commit 30e41798f40c684be57d7ccfebf5c6ad94c0ff97
Author: Petr Viktorin <pviktori@redhat.com>
Date:   Wed Jan 20 15:21:43 2021 +0100

    CVE-2021-3177: Replace snprintf with Python unicode formatting in ctypes param reprs
    
    Backport of Python3 commit 916610ef90a0d0761f08747f7b0905541f0977c7:
    https://bugs.python.org/issue42938
    https://github.com/python/cpython/pull/24239

diff --git a/Lib/ctypes/test/test_parameters.py b/Lib/ctypes/test/test_parameters.py
index 23c1b6e2259..77300d71ae1 100644
--- a/Lib/ctypes/test/test_parameters.py
+++ b/Lib/ctypes/test/test_parameters.py
@@ -206,6 +206,49 @@ class SimpleTypesTestCase(unittest.TestCase):
         with self.assertRaises(ZeroDivisionError):
             WorseStruct().__setstate__({}, b'foo')
 
+    def test_parameter_repr(self):
+        from ctypes import (
+            c_bool,
+            c_char,
+            c_wchar,
+            c_byte,
+            c_ubyte,
+            c_short,
+            c_ushort,
+            c_int,
+            c_uint,
+            c_long,
+            c_ulong,
+            c_longlong,
+            c_ulonglong,
+            c_float,
+            c_double,
+            c_longdouble,
+            c_char_p,
+            c_wchar_p,
+            c_void_p,
+        )
+        self.assertRegexpMatches(repr(c_bool.from_param(True)), r"^<cparam '\?' at 0x[A-Fa-f0-9]+>$")
+        self.assertEqual(repr(c_char.from_param('a')), "<cparam 'c' ('a')>")
+        self.assertRegexpMatches(repr(c_wchar.from_param('a')), r"^<cparam 'u' at 0x[A-Fa-f0-9]+>$")
+        self.assertEqual(repr(c_byte.from_param(98)), "<cparam 'b' (98)>")
+        self.assertEqual(repr(c_ubyte.from_param(98)), "<cparam 'B' (98)>")
+        self.assertEqual(repr(c_short.from_param(511)), "<cparam 'h' (511)>")
+        self.assertEqual(repr(c_ushort.from_param(511)), "<cparam 'H' (511)>")
+        self.assertRegexpMatches(repr(c_int.from_param(20000)), r"^<cparam '[li]' \(20000\)>$")
+        self.assertRegexpMatches(repr(c_uint.from_param(20000)), r"^<cparam '[LI]' \(20000\)>$")
+        self.assertRegexpMatches(repr(c_long.from_param(20000)), r"^<cparam '[li]' \(20000\)>$")
+        self.assertRegexpMatches(repr(c_ulong.from_param(20000)), r"^<cparam '[LI]' \(20000\)>$")
+        self.assertRegexpMatches(repr(c_longlong.from_param(20000)), r"^<cparam '[liq]' \(20000\)>$")
+        self.assertRegexpMatches(repr(c_ulonglong.from_param(20000)), r"^<cparam '[LIQ]' \(20000\)>$")
+        self.assertEqual(repr(c_float.from_param(1.5)), "<cparam 'f' (1.5)>")
+        self.assertEqual(repr(c_double.from_param(1.5)), "<cparam 'd' (1.5)>")
+        self.assertEqual(repr(c_double.from_param(1e300)), "<cparam 'd' (1e+300)>")
+        self.assertRegexpMatches(repr(c_longdouble.from_param(1.5)), r"^<cparam ('d' \(1.5\)|'g' at 0x[A-Fa-f0-9]+)>$")
+        self.assertRegexpMatches(repr(c_char_p.from_param(b'hihi')), "^<cparam 'z' \(0x[A-Fa-f0-9]+\)>$")
+        self.assertRegexpMatches(repr(c_wchar_p.from_param('hihi')), "^<cparam 'Z' \(0x[A-Fa-f0-9]+\)>$")
+        self.assertRegexpMatches(repr(c_void_p.from_param(0x12)), r"^<cparam 'P' \(0x0*12\)>$")
+
 ################################################################
 
 if __name__ == '__main__':
diff --git a/Misc/NEWS.d/next/Security/2021-01-18-09-27-31.bpo-42938.4Zn4Mp.rst b/Misc/NEWS.d/next/Security/2021-01-18-09-27-31.bpo-42938.4Zn4Mp.rst
new file mode 100644
index 00000000000..7df65a156fe
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2021-01-18-09-27-31.bpo-42938.4Zn4Mp.rst
@@ -0,0 +1,2 @@
+Avoid static buffers when computing the repr of :class:`ctypes.c_double` and
+:class:`ctypes.c_longdouble` values.
diff --git a/Modules/_ctypes/callproc.c b/Modules/_ctypes/callproc.c
index 066fefc0cca..5cc3c4cf685 100644
--- a/Modules/_ctypes/callproc.c
+++ b/Modules/_ctypes/callproc.c
@@ -460,50 +460,62 @@ PyCArg_dealloc(PyCArgObject *self)
 static PyObject *
 PyCArg_repr(PyCArgObject *self)
 {
-    char buffer[256];
     switch(self->tag) {
     case 'b':
     case 'B':
-        sprintf(buffer, "<cparam '%c' (%d)>",
+        return PyString_FromFormat("<cparam '%c' (%d)>",
             self->tag, self->value.b);
-        break;
     case 'h':
     case 'H':
-        sprintf(buffer, "<cparam '%c' (%d)>",
+        return PyString_FromFormat("<cparam '%c' (%d)>",
             self->tag, self->value.h);
-        break;
     case 'i':
     case 'I':
-        sprintf(buffer, "<cparam '%c' (%d)>",
+        return PyString_FromFormat("<cparam '%c' (%d)>",
             self->tag, self->value.i);
-        break;
     case 'l':
     case 'L':
-        sprintf(buffer, "<cparam '%c' (%ld)>",
+        return PyString_FromFormat("<cparam '%c' (%ld)>",
             self->tag, self->value.l);
-        break;
 
 #ifdef HAVE_LONG_LONG
     case 'q':
     case 'Q':
-        sprintf(buffer,
-            "<cparam '%c' (%" PY_FORMAT_LONG_LONG "d)>",
+        return PyString_FromFormat("<cparam '%c' (%lld)>",
             self->tag, self->value.q);
-        break;
 #endif
     case 'd':
-        sprintf(buffer, "<cparam '%c' (%f)>",
-            self->tag, self->value.d);
-        break;
-    case 'f':
-        sprintf(buffer, "<cparam '%c' (%f)>",
-            self->tag, self->value.f);
-        break;
-
+    case 'f': {
+        PyObject *s = PyString_FromFormat("<cparam '%c' (", self->tag);
+        if (s == NULL) {
+            return NULL;
+        }
+        PyObject *f = PyFloat_FromDouble((self->tag == 'f') ? self->value.f : self->value.d);
+        if (f == NULL) {
+            Py_DECREF(s);
+            return NULL;
+        }
+        PyObject *r = PyObject_Repr(f);
+        Py_DECREF(f);
+        if (r == NULL) {
+            Py_DECREF(s);
+            return NULL;
+        }
+        PyString_ConcatAndDel(&s, r);
+        if (s == NULL) {
+            return NULL;
+        }
+        r = PyString_FromString(")>");
+        if (r == NULL) {
+            Py_DECREF(s);
+            return NULL;
+        }
+        PyString_ConcatAndDel(&s, r);
+        return s;
+    }
     case 'c':
-        sprintf(buffer, "<cparam '%c' (%c)>",
+        return PyString_FromFormat("<cparam '%c' ('%c')>",
             self->tag, self->value.c);
-        break;
 
 /* Hm, are these 'z' and 'Z' codes useful at all?
    Shouldn't they be replaced by the functionality of c_string
@@ -512,16 +524,13 @@ PyCArg_repr(PyCArgObject *self)
     case 'z':
     case 'Z':
     case 'P':
-        sprintf(buffer, "<cparam '%c' (%p)>",
+        return PyUnicode_FromFormat("<cparam '%c' (%p)>",
             self->tag, self->value.p);
-        break;
 
     default:
-        sprintf(buffer, "<cparam '%c' at %p>",
-            self->tag, self);
-        break;
+        return PyString_FromFormat("<cparam '%c' at %p>",
+            (unsigned char)self->tag, (void *)self);
     }
-    return PyString_FromString(buffer);
 }
 
 static PyMemberDef PyCArgType_members[] = {