Who cares about GPUs?
You, me, and the whole ecosystem! GPUs (graphics processing models) are vital in delivering wealthy visible experiences on cell units. Nonetheless, the GPU software program and firmware stack has turn into 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 sweet hygiene as it could actually result in the detection and backbone of recent vulnerabilities earlier than they’re exploited. It’s additionally probably the most 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 house owners and OEMs, we are able to make sure the Android ecosystem retains a powerful measure of integrity.
Why examine GPUs?
When researching vulnerabilities, GPUs are a well-liked goal on account of:
- Performance vs. Safety Tradeoffs
No person needs a gradual, unresponsive gadget; any hits to GPU efficiency might end in 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 house drivers speaking with the GPU kernel mode module are operating instantly inside the context of apps, thus avoiding IPC (interprocess communication). This opens the door for doubtlessly untrusted code from a 3rd get together app having the ability to instantly entry the interface uncovered by the GPU kernel module. If there are any vulnerabilities within the module, the third get together app has an avenue to use them. Because of this, a doubtlessly untrusted code operating within the context of the third get together software is ready to instantly 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 usually written in reminiscence unsafe languages corresponding 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 elements 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 often collaborates with third get together software program and {hardware} suppliers to investigate and perceive proprietary and “closed supply” code repositories and related supply code which can be utilized by Android merchandise with the only goal to determine safety dangers and potential vulnerabilities earlier than they are often exploited by adversaries outdoors of Android. This 12 months, the Android Purple Crew collaborated instantly 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 apply throughout the corporate. Additionally they have devoted product safety specialists embedded in engineering groups. Arm operates a scientific method which is designed to stop, uncover, and get rid of safety vulnerabilities. This features a Safety Improvement Lifecycle (SDL), a Monitoring functionality, and Incident Response. For this collaboration the Android Purple Groups had been supported by the embedded safety specialists based mostly 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 often sharing experience, the Android Purple Crew and Arm had been in a position to speed up detection and backbone. Investigations of recognized vulnerabilities, potential remediation methods, and hardening measures drove detailed analyses and the implementation of fixes the place related.
Latest analysis centered on the Mali GPU as a result of it’s the most well-liked 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 unique gear producers (OEMs) and is discovered in lots of standard cell units. By specializing in the Arm Mali GPU, the Android Purple Crew might assess the safety of a GPU implementation operating 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 all the time difficult to make, and may typically introduce safety vulnerabilities that aren’t current within the unique model of the GPU upstream. On this particular occasion, the Google Pixel group actively labored with the Android Purple Crew to higher perceive and safe the modifications they made for Pixel units.
Enhancements
Investigations have led to vital 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 part of the GPU subsystem is its kernel mode driver. Throughout this engagement, each the Android Purple Crew and Arm invested vital effort wanting on the Mali kbase kernel driver. As a consequence of its complexity, fuzzing was chosen as the first testing method for this space. Fuzzing automates and scales vulnerability discovery in a manner not potential through guide strategies. With assist from Arm, the Android Purple Crew added extra syzkaller fuzzing descriptions to match the most recent Mali kbase driver implementation.
The group constructed just a few 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 in a position to arrange fuzzing on precise Pixel units. By the mixture of cloud-based fuzzing, Pixel-based fuzzing, and guide assessment, we had been in a position to 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 fastidiously alongside different vulnerabilities, these points might result in kernel privilege escalation from consumer house. Each points had been fastened 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 circumstances, firmware performance is instantly/not directly accessible from the appliance. So “software ⇒ kernel ⇒ firmware ⇒ kernel” is a recognized assault circulation on this space. Additionally, typically, firmware runs on embedded microcontrollers with restricted assets. Generally used safety kernel mitigations (ASLR, stack safety, heap safety, sure sanitizers, and so forth.) won’t be relevant to firmware on account of useful resource constraints and efficiency impression. This could make compromising firmware simpler, in some circumstances, than instantly compromising kernel drivers from consumer house. 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 guide evaluation. This multi-pronged method 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 dimension checks earlier than the copy operation. Nonetheless, below very particular circumstances, an out-of-bounds write occurs to the vacation spot buffer, resulting in a buffer overflow. When fastidiously manipulated, this overflow will overwrite another essential buildings following the buffer, inflicting code execution within the GPU firmware.
The circumstances needed to succeed in and doubtlessly exploit this problem 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 in a position to 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 wonderful job to reply rapidly and remediate the problem. Altogether, this highlights the energy 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 cut back danger of exploitation and shield customers. On account of this engagement, 9 new Safety Check suite (STS) assessments had been constructed to assist companions routinely examine 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, as an illustration, gives useful enablement that drives greatest practices and product excellence. Constructing on this collaborative method, Arm is complementing its product safety assurance capabilities with a bug bounty program. This funding will broaden Arm’s efforts to determine potential vulnerabilities. For extra info 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, fast fixing, and updates to the safety check suite, we’re bettering the ecosystem for Android customers. The Android Purple Crew appears to be like ahead to replicating this working relationship with different ecosystem companions to make units safer.
