gh-143378: Fix UAF when BytesIO is concurrently mutated during write operations

Lib/_pyio.py

@@ -949,12 +949,12 @@ def read1(self, size=-1):
         return self.read(size)
 
     def write(self, b):
-        if self.closed:
-            raise ValueError("write to closed file")
         if isinstance(b, str):
             raise TypeError("can't write str to binary stream")
         with memoryview(b) as view:
             n = view.nbytes  # Size of any bytes-like object
+            if self.closed:
+                raise ValueError("write to closed file")
         if n == 0:
             return 0
 

Lib/test/test_io/test_memoryio.py

@@ -587,6 +587,48 @@ def test_issue5449(self):
         self.ioclass(initial_bytes=buf)
         self.assertRaises(TypeError, self.ioclass, buf, foo=None)
 
+    def test_write_concurrent_close(self):
+        class B:
+            def __buffer__(self, flags):
+                memio.close()
+                return memoryview(b"A")
+
+        memio = self.ioclass()
+        self.assertRaises(ValueError, memio.write, B())
+
+    # Prevent crashes when memio.write() or memio.writelines()
+    # concurrently mutates (e.g., closes or exports) 'memio'.
+    # See: https://github.com/python/cpython/issues/143378.
+
+    def test_writelines_concurrent_close(self):
+        class B:
+            def __buffer__(self, flags):
+                memio.close()
+                return memoryview(b"A")
+
+        memio = self.ioclass()
+        self.assertRaises(ValueError, memio.writelines, [B()])
+
+    def test_write_concurrent_export(self):
+        class B:
+            buf = None
+            def __buffer__(self, flags):
+                self.buf = memio.getbuffer()
+                return memoryview(b"A")
+
+        memio = self.ioclass()
+        self.assertRaises(BufferError, memio.write, B())
+
+    def test_writelines_concurrent_export(self):
+        class B:
+            buf = None
+            def __buffer__(self, flags):
+                self.buf = memio.getbuffer()
+                return memoryview(b"A")
+
+        memio = self.ioclass()
+        self.assertRaises(BufferError, memio.writelines, [B()])
+
 
 class TextIOTestMixin:
 

Misc/NEWS.d/next/Library/2026-01-03-19-41-36.gh-issue-143378.29AvE7.rst

@@ -0,0 +1 @@
+Fix use-after-free crashes when a :class:`~io.BytesIO` object is concurrently mutated during :meth:`~io.RawIOBase.write` or :meth:`~io.IOBase.writelines`.

Modules/_io/bytesio.c

@@ -194,18 +194,18 @@ write_bytes_lock_held(bytesio *self, PyObject *b)
 {
     _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
 
-    if (check_closed(self)) {
-        return -1;
-    }
-    if (check_exports(self)) {
-        return -1;
-    }
-
     Py_buffer buf;
+    Py_ssize_t len;
     if (PyObject_GetBuffer(b, &buf, PyBUF_CONTIG_RO) < 0) {
         return -1;
     }
-    Py_ssize_t len = buf.len;
+
+    if (check_closed(self) || check_exports(self)) {
+        len = -1;
+        goto done;
+    }
+
+    len = buf.len;
     if (len == 0) {
         goto done;
     }