CVE-2026-90305 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ARM: 9483/1: select HAVE_POSIX_CPU_TIMERS_TASK_WORK
Commit c6e61c06d606 ("ARM: 9463/1: Allow to enable RT") enabled PREEMPT_RT on ARM but did not select HAVE_POSIX_CPU_TIMERS_TASK_WORK. This leaves CONFIG_POSIX_CPU_TIMERS_TASK_WORK disabled, so CPU timers expire in hard IRQ context.
On PREEMPT_RT this makes run_posix_cpu_timers() take the sleeping sighand->siglock:
BUG: sleeping function called from invalid context at spinlock_rt.c:48 rt_spin_lock from lock_task_sighand lock_task_sighand from run_posix_cpu_timers run_posix_cpu_timers from update_process_times
ARM handles TIF_NOTIFY_RESUME on all return-to-user paths, including v7-M. ARM32 KVM host support was removed by commit 541ad0150ca4 ("arm: Remove 32bit KVM host support"), so the select need not be conditional on KVM.
Select it to defer POSIX CPU timer expiry to task context.
Reproduced with setrlimit(RLIMIT_CPU, ...) and a busy loop. The same path is used by setitimer(ITIMER_PROF or ITIMER_VIRTUAL) and POSIX CPU timers created with timer_create().
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability described involves an incorrect configuration dependency within the ARM architecture support of the Linux kernel, specifically concerning the handling of POSIX CPU timers under the PREEMPT_RT real-time patchset. The root cause stems from commit c6e61c06d606 which enabled PREEMPT_RT on ARM architectures but failed to select the HAVE_POSIX_CPU_TIMERS_TASK_WORK configuration option. This omission results in CONFIG_POSIX_CPU_TIMERS_TASK_WORK remaining disabled, which forces CPU timers to expire within hard interrupt context rather than being deferred to task context as required by real-time scheduling constraints.
In standard Linux kernel operation, POSIX CPU timer expirations are typically handled in a manner that respects the sleeping capabilities of the current execution context. However, when PREEMPT_RT is enabled, certain spinlocks such as sighand->siglock become mutexes that allow sleeping. The function run_posix_cpu_timers() attempts to acquire this lock while executing in hard IRQ context due to the missing configuration select. This leads to a critical violation of kernel locking rules because interrupt contexts are not allowed to sleep or block on locks that might cause task switching.
The operational impact is severe, manifesting as a BUG message indicating a sleeping function called from an invalid context at spinlock_rt.c:48. The call trace reveals the sequence rt_spin_lock leading into lock_task_sighand and subsequently run_posix_cpu_timers invoked by update_process_times. This condition effectively causes kernel panics or system instability whenever CPU timers are triggered, such as through setrlimit with RLIMIT_CPU, setitimer for ITIMER_PROF or ITIMER_VIRTUAL, or POSIX timers created via timer_create(). The vulnerability is reproducible under load conditions where these timers expire frequently.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation regarding configuration parameters and CWE-674 Uncontrolled Resource Consumption if the instability leads to denial of service through system crashes. In terms of ATT&CK mapping, while not an exploit vector per se, it represents a weakness in System Configuration that could be leveraged for Denial of Service by triggering timer expirations on affected systems running PREEMPT_RT kernels.
The resolution involves selecting HAVE_POSIX_CPU_TIMERS_TASK_WORK to ensure POSIX CPU timer expiry is deferred to task context where sleeping locks are permitted. This change does not need to be conditional on KVM support since ARM32 KVM host support was previously removed, simplifying the dependency logic. Mitigation for systems unable to immediately patch involves avoiding PREEMPT_RT configurations or disabling specific timer mechanisms that trigger this code path until an updated kernel is deployed.