closes bpo-39510: Fix use-after-free in BufferedReader.readinto() (GH-18295)

miss-islington · phi-gamma · web-flow · commit 97d2a9832494 · 2020-02-04T13:43:07.000-08:00
When called on a closed object, readinto() segfaults on account of a write to a freed buffer: ==220553== Process terminating with default action of signal 11 (SIGSEGV): dumping core ==220553== Access not within mapped region at address 0x2A ==220553== at 0x48408A0: memmove (vg_replace_strmem.c:1272) ==220553== by 0x58DB0C: _buffered_readinto_generic (bufferedio.c:972) ==220553== by 0x58DCBA: _io__Buffered_readinto_impl (bufferedio.c:1053) ==220553== by 0x58DCBA: _io__Buffered_readinto (bufferedio.c.h:253) Reproducer: reader = open ("/dev/zero", "rb") _void = reader.read (42) reader.close () reader.readinto (bytearray (42)) GH-GH-GH- BANG! The problem exists since 2012 when commit dc46945 added code to free the read buffer on close(). Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com> (cherry picked from commit cb1c074) Co-authored-by: Philipp Gesang <phg@phi-gamma.net>
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -726,6 +726,11 @@ def test_read_closed(self):
             file.seek(0)
             file.close()
             self.assertRaises(ValueError, file.read)
+        with self.open(support.TESTFN, "rb") as f:
+            file = self.open(f.fileno(), "rb", closefd=False)
+            self.assertEqual(file.read()[:3], b"egg")
+            file.close()
+            self.assertRaises(ValueError, file.readinto, bytearray(1))
 
     def test_no_closefd_with_filename(self):
         # can't use closefd in combination with a file name
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-02-04-10-27-41.bpo-39510.PMIh-f.rst b/Misc/NEWS.d/next/Core and Builtins/2020-02-04-10-27-41.bpo-39510.PMIh-f.rst
@@ -0,0 +1 @@
+Fix segfault in ``readinto()`` method on closed BufferedReader.
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
@@ -974,6 +974,7 @@ _buffered_readinto_generic(buffered *self, Py_buffer *buffer, char readinto1)
     PyObject *res = NULL;
 
     CHECK_INITIALIZED(self)
+    CHECK_CLOSED(self, "readinto of closed file")
 
     n = Py_SAFE_DOWNCAST(READAHEAD(self), Py_off_t, Py_ssize_t);
     if (n > 0) {

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Fix segfault in ``readinto()`` method on closed BufferedReader.