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gh-156939: Fix memory corruption check in fcntl by vstinner · Pull Request #157103 · python/cpython · GitHub

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gh-156939: Fix memory corruption check in fcntl #157103

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vstinner wants to merge 4 commits into from
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gh-156939: Fix memory corruption check in fcntl #157103

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6627b40
gh-156939: Fix buffer overflow in fcntl
vstinner Sep 7, 2026
d90a253
Replace canary byte with guard bytes
vstinner Sep 7, 2026
db05828
Update comment
vstinner Sep 7, 2026
c303c1e
Fix typo
vstinner Sep 7, 2026
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16 changes: 10 additions & 6 deletions Modules/fcntlmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -121,13 +121,14 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
return PyBytes_FromStringAndSize(buf, len);
}
else {
PyBytesWriter *writer = PyBytesWriter_Create(len);
PyBytesWriter *writer = PyBytesWriter_Create(len + GUARDSZ);
if (writer == NULL) {
PyBuffer_Release(&view);
return NULL;
}
char *ptr = PyBytesWriter_GetData(writer);
memcpy(ptr, view.buf, len);
memcpy(ptr + len, guard, GUARDSZ);
PyBuffer_Release(&view);

do {
Expand All @@ -142,7 +143,7 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
PyBytesWriter_Discard(writer);
return NULL;
}
if (ptr[len] != '\0') {
if (memcmp(ptr + len, guard, GUARDSZ) != 0) {
PyErr_SetString(PyExc_SystemError,
"Memory corruption in fcntl() due to "
"buffer overflow. "
Expand All @@ -151,7 +152,8 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
PyBytesWriter_Discard(writer);
return NULL;
}
return PyBytesWriter_Finish(writer);
// Truncate the trailing guard bytes
return PyBytesWriter_FinishWithSize(writer, len);
}
#undef FCNTL_BUFSZ
}
Expand Down Expand Up @@ -316,13 +318,14 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
return PyBytes_FromStringAndSize(buf, len);
}
else {
PyBytesWriter *writer = PyBytesWriter_Create(len);
PyBytesWriter *writer = PyBytesWriter_Create(len + GUARDSZ);
if (writer == NULL) {
PyBuffer_Release(&view);
return NULL;
}
char *ptr = PyBytesWriter_GetData(writer);
memcpy(ptr, view.buf, len);
memcpy(ptr + len, guard, GUARDSZ);
PyBuffer_Release(&view);

do {
Expand All @@ -337,7 +340,7 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
PyBytesWriter_Discard(writer);
return NULL;
}
if (ptr[len] != '\0') {
if (memcmp(ptr + len, guard, GUARDSZ) != 0) {
PyErr_SetString(PyExc_SystemError,
"Memory corruption in ioctl() due to "
"buffer overflow. "
Expand All @@ -346,7 +349,8 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
PyBytesWriter_Discard(writer);
return NULL;
}
return PyBytesWriter_Finish(writer);
// Truncate the trailing guard bytes
return PyBytesWriter_FinishWithSize(writer, len);
}
#undef IOCTL_BUFSZ
}
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