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Date: Fri, 16 Jun 2023 11:43:49 +0200
From: Tamás Koczka <poprdi@...omium.org>
To: oss-security@...ts.openwall.com
Subject: Our learnings from 42 Linux kernel exploits, we are limiting io_uring
Hello everyone,
We've posted the following article to the Google Security Blog which
contains some of our learnings from 42 Linux kernel exploits we got so
far on our kCTF VRP and the actions we are taking based on these
learnings (tl;dr: we are limiting io_uring in our products):
=======================
In 2020[1], we integrated kCTF into Google's Vulnerability Rewards
Program (VRP) to support researchers evaluating the security of Google
Kubernetes Engine (GKE) and the underlying Linux kernel. As the Linux
kernel is a key component not just for Google, but for the Internet,
we started heavily investing in this area. We extended the VRP's scope
and maximum reward in 2021[2] (to $50k), then again in February
2022[3] (to $91k), and finally in August 2022[4] (to $133k). In 2022,
we also summarized our learnings to date in our cookbook[5], and
introduced our experimental mitigations[6] for the most common
exploitation techniques.
In this post, we'd like to share our learnings and statistics about
the latest Linux kernel exploit submissions, how effective our
mitigations[7] are against them, what we do to protect our users, and,
finally, how we are changing our program to align incentives to the
areas we are most interested in.
= Learnings and Statistics =
Since its inception, the program has rewarded researchers with a total
of 1.8 million USD, and in the past year, there has been a clear
trend: 60% of the submissions[8] exploited the io_uring component of
the Linux kernel (we paid out around 1 million USD for io_uring
alone). Furthermore, io_uring vulnerabilities were used in all the
submissions which bypassed our mitigations.
= Limiting io_uring =
To protect our users, we decided to limit the usage of io_uring in
Google products:
* ChromeOS: We disabled[9] io_uring (while we explore new ways to sandbox it).
* Android: Our seccomp-bpf filter[10] ensures that io_uring is
unreachable to apps. Future Android releases will use SELinux to limit
io_uring access to a select few system processes[11].
* GKE AutoPilot: We are investigating disabling io_uring by default.
* It is disabled on production Google servers.
While io_uring brings performance benefits, and promptly reacts to
security issues with comprehensive security fixes (like
backporting[12] the 5.15 version to the 5.10 stable tree), it is a
fairly new part of the kernel. As such, io_uring continues to be
actively developed, but it is still affected by severe vulnerabilities
and also provides strong exploitation primitives. For these reasons,
we currently consider it safe only for use by trusted components.
= Transparency =
Currently, we make vulnerability details public on our spreadsheet[8]
(which now also includes CVE details), and we have summarized
different exploitation techniques in our cookbook[5]. In the future,
to make our efforts more transparent and give faster feedback to the
community, we will ask researchers to open-source their
submissions[13], including the code they used.
= Introducing kernelCTF =
To better align incentives with our areas of interest, we are shifting
our focus from GKE and kCTF to the latest stable kernel and our
mitigations. As a result, starting today we will handle kernel exploit
submissions under a new name, "kernelCTF," with its own reward
structure and submission process[14]. The maximum total payout for
kernelCTF is still $133,337 per submission. While the specific GKE
kernel configuration is still covered by the new kernelCTF, exploits
affecting non-kernel components like the full GKE stack (including
Kubernetes), the container runtime, and GKE itself, are now separately
eligible for vulnerability rewards under the kCTF VRP which is
returning to its original reward amounts and conditions.
= Conclusion =
Our goal remains the same: we are building a pipeline to analyze,
experiment, measure, and build security mitigations to make the Linux
kernel as safe as possible, with the help of the security community.
We hope that over time, we will be able to implement security
mitigations that make it more difficult to exploit Linux kernel
vulnerabilities.
With the name change, we have moved our communication channel to
#kernelctf on Discord[15], with a separate #kernelctf-announcements
channel[16]. Please join us there for the latest updates regarding
kernelCTF.
[1] https://security.googleblog.com/2020/05/expanding-our-work-with-open-source.html
[2] https://security.googleblog.com/2021/11/trick-treat-paying-leets-and-sweets-for.html
[3] https://security.googleblog.com/2022/02/roses-are-red-violets-are-blue-giving.html
[4] https://security.googleblog.com/2022/08/making-linux-kernel-exploit-cooking.html
[5] https://docs.google.com/document/d/1a9uUAISBzw3ur1aLQqKc5JOQLaJYiOP5pe_B4xCT1KA/edit
[6] https://security.googleblog.com/2022/08/making-linux-kernel-exploit-cooking.html#:~:text=The%20mitigations%20we%27ve%20built%20attempt%20to%20tackle%20the%20following%20exploit%20primitives
[7] https://github.com/thejh/linux/blob/slub-virtual/MITIGATION_README
[8] https://docs.google.com/spreadsheets/d/e/2PACX-1vS1REdTA29OJftst8xN5B5x8iIUcxuK6bXdzF8G1UXCmRtoNsoQ9MbebdRdFnj6qZ0Yd7LwQfvYC2oF/pubhtml
[9] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4228112
[10] https://cs.android.com/android/platform/superproject/+/master:bionic/libc/SECCOMP_ALLOWLIST_COMMON.TXT
[11] https://android-review.googlesource.com/c/platform/system/sepolicy/+/2302679
[12] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=788d0824269bef539fe31a785b1517882eafed93
[13] https://google.github.io/security-research/kernelctf/rules#note-about-making-the-exploit-public
[14] https://google.github.io/security-research/kernelctf/rules
[15] https://discord.gg/A3qZcyaZ69
[16] https://discord.gg/AjGJ3acF2e
=======================
The article can also be read on our blog:
https://security.googleblog.com/2023/06/learnings-from-kctf-vrps-42-linux.html
--
Tamas
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