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Date: Fri, 14 Jul 2023 20:06:56 +0200
From: Solar Designer <>
To: Tamas Koczka <>
Subject: Re: Our learnings from 42 Linux kernel exploits, we are limiting io_uring


Thank you for bringing this to oss-security back then.  I have a few
questions below that I think you could clarify for everyone.  I'll quote
more of your message than I normally do since it's been a while.

On Fri, Jun 16, 2023 at 11:43:49AM +0200, Tamas Koczka wrote:
> 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.

There's a recent write-up on an exploitation technique that also
partially describes CVE-2023-21400, "a double free vulnerability in
io_uring [...] found by Ye Zhang and [Nicolas Wu] last year, affecting
kernel 5.10. [...] we exploit CVE-2023-21400 with Dirty Pagetable on
Google Pixel 7."

Dirty Pagetable: A Novel Exploitation Technique To Rule Linux Kernel

I wish this vulnerability and exploitation technique were properly
brought to oss-security on its own, and in a context not limited to
Google Pixel.  Maybe it will be once the full description is made
public, as right now the write-up above omits vulnerability detail.

It appears that this got patched in the July 5 update for Google Pixel:

Pixel Update Bulletin - July 2023
Published July 5, 2023

"For Google devices, security patch levels of 2023-07-05 or later
address all issues in this bulletin and all issues in the July 2023
Android Security Bulletin."

"CVE-2023-21400	A-264663832 *	EoP	Moderate	Kernel io_uring"

Nothing is mentioned about seccomp-bpf on either of the above web pages,
although maybe it's factored into the Moderate severity rating?

I understand that with vulnerability detail still not public you might
not be able to tell much, but I am wondering whether there's any
inconsistency here (seccomp-bpf on Android was meant to prevent this,
but did not?) or just a misunderstanding or something else.  I wonder
if a vulnerability in io_uring could be such that it's exploitable
without io_uring access directly from the attacking app.

> = 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.

For archival and relevant discussions on linux-distros list policy, let
me quote what [13] currently says:

Quote start:
> Note about making the exploit public
> You can publish your exploit at any time you would like to, but we
> recommend publishing the exploit 30 days after the vulnerability was
> disclosed. This gives the industry time to apply patches. Read our
> stance on the topic in Google's disclosure policy.
> We only process submissions after the exploit is public (and we can only
> issue rewards when the submission was processed), but not sooner than 30
> days after the vulnerability disclosure.
> If you publish sooner than 30 days, you won't get the reward faster. If
> you want to delay the publication (disclose later than 30 days), you
> could do that, but you would get the money later (we want to encourage
> you to publish the exploit details sooner than later).
> The above is about the exploit itself, not the vulnerability. We
> automatically share some limited vulnerability details of the
> submissions on our public submission spreadsheet, as a CVE, and as soon
> as you submit the vulnerability details via the form.
Quote end.

In the above, do you mean 30 days after _public_ disclosure (or e.g.
disclosure to Google, to upstream, or something else)?  I suggest you
clarify this.

> = 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.

Are there separate bug bounty programs for ChromeOS and Android, which
would also cover relevant Linux kernel issues?  If so, a Linux kernel
bug can potentially be eligible for up to 4 Google bug bounty programs,
right?  Are the program terms compatible?

> = 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]
> [2]
> [3]
> [4]
> [5]
> [6]
> [7]
> [8]
> [9]
> [10]
> [11]
> [12]
> [13]
> [14]
> [15]
> [16]
> =======================
> The article can also be read on our blog:

Thank you for your efforts, and for the transparency.


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