A critical race condition vulnerability in the Linux kernel’s POSIX CPU timers has been exposed through a detailed proof-of-concept, one of the most sophisticated kernel exploits targeting Android devices.
CVE-2025-38352 represents a use-after-free (UAF) vulnerability in the Linux kernel’s POSIX CPU timers implementation.
The flaw was previously reported under limited, targeted exploitation in real-world Android environments.
Now, security researchers have released a fully functional proof of concept demonstrating the vulnerability’s mechanics.
The vulnerability exists in the handle_posix_cpu_timers() kernel function, which processes fired timers during CPU scheduler ticks.
The critical flaw occurs when a task transitions to zombie state a process state that occurs after a thread exits but before its parent process collects its exit status.
Technical Exploitation Mechanics
The attack exploits a race condition between two critical kernel operations.
When handle_posix_cpu_timers() releases its lock after collecting firing timers, an attacker can simultaneously reap the zombie task and delete the timer, causing the kernel to free the timer structure via RCU (Read-Copy-Update) mechanism while it’s still being accessed. This creates a classic use-after-free scenario.
The exploitation requires precise timing coordination. A child process creates a non-main thread with a POSIX CPU timer set to fire immediately after the thread transitions to zombie state.
A parent process, running via ptrace, reaps the zombie thread while the child process simultaneously calls timer_delete().
When the race window is won, the freed timer is accessed in handle_posix_cpu_timers(), triggering kernel memory corruption.
Notably, this vulnerability exclusively affects 32-bit Android devices. Modern 64-bit Linux kernels and x86 architectures include the CONFIG_POSIX_CPU_TIMERS_TASK_WORK configuration option, which blocks exploitation.
Only 32-bit ARM devices lack this protection, explaining why exploits remain limited to specific Android implementations.
Testing requires careful environmental setup: kernel version 6.12.33 (latest vulnerable LTS), multi-CPU configuration, and disabled KASAN detection.
Researchers demonstrated successful exploitation both with and without kernel address sanitizer, producing distinct crash signatures.
Proof of Concept Details
The working PoC combines parent-child process coordination through ptrace attachment, multi-threaded synchronization using barriers, and CPU affinity pinning.
The exploit achieves a success rate of approximately 50% on properly configured systems, validating the triggering methodology of the race condition, as reported by Security Researcher Faraz.
Security impact manifests through KASAN-detected slab-use-after-free writes and kernel warnings in send_sigqueue(). Both demonstrate successful memory corruption within kernel structures.
While current PoCs remain largely proof-of-concept, researchers note potential for privilege escalation through cross-cache heap exploitation techniques.
However, extending the limited race window within interrupt context presents engineering challenges.
The Linux kernel community has already deployed patches, and affected systems should prioritize immediate updates to mitigated kernel versions.
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