gh-97514: Don't use Linux abstract sockets for multiprocessing (#98501)

Linux abstract sockets are insecure as they lack any form of filesystem
permissions so their use allows anyone on the system to inject code into
the process.

This removes the default preference for abstract sockets in
multiprocessing introduced in Python 3.9+ via
#18866 while fixing
#84031.

Explicit use of an abstract socket by a user now generates a
RuntimeWarning.  If we choose to keep this warning, it should be
backported to the 3.7 and 3.8 branches.

Lib/multiprocessing/connection.py

@@ -73,11 +73,6 @@ def arbitrary_address(family):
     if family == 'AF_INET':
         return ('localhost', 0)
     elif family == 'AF_UNIX':
-        # Prefer abstract sockets if possible to avoid problems with the address
-        # size.  When coding portable applications, some implementations have
-        # sun_path as short as 92 bytes in the sockaddr_un struct.
-        if util.abstract_sockets_supported:
-            return f"\0listener-{os.getpid()}-{next(_mmap_counter)}"
         return tempfile.mktemp(prefix='listener-', dir=util.get_temp_dir())
     elif family == 'AF_PIPE':
         return tempfile.mktemp(prefix=r'\\.\pipe\pyc-%d-%d-' %

Misc/NEWS.d/next/Security/2022-09-07-10-42-00.gh-issue-97514.Yggdsl.rst

@@ -0,0 +1,15 @@
+On Linux the :mod:`multiprocessing` module returns to using filesystem backed
+unix domain sockets for communication with the *forkserver* process instead of
+the Linux abstract socket namespace.  Only code that chooses to use the
+:ref:`"forkserver" start method <multiprocessing-start-methods>` is affected.
+
+Abstract sockets have no permissions and could allow any user on the system in
+the same `network namespace
+<https://man7.org/linux/man-pages/man7/network_namespaces.7.html>`_ (often the
+whole system) to inject code into the multiprocessing *forkserver* process.
+This was a potential privilege escalation. Filesystem based socket permissions
+restrict this to the *forkserver* process user as was the default in Python 3.8
+and earlier.
+
+This prevents Linux `CVE-2022-42919
+<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-42919>`_.