Skip to content

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

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
vstinner wants to merge 4 commits into
base: main
Choose a base branch
from
+10 −6
Draft

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

Changes from all commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
16 changes: 10 additions & 6 deletions Modules/fcntlmodule.c
View file
Open in desktop
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
}
Expand Down
Loading