Who cares about GPUs?
You, me, and the complete ecosystem! GPUs (graphics processing models) are important in delivering wealthy visible experiences on cellular units. Nevertheless, the GPU software program and firmware stack has turn out to be a manner for attackers to achieve permissions and entitlements (privilege escalation) to Android-based units. There are many points on this class that may have an effect on all main GPU manufacturers, for instance, CVE-2023-4295, CVE-2023-21106, CVE-2021-0884, and extra. Most exploitable GPU vulnerabilities are within the implementation of the GPU kernel mode modules. These modules are items of code that load/unload throughout runtime, extending performance with out the necessity to reboot the gadget.
Proactive testing is nice hygiene as it will possibly result in the detection and determination of latest vulnerabilities earlier than they’re exploited. It’s additionally one of the vital advanced investigations to do as you don’t essentially know the place the vulnerability will seem (that’s the purpose!). By combining the experience of Google’s engineers with IP homeowners and OEMs, we are able to make sure the Android ecosystem retains a robust measure of integrity.
Why examine GPUs?
When researching vulnerabilities, GPUs are a well-liked goal resulting from:
- Performance vs. Safety Tradeoffs
No person desires a gradual, unresponsive gadget; any hits to GPU efficiency may lead to a noticeably degraded consumer expertise. As such, the GPU software program stack in Android depends on an in-process HAL mannequin the place the API & consumer area drivers speaking with the GPU kernel mode module are working straight throughout the context of apps, thus avoiding IPC (interprocess communication). This opens the door for doubtlessly untrusted code from a 3rd celebration app with the ability to straight entry the interface uncovered by the GPU kernel module. If there are any vulnerabilities within the module, the third celebration app has an avenue to use them. In consequence, a doubtlessly untrusted code working within the context of the third celebration software is ready to straight entry the interface uncovered by the GPU kernel module and exploit potential vulnerabilities within the kernel module.
- Selection & Reminiscence Security
Moreover, the implementation of GPU subsystems (and kernel modules particularly) from main OEMs are more and more advanced. Kernel modules for many GPUs are sometimes written in reminiscence unsafe languages similar to C, that are prone to reminiscence corruption vulnerabilities like buffer overflow.
Can somebody do one thing about this?
Nice information, we have already got! Who’s we? The Android Purple Crew and Arm! We’ve labored collectively to run an engagement on the Mali GPU (extra on that beneath), however first, a quick introduction:
Android Purple Crew
The Android Purple Crew performs time-bound safety evaluation engagements on all points of the Android open supply codebase and conducts common safety evaluations and assessments of inner Android parts. All through these engagements, the Android Purple Crew usually collaborates with third celebration software program and {hardware} suppliers to research and perceive proprietary and “closed supply” code repositories and related supply code which might be utilized by Android merchandise with the only real goal to determine safety dangers and potential vulnerabilities earlier than they are often exploited by adversaries exterior of Android. This yr, the Android Purple Crew collaborated straight with our trade associate, Arm, to conduct the Mali GPU engagement and additional safe hundreds of thousands of Android units.
Arm Product Safety and GPU Groups
Arm has a central product safety group that units the coverage and follow throughout the corporate. Additionally they have devoted product safety specialists embedded in engineering groups. Arm operates a scientific strategy which is designed to forestall, uncover, and get rid of safety vulnerabilities. This features a Safety Growth Lifecycle (SDL), a Monitoring functionality, and Incident Response. For this collaboration the Android Purple Groups had been supported by the embedded safety specialists primarily based in Arm’s GPU engineering group.
Working collectively to safe Android units
Google’s Android Safety groups and Arm have been working collectively for a very long time. Safety necessities are by no means static, and challenges exist with all GPU distributors. By steadily sharing experience, the Android Purple Crew and Arm had been capable of speed up detection and determination. Investigations of recognized vulnerabilities, potential remediation methods, and hardening measures drove detailed analyses and the implementation of fixes the place related.
Latest analysis targeted on the Mali GPU as a result of it’s the preferred GPU in right this moment’s Android units. Collaborating on GPU safety allowed us to:
- Assess the impression on the broadest phase of the Android Ecosystem: The Arm Mali GPU is among the most used GPUs by authentic gear producers (OEMs) and is discovered in lots of well-liked cellular units. By specializing in the Arm Mali GPU, the Android Purple Crew may assess the safety of a GPU implementation working on hundreds of thousands of Android units worldwide.
- Consider the reference implementation and vendor-specific adjustments: Cellphone producers usually modify the upstream implementation of GPUs. This tailors the GPU to the producer’s particular gadget(s). These modifications and enhancements are at all times difficult to make, and might typically introduce safety vulnerabilities that aren’t current within the authentic model of the GPU upstream. On this particular occasion, the Google Pixel group actively labored with the Android Purple Crew to raised perceive and safe the modifications they made for Pixel units.
Enhancements
Investigations have led to important enhancements, leveling up the safety of the GPU software program/firmware stack throughout a large phase of the Android ecosystem.
Testing the kernel driver
One key element of the GPU subsystem is its kernel mode driver. Throughout this engagement, each the Android Purple Crew and Arm invested important effort wanting on the Mali kbase kernel driver. Because of its complexity, fuzzing was chosen as the first testing strategy for this space. Fuzzing automates and scales vulnerability discovery in a manner not potential through handbook strategies. With assist from Arm, the Android Purple Crew added extra syzkaller fuzzing descriptions to match the newest Mali kbase driver implementation.
The group constructed a couple of customizations to allow fuzzing the Mali kbase driver within the cloud, with out bodily {hardware}. This offered an enormous enchancment to fuzzing efficiency and scalability. With the Pixel group’s help, we additionally had been capable of arrange fuzzing on precise Pixel units. By way of the mixture of cloud-based fuzzing, Pixel-based fuzzing, and handbook evaluate, we had been capable of uncover two reminiscence points in Pixel’s customization of driver code (CVE-2023-48409 and CVE-2023-48421).
Each points occurred within the gpu_pixel_handle_buffer_liveness_update_ioctl perform, which is applied by the Pixel group as a part of gadget particular customization. These are each reminiscence points brought on by integer overflow issues. If exploited rigorously alongside different vulnerabilities, these points may result in kernel privilege escalation from consumer area. Each points had been mounted and the patch was launched to affected units in Pixel safety bulletin 2023-12-01.
Testing the firmware
Firmware is one other basic constructing block of the GPU subsystem. It’s the middleman working with kernel drivers and GPU {hardware}. In lots of instances, firmware performance is straight/not directly accessible from the applying. So “software ⇒ kernel ⇒ firmware ⇒ kernel” is a recognized assault stream on this space. Additionally, typically, firmware runs on embedded microcontrollers with restricted sources. Generally used safety kernel mitigations (ASLR, stack safety, heap safety, sure sanitizers, and many others.) won’t be relevant to firmware resulting from useful resource constraints and efficiency impression. This may make compromising firmware simpler, in some instances, than straight compromising kernel drivers from consumer area. To check the integrity of present firmware, the Android Purple Crew and Arm labored collectively to carry out each fuzzing and formal verification together with handbook evaluation. This multi-pronged strategy led to the invention of CVE-2024-0153, which had a patch launched in the July 2024 Android Safety Bulletin.
CVE-2024-0153 occurs when GPU firmware handles sure directions. When dealing with such directions, the firmware copies register content material right into a buffer. There are measurement checks earlier than the copy operation. Nevertheless, beneath very particular circumstances, an out-of-bounds write occurs to the vacation spot buffer, resulting in a buffer overflow. When rigorously manipulated, this overflow will overwrite another necessary constructions following the buffer, inflicting code execution within the GPU firmware.
The circumstances mandatory to achieve and doubtlessly exploit this subject are very advanced because it requires a deep understanding of how directions are executed. With collective experience, the Android Purple Crew and Arm had been capable of confirm the exploitation path and leverage the problem to achieve restricted management of GPU firmware. This ultimately circled again to the kernel to acquire privilege escalation. Arm did a superb job to reply shortly and remediate the problem. Altogether, this highlights the power of collaboration between each groups to dive deeper.
Time to Patch
It’s recognized that attackers exploit GPU vulnerabilities within the wild, and time to patch is essential to scale back danger of exploitation and defend customers. Because of this engagement, 9 new Safety Check suite (STS) assessments had been constructed to assist companions mechanically verify their builds for lacking Mali kbase patches. (Safety Check Suite is software program offered by Google to assist companions automate the method of checking their builds for lacking safety patches.)
What’s Subsequent?
The Arm Product Safety Crew is actively concerned in security-focused trade communities and collaborates intently with its ecosystem companions. The engagement with the Android Purple Crew, for example, offers priceless enablement that drives greatest practices and product excellence. Constructing on this collaborative strategy, Arm is complementing its product safety assurance capabilities with a bug bounty program. This funding will develop Arm’s efforts to determine potential vulnerabilities. For extra data on Arm’s product safety initiatives, please go to this product safety web page.
The Android Purple Crew and Arm proceed to work collectively to proactively elevate the bar on GPU safety. With thorough testing, speedy fixing, and updates to the safety take a look at suite, we’re enhancing the ecosystem for Android customers. The Android Purple Crew seems to be ahead to replicating this working relationship with different ecosystem companions to make units safer.