
 Fetches several rows from the resultset.
 [clinic start generated code]*/
 
 static PyObject *
-pysqlite_cursor_fetchmany_impl(pysqlite_Cursor *self, int maxrows)
-/*[clinic end generated code: output=a8ef31fea64d0906 input=035dbe44a1005bf2]*/
+pysqlite_cursor_fetchmany_impl(pysqlite_Cursor *self, uint32_t maxrows)
+/*[clinic end generated code: output=3325f2b477c71baf input=a509c412aa70b27e]*/
 {
     PyObject* row;
     PyObject* list;
-    int counter = 0;
 
     list = PyList_New(0);
     if (!list) {
         return NULL;
     }
 
-    while ((row = pysqlite_cursor_iternext((PyObject *)self))) {
-        if (PyList_Append(list, row) < 0) {
-            Py_DECREF(row);
-            break;
-        }
+    while (maxrows > 0 && (row = pysqlite_cursor_iternext((PyObject *)self))) {
+        int rc = PyList_Append(list, row);
         Py_DECREF(row);
-
-        if (++counter == maxrows) {
+        if (rc < 0) {
             break;
         }
+        maxrows--;
     }
 
     if (PyErr_Occurred()) {
@@ -1301,6 +1297,30 @@ pysqlite_cursor_close_impl(pysqlite_Cursor *self)
     Py_RETURN_NONE;
 }
 
+/*[clinic input]
+@getter
+_sqlite3.Cursor.arraysize
+[clinic start generated code]*/
+
+static PyObject *
+_sqlite3_Cursor_arraysize_get_impl(pysqlite_Cursor *self)
+/*[clinic end generated code: output=e0919d97175e6c50 input=3278f8d3ecbd90e3]*/
+{
+    return PyLong_FromUInt32(self->arraysize);
+}
+
+/*[clinic input]
+@setter
+_sqlite3.Cursor.arraysize
+[clinic start generated code]*/
+
+static int
+_sqlite3_Cursor_arraysize_set_impl(pysqlite_Cursor *self, PyObject *value)
+/*[clinic end generated code: output=af59a6b09f8cce6e input=ace48cb114e26060]*/
+{
+    return PyLong_AsUInt32(value, &self->arraysize);
+}
+
 static PyMethodDef cursor_methods[] = {
     PYSQLITE_CURSOR_CLOSE_METHODDEF
     PYSQLITE_CURSOR_EXECUTEMANY_METHODDEF
@@ -1318,14 +1338,18 @@ static struct PyMemberDef cursor_members[] =
 {
     {"connection", _Py_T_OBJECT, offsetof(pysqlite_Cursor, connection), Py_READONLY},
     {"description", _Py_T_OBJECT, offsetof(pysqlite_Cursor, description), Py_READONLY},
-    {"arraysize", Py_T_INT, offsetof(pysqlite_Cursor, arraysize), 0},
     {"lastrowid", _Py_T_OBJECT, offsetof(pysqlite_Cursor, lastrowid), Py_READONLY},
     {"rowcount", Py_T_LONG, offsetof(pysqlite_Cursor, rowcount), Py_READONLY},
     {"row_factory", _Py_T_OBJECT, offsetof(pysqlite_Cursor, row_factory), 0},
     {"__weaklistoffset__", Py_T_PYSSIZET, offsetof(pysqlite_Cursor, in_weakreflist), Py_READONLY},
     {NULL}
 };
 
+static struct PyGetSetDef cursor_getsets[] = {
+    _SQLITE3_CURSOR_ARRAYSIZE_GETSETDEF
+    {NULL},
+};
+
 static const char cursor_doc[] =
 PyDoc_STR("SQLite database cursor class.");
 
@@ -1336,6 +1360,7 @@ static PyType_Slot cursor_slots[] = {
     {Py_tp_iternext, pysqlite_cursor_iternext},
     {Py_tp_methods, cursor_methods},
     {Py_tp_members, cursor_members},
+    {Py_tp_getset, cursor_getsets},
     {Py_tp_init, pysqlite_cursor_init},
     {Py_tp_traverse, cursor_traverse},
     {Py_tp_clear, cursor_clear},

Modules/_sqlite/cursor.c

@@ -35,7 +35,7 @@ typedef struct
     pysqlite_Connection* connection;
     PyObject* description;
     PyObject* row_cast_map;
-    int arraysize;
+    uint32_t arraysize;
     PyObject* lastrowid;
     long rowcount;
     PyObject* row_factory;