From 86998f2ba61c4aef489dc7cb8e0c0e8e07f0396f Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 29 Jan 2024 12:17:12 +0200
Subject: [PATCH] gh-114685: Check flags in PyObject_GetBuffer()

PyObject_GetBuffer() now raises a SystemError if called with
PyBUF_READ or PyBUF_WRITE as flags. These flags should
only be used with the PyMemoryView_* C API.
---
 Lib/test/test_buffer.py                                     | 6 ++++++
 .../C API/2024-01-29-12-13-24.gh-issue-114685.B07RME.rst    | 3 +++
 Modules/_testcapi/buffer.c                                  | 6 ++++--
 Objects/abstract.c                                          | 6 ++++++
 4 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/C API/2024-01-29-12-13-24.gh-issue-114685.B07RME.rst

diff --git a/Lib/test/test_buffer.py b/Lib/test/test_buffer.py
index 72a06d6af450e3..535b795f508a24 100644
--- a/Lib/test/test_buffer.py
+++ b/Lib/test/test_buffer.py
@@ -4585,6 +4585,12 @@ def test_c_buffer(self):
             buf.__release_buffer__(mv)
         self.assertEqual(buf.references, 0)
 
+    @unittest.skipIf(_testcapi is None, "requires _testcapi")
+    def test_c_buffer_invalid_flags(self):
+        buf = _testcapi.testBuf()
+        self.assertRaises(SystemError, buf.__buffer__, PyBUF_READ)
+        self.assertRaises(SystemError, buf.__buffer__, PyBUF_WRITE)
+
     def test_inheritance(self):
         class A(bytearray):
             def __buffer__(self, flags):
diff --git a/Misc/NEWS.d/next/C API/2024-01-29-12-13-24.gh-issue-114685.B07RME.rst b/Misc/NEWS.d/next/C API/2024-01-29-12-13-24.gh-issue-114685.B07RME.rst
new file mode 100644
index 00000000000000..55b02d1d8e1e9f
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2024-01-29-12-13-24.gh-issue-114685.B07RME.rst	
@@ -0,0 +1,3 @@
+:c:func:`PyObject_GetBuffer` now raises a :exc:`SystemError` if called with
+:c:macro:`PyBUF_READ` or :c:macro:`PyBUF_WRITE` as flags. These flags should
+only be used with the ``PyMemoryView_*`` C API.
diff --git a/Modules/_testcapi/buffer.c b/Modules/_testcapi/buffer.c
index 942774156c6c47..7e2f6e5e29482c 100644
--- a/Modules/_testcapi/buffer.c
+++ b/Modules/_testcapi/buffer.c
@@ -54,8 +54,10 @@ static int
 testbuf_getbuf(testBufObject *self, Py_buffer *view, int flags)
 {
     int buf = PyObject_GetBuffer(self->obj, view, flags);
-    Py_SETREF(view->obj, Py_NewRef(self));
-    self->references++;
+    if (buf == 0) {
+        Py_SETREF(view->obj, Py_NewRef(self));
+        self->references++;
+    }
     return buf;
 }
 
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 1ec5c5b8c3dc2f..daf04eb4ab2cda 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -425,6 +425,12 @@ PyObject_AsWriteBuffer(PyObject *obj,
 int
 PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)
 {
+    if (flags != PyBUF_SIMPLE) {  /* fast path */
+        if (flags == PyBUF_READ || flags == PyBUF_WRITE) {
+            PyErr_BadInternalCall();
+            return -1;
+        }
+    }
     PyBufferProcs *pb = Py_TYPE(obj)->tp_as_buffer;
 
     if (pb == NULL || pb->bf_getbuffer == NULL) {