gh-132915: Try to detect a buffer overflow in fcntl() and ioctl() (GH-132919)

SystemError is raised when buffer overflow is detected.
The stack and memory can already be corrupted, so treat this error as fatal.

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2025-04-25-12-55-06.gh-issue-132915.XuKCXn.rst

@@ -0,0 +1,3 @@
+:func:`fcntl.fcntl` and :func:`fcntl.ioctl` can now detect a buffer overflow
+and raise :exc:`SystemError`. The stack and memory can be corrupted in such
+case, so treat this error as fatal.

Modules/fcntlmodule.c

@@ -22,6 +22,10 @@
 #  include <stropts.h>            // I_FLUSHBAND
 #endif
 
+#define GUARDSZ 8
+// NUL followed by random bytes.
+static const char guard[GUARDSZ] = "\x00\xfa\x69\xc4\x67\xa3\x6c\x58";
+
 /*[clinic input]
 module fcntl
 [clinic start generated code]*/
@@ -80,9 +84,10 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
         return PyLong_FromLong(ret);
     }
     if (PyUnicode_Check(arg) || PyObject_CheckBuffer(arg)) {
-#define FCNTL_BUFSZ 1024
         Py_buffer view;
-        char buf[FCNTL_BUFSZ+1];  /* argument plus NUL byte */
+#define FCNTL_BUFSZ 1024
+        /* argument plus NUL byte plus guard to detect a buffer overflow */
+        char buf[FCNTL_BUFSZ+GUARDSZ];
 
         if (!PyArg_Parse(arg, "s*", &view)) {
             return NULL;
@@ -95,7 +100,7 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
             return NULL;
         }
         memcpy(buf, view.buf, len);
-        buf[len] = '\0';
+        memcpy(buf + len, guard, GUARDSZ);
         PyBuffer_Release(&view);
 
         do {
@@ -106,6 +111,10 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
         if (ret < 0) {
             return !async_err ? PyErr_SetFromErrno(PyExc_OSError) : NULL;
         }
+        if (memcmp(buf + len, guard, GUARDSZ) != 0) {
+            PyErr_SetString(PyExc_SystemError, "buffer overflow");
+            return NULL;
+        }
         return PyBytes_FromStringAndSize(buf, len);
 #undef FCNTL_BUFSZ
     }
@@ -199,34 +208,37 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
     if (PyUnicode_Check(arg) || PyObject_CheckBuffer(arg)) {
         Py_buffer view;
 #define IOCTL_BUFSZ 1024
-        char buf[IOCTL_BUFSZ+1];  /* argument plus NUL byte */
+        /* argument plus NUL byte plus guard to detect a buffer overflow */
+        char buf[IOCTL_BUFSZ+GUARDSZ];
         if (mutate_arg && !PyBytes_Check(arg) && !PyUnicode_Check(arg)) {
             if (PyObject_GetBuffer(arg, &view, PyBUF_WRITABLE) == 0) {
-                if (view.len <= IOCTL_BUFSZ) {
-                    memcpy(buf, view.buf, view.len);
-                    buf[view.len] = '\0';
-                    do {
-                        Py_BEGIN_ALLOW_THREADS
-                        ret = ioctl(fd, code, buf);
-                        Py_END_ALLOW_THREADS
-                    } while (ret == -1 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
-                    memcpy(view.buf, buf, view.len);
-                }
-                else {
-                    do {
-                        Py_BEGIN_ALLOW_THREADS
-                        ret = ioctl(fd, code, view.buf);
-                        Py_END_ALLOW_THREADS
-                    } while (ret == -1 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
+                Py_ssize_t len = view.len;
+                void *ptr = view.buf;
+                if (len <= IOCTL_BUFSZ) {
+                    memcpy(buf, ptr, len);
+                    memcpy(buf + len, guard, GUARDSZ);
+                    ptr = buf;
                 }
+                do {
+                    Py_BEGIN_ALLOW_THREADS
+                    ret = ioctl(fd, code, ptr);
+                    Py_END_ALLOW_THREADS
+                } while (ret == -1 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
                 if (ret < 0) {
                     if (!async_err) {
                         PyErr_SetFromErrno(PyExc_OSError);
                     }
                     PyBuffer_Release(&view);
                     return NULL;
                 }
+                if (ptr == buf) {
+                    memcpy(view.buf, buf, len);
+                }
                 PyBuffer_Release(&view);
+                if (ptr == buf && memcmp(buf + len, guard, GUARDSZ) != 0) {
+                    PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                    return NULL;
+                }
                 return PyLong_FromLong(ret);
             }
             if (!PyErr_ExceptionMatches(PyExc_BufferError)) {
@@ -246,7 +258,7 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
             return NULL;
         }
         memcpy(buf, view.buf, len);
-        buf[len] = '\0';
+        memcpy(buf + len, guard, GUARDSZ);
         PyBuffer_Release(&view);
 
         do {
@@ -257,6 +269,10 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
         if (ret < 0) {
             return !async_err ? PyErr_SetFromErrno(PyExc_OSError) : NULL;
         }
+        if (memcmp(buf + len, guard, GUARDSZ) != 0) {
+            PyErr_SetString(PyExc_SystemError, "buffer overflow");
+            return NULL;
+        }
         return PyBytes_FromStringAndSize(buf, len);
 #undef IOCTL_BUFSZ
     }