gh-153539: Fix use-after-free in TextIOWrapper.tell() with a reentrant decoder (GH-153540)

TextIOWrapper.tell() used a borrowed next_input from the snapshot across the
decoder's getstate/decode/setstate calls, so a decoder that reenters seek()
from getstate could free it and leave tell() reading freed memory. Own the
reference across those calls, matching the sibling textiowrapper_read_chunk.

(cherry picked from commit fa0ec86)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>

Author: tonghuaroot

Lib/test/test_io/test_textio.py

@@ -1448,6 +1448,49 @@ def _to_memoryview(buf):
 class CTextIOWrapperTest(TextIOWrapperTest, CTestCase):
     shutdown_error = "LookupError: unknown encoding: ascii"
 
+    def test_reentrant_seek_during_tell(self):
+        # gh-153539: reading short of _CHUNK_SIZE leaves residual bytes in the
+        # snapshot, so tell() re-decodes and calls the decoder's getstate(); a
+        # reentrant seek() there must not free the snapshot tell() still uses.
+        # C-only: _pyio binds next_input as a strong local and cannot crash.
+        wrapper = None
+        armed = False
+
+        class ReentrantDecoder(codecs.IncrementalDecoder):
+            def decode(self, input, final=False):
+                return bytes(input).decode("latin-1")
+            def getstate(self):
+                nonlocal armed
+                if wrapper is not None and armed:
+                    armed = False
+                    wrapper.seek(0)
+                return (b"", 0)
+            def setstate(self, state):
+                pass
+
+        def search(name):
+            if name != "reentrant_tell_test":
+                return None
+            return codecs.CodecInfo(
+                name=name,
+                encode=lambda s, e='strict': (s.encode("latin-1"), len(s)),
+                decode=lambda b, e='strict': (bytes(b).decode("latin-1"), len(b)),
+                incrementaldecoder=ReentrantDecoder)
+
+        codecs.register(search)
+        self.addCleanup(codecs.unregister, search)
+        raw = self.BytesIO(b"abcdefghijklmnop" * 8)
+        wrapper = self.TextIOWrapper(self.BufferedReader(raw),
+                                     encoding="reentrant_tell_test", newline="")
+        wrapper._CHUNK_SIZE = 8
+        wrapper.read(5)
+        armed = True
+        self.assertIsInstance(wrapper.tell(), int)
+        # tell() at the snapshot boundary takes the early return that owns and
+        # must release next_input; exercise it too (leak-checked under -R).
+        wrapper.seek(0)
+        self.assertIsInstance(wrapper.tell(), int)
+
     def test_initialization(self):
         r = self.BytesIO(b"\xc3\xa9\n\n")
         b = self.BufferedReader(r, 1000)

Misc/NEWS.d/next/Library/2026-07-11-12-21-12.gh-issue-153539.gBs2T5.rst

@@ -0,0 +1,3 @@
+Fix a crash in the C implementation of :meth:`io.TextIOWrapper.tell` when the
+decoder's ``getstate`` method triggers a reentrant seek, or when another thread
+seeks the same stream concurrently. Patch by tonghuaroot.

Modules/_io/textio.c

@@ -2745,7 +2745,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
     PyObject *res;
     PyObject *posobj = NULL;
     cookie_type cookie = {0,0,0,0,0};
-    PyObject *next_input;
+    PyObject *next_input = NULL;
     Py_ssize_t chars_to_skip, chars_decoded;
     Py_ssize_t skip_bytes, skip_back;
     PyObject *saved_state = NULL;
@@ -2797,11 +2797,15 @@ _io_TextIOWrapper_tell_impl(textio *self)
 
     assert (PyBytes_Check(next_input));
 
+    /* Own next_input: a reentrant or concurrent seek can drop the snapshot. */
+    Py_INCREF(next_input);
+
     cookie.start_pos -= PyBytes_GET_SIZE(next_input);
 
     /* How many decoded characters have been used up since the snapshot? */
     if (self->decoded_chars_used == 0)  {
         /* We haven't moved from the snapshot point. */
+        Py_DECREF(next_input);
         return textiowrapper_build_cookie(&cookie);
     }
 
@@ -2942,6 +2946,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
     }
 
 finally:
+    Py_XDECREF(next_input);
     res = PyObject_CallMethodOneArg(
             self->decoder, &_Py_ID(setstate), saved_state);
     Py_DECREF(saved_state);
@@ -2954,6 +2959,7 @@ _io_TextIOWrapper_tell_impl(textio *self)
     return textiowrapper_build_cookie(&cookie);
 
 fail:
+    Py_XDECREF(next_input);
     if (saved_state) {
         PyObject *exc = PyErr_GetRaisedException();
         res = PyObject_CallMethodOneArg(