Author: Armin Rigo <ar...@tunes.org>
Branch:
Changeset: r3239:6d435908617d
Date: 2019-03-05 10:11 +0100
http://bitbucket.org/cffi/cffi/changeset/6d435908617d/
Log: Document issue 391
diff --git a/c/_cffi_backend.c b/c/_cffi_backend.c
--- a/c/_cffi_backend.c
+++ b/c/_cffi_backend.c
@@ -6019,8 +6019,8 @@
PyErr_SetString(PyExc_MemoryError,
"Cannot allocate write+execute memory for ffi.callback(). "
"You might be running on a system that prevents this. "
- "For more information, see https://cffi.readthedocs.io/en/latest";
- "/using.html#callbacks-old-style");
+ "For more information, see "
+ "https://cffi.readthedocs.io/en/latest/using.html#callbacks";);
return NULL;
}
cd = PyObject_GC_New(CDataObject_closure, &CDataOwningGC_Type);
diff --git a/doc/source/using.rst b/doc/source/using.rst
--- a/doc/source/using.rst
+++ b/doc/source/using.rst
@@ -876,11 +876,27 @@
protections can interfere (for example, on SELinux you need to
run with ``deny_execmem`` set to ``off``).
- Note also that a cffi fix for the latter issue was attempted---see
+ - `On Mac OS X,`__ you need to give your application the entitlement
+ ``com.apple.security.cs.allow-unsigned-executable-memory``.
+
+ Note also that a cffi fix for this issue was attempted---see
the ``ffi_closure_alloc`` branch---but was not merged because it
creates potential `memory corruption`__ with ``fork()``.
+ In other words: yes, it is dangerous to allow write+execute memory in your
+ program; that's why the various "hardening" options above exist. But at
+ the same time, these options open wide the door to another attack: if the
+ program forks and then attempts to call any of the ``ffi.callback()``, then
+ this immediately results in a crash---or, with a minimal amount of work
+ from an attacker, arbitrary code execution. To me it sounds even more
+ dangerous than the original problem, and that's why cffi is not playing
+ along.
+
+ To fix the issue once and for all on the affected platforms, you need
+ to refactor the involved code so that it no longer uses ``ffi.callback()``.
+
.. __: https://github.com/pyca/pyopenssl/issues/596
+.. __: https://bitbucket.org/cffi/cffi/issues/391/
.. __: https://bugzilla.redhat.com/show_bug.cgi?id=1249685
Warning: like ffi.new(), ffi.callback() returns a cdata that has
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