Coverity scan reports this for bufferedio.c :

Error: RESOURCE_LEAK (CWE-772): [#def23]
Python-2.7.15/Modules/_io/bufferedio.c:1353: alloc_fn: Storage is returned from allocation function "PyString_FromStringAndSize".
Python-2.7.15/Objects/stringobject.c:88:5: alloc_fn: Storage is returned from allocation function "PyObject_Malloc".
Python-2.7.15/Objects/obmalloc.c:982:5: alloc_fn: Storage is returned from allocation function "malloc".
Python-2.7.15/Objects/obmalloc.c:982:5: return_alloc_fn: Directly returning storage allocated by "malloc".
Python-2.7.15/Objects/stringobject.c:88:5: var_assign: Assigning: "op" = "PyObject_Malloc(37UL + size)".
Python-2.7.15/Objects/stringobject.c:111:5: return_alloc: Returning allocated memory "op".
Python-2.7.15/Modules/_io/bufferedio.c:1353: var_assign: Assigning: "data" = storage returned from "PyString_FromStringAndSize(self->buffer + self->pos, current_size)".
Python-2.7.15/Modules/_io/bufferedio.c:1366: leaked_storage: Variable "data" going out of scope leaks the storage it points to.
 1364|           if (res == NULL) {
 1365|               Py_DECREF(chunks);
 1366|->             return NULL;
 1367|           }
 1368|           Py_CLEAR(res);

New changeset 2dd6e079ae71f3723fbea2582ac080be06a6968f by Victor Stinner (stratakis) in branch '2.7':
[2.7] bpo-36289: Fix a possible reference leak in the io module (GH-12329)
https://github.com/python/cpython/commit/2dd6e079ae71f3723fbea2582ac080be06a6968f

Thanks Charalampos, I merged your PR.

Python 3.7 and master are not affected: _bufferedreader_read_all() has been refactored to add a "cleanup" label which clears all local data including the 'data' variable.