Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 #97514

gpshead · 2022-09-23T19:24:04Z

TL;DR

Python 3.9, 3.10, and 3.11.0rc2 on Linux may allow for a local privilege escalation attack in a non-default configuration when code uses the multiprocessing module and configures multiprocessing to use the forkserver start method.

Details

The Python multiprocessing library, when used with the forkserver start method on Linux, allows Python pickles to be deserialized from any user in the same machine local network namespace, which in many system configurations means any user on the same machine. Pickles can execute arbitrary code. Thus, this allows for local user privilege escalation to the user that any Python multiprocessing forkserver process is running as.

The forkserver start method for multiprocessing is not the default start method. This issue is Linux specific because only Linux supports abstract namespace sockets.

CPython before 3.9 does not make use of Linux abstract namespace sockets by default.

This issue has been assigned CVE-2022-42919.

Credit: This issue was discovered by Devin Jeanpierre (@ssbr) of Google.

Are Python 3.7 and 3.8 affected?

Not by default.

Support for users manually specifying an abstract namespace AF_UNIX socket was added as a bugfix in 3.7.8 and 3.8.4, but users would need to make specific uncommon multiprocessing API calls specifying their own forkserver control socket path in order to do that in CPython before 3.9.

What about code that explicitly asks for an abstract socket?

Applications found to be making the uncommon multiprocessing API calls to explicitly use Linux abstract namespace sockets with a forkserver are believed to be rare and should have their own specific security issues filed.

Workarounds

From Python application or library code:

import multiprocessing.util
multiprocessing.util.abstract_sockets_supported = False

This disables their use by default. You must execute that before anything else in your process has started making use of multiprocessing.

If you can patch your CPython runtime itself:

Remove these two lines from CPython's Lib/multiprocessing/connection.py:

-        if util.abstract_sockets_supported:
-            return f"\0listener-{os.getpid()}-{next(_mmap_counter)}"

(that is what our security bug fix commits do).

Or, similar to the application level fix, edit Lib/multiprocessing/util.py to always set:

- abstract_sockets_supported = _platform_supports_abstract_sockets()
+ abstract_sockets_supported = False

Alternatives to avoid the problem

If your Linux Python application can be switched from multiprocessing's .set_start_method("forkserver") to a start method such as "spawn" that will also avoid this issue.

Scope of the bug fixes

We are changing the default in Python 3.9 and higher to not use the Linux abstract namespace sockets by default.

It would be ideal to add authentication to the forkserver control socket so that it isn't even relying on filesystem permissions. This is a more complicated change and is expected to be done as a feature in 3.12.

Tasks

Cherry pick the 3.11 commit to the 3.11.0 release. https://github.com/python/cpython/commit/4686d77a04570a663164c03193d9def23c89b122
Merge the 3.9 PR. https://github.com/python/cpython/pull/98504
After 3.11.0 is out, make sure 3.11.1 won't have a duplicate news entry about this due to the branch vs 3.11.0 release branch
Push @gpshead 's PR(s) for proper forkserver socket authentication in 3.12.

The text was updated successfully, but these errors were encountered:

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 python#18866 while fixing python#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.

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.

…ythonGH-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 python#18866 while fixing python#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. (cherry picked from commit 49f6106) Co-authored-by: Gregory P. Smith <greg@krypto.org>

…GH-98501) (GH-98502) 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. (cherry picked from commit 49f6106) Co-authored-by: Gregory P. Smith <greg@krypto.org> Automerge-Triggered-By: GH:gpshead

…GH-98501) (GH-98503) 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. (cherry picked from commit 49f6106) Co-authored-by: Gregory P. Smith <greg@krypto.org> Automerge-Triggered-By: GH:gpshead

…GH-98501) (GH-98502) 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. (cherry picked from commit 49f6106) Co-authored-by: Gregory P. Smith <greg@krypto.org> Automerge-Triggered-By: GH:gpshead

…H-98501) (#98504) 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. (cherry picked from commit 49f6106) Co-authored-by: Gregory P. Smith <greg@krypto.org>

vstinner · 2022-11-03T13:27:27Z

I created https://python-security.readthedocs.io/vuln/multiprocessing-abstract-socket.html to track this vulnerability.

gpshead added type-bug An unexpected behavior, bug, or error type-security A security issue stdlib Python modules in the Lib dir labels Sep 23, 2022

gpshead self-assigned this Sep 23, 2022

gpshead added 3.11 3.10 3.9 3.12 labels Oct 20, 2022

gpshead changed the title ~~Placeholder issue for a specific pending security fix.~~ Linux specific local priviledge escalation via the multiprocessing forkserver start method - CVE-2022-42919 Oct 20, 2022

bedevere-bot mentioned this issue Oct 20, 2022

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

Merged

gpshead mentioned this issue Oct 20, 2022

multiprocessing.connection.Listener fails to close with null byte in AF_UNIX socket name. #84031

Closed

gpshead added the release-blocker label Oct 20, 2022

bedevere-bot mentioned this issue Oct 20, 2022

[3.11] gh-97514: Don't use Linux abstract sockets for multiprocessing (GH-98501) #98502

Merged

This was referenced Oct 20, 2022

[3.10] gh-97514: Don't use Linux abstract sockets for multiprocessing (GH-98501) #98503

Merged

[3.9] gh-97514: Don't use Linux abstract sockets for multiprocessing (GH-98501) #98504

Merged

gpshead changed the title ~~Linux specific local priviledge escalation via the multiprocessing forkserver start method - CVE-2022-42919~~ Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 Oct 20, 2022

gpshead assigned pablogsal and ambv Oct 20, 2022

gpshead mentioned this issue Oct 23, 2022

preloading '__main__' with forkserver has been broken for a long time #98552

Open

gpshead closed this as completed Nov 2, 2022

Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 #97514

Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 #97514

gpshead commented Sep 23, 2022 •

edited

Tasks

vstinner commented Nov 3, 2022

Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 #97514

Linux specific local privilege escalation via the multiprocessing forkserver start method - CVE-2022-42919 #97514

Comments

gpshead commented Sep 23, 2022 • edited

TL;DR

Details

Are Python 3.7 and 3.8 affected?

What about code that explicitly asks for an abstract socket?

Workarounds

From Python application or library code:

If you can patch your CPython runtime itself:

Alternatives to avoid the problem

Scope of the bug fixes

Tasks

vstinner commented Nov 3, 2022

gpshead commented Sep 23, 2022 •

edited