CVE-2026-90338 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
serial: amba-pl011: keep console clock enabled for atomic writes
pl011_console_write_atomic() runs from nbcon atomic context, where sleeping is not allowed. It calls clk_enable(), which takes the common-clk enable_lock. Under PREEMPT_RT that is a sleeping lock: clk_enable_lock() first tries spin_trylock_irqsave(), but on contention falls back to spin_lock_irqsave(). Therefore, an atomic-context printk on an RT kernel with a clk-backed pl011 can trip:
BUG: sleeping function called from invalid context at spinlock_rt.c:48 __might_resched from rt_spin_lock rt_spin_lock from clk_enable_lock clk_enable_lock from clk_enable clk_enable from pl011_console_write_atomic ... from vprintk_emit
This was found and reproduced on PREEMPT_RT. Arm32 and arm64 DT SoCs are affected; arm64 SBSA/ACPI has no clk, so clk_enable(NULL) short-circuits before the lock. In addition, write_atomic() may be invoked from NMI context and is documented to avoid locking. Removing clk_enable() from the callback also avoids a potentially unsafe NMI acquisition of the common-clock enable_lock.
An nbcon atomic-capable console must be printable from any context, so the clock cannot be gated between writes. Enable the clock while the console is available for output: use clk_prepare_enable() in pl011_console_setup(), release it via clk_disable_unprepare() in the console .exit() callback, and drop the per-write clk_enable()/clk_disable() pairs from write_atomic() and write_thread().
When printk suspends consoles, drop the reference after uart_suspend_port() stops console access and restore it before uart_resume_port() -- but only if suspend actually marked the port suspended (a wake-capable tty stays running and must keep its clock), and keep it when console_suspend_enabled is false so no_console_suspend works.
The active power cost of keeping the clock enabled is platform-dependent: none where the UART clock is a fixed always-on oscillator, real where it is a gateable clock branch, which then cannot be gated (nor possibly can its parent clocks) while the console is available for output. When serial core actually suspends the port, the reference is released so the clock provider can gate the clock tree.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified in the amba-pl011 UART driver involves an improper use of sleeping functions within atomic execution contexts, specifically affecting systems running with PREEMPT_RT patches on Arm32 and arm64 platforms that utilize device tree descriptions for serial controllers. The core technical flaw resides in the pl011_console_write_atomic function, which is designed to operate from nbcon atomic context where preemption and sleeping are strictly prohibited due to the non-blocking nature required for reliable console output during critical system events. Within this constrained environment, the original implementation invoked clk_enable() to ensure the UART hardware clock was active before writing data. However, on PREEMPT_RT kernels, the underlying common-clock enable_lock is implemented as a sleeping lock rather than a spinlock. When contention occurs, the locking mechanism falls back to a sleepable operation via rt_spin_lock_irqsave(), which directly violates atomic context constraints by potentially causing task rescheduling or blocking while interrupts are disabled. This violation triggers a kernel bug report indicating that a sleeping function was called from an invalid context, effectively crashing the system or halting console output during critical debugging scenarios such as early boot failures or panic states where reliable logging is paramount.
The operational impact of this vulnerability extends beyond simple stability issues to include potential security and reliability degradation in high-availability environments. By introducing sleepable locks into atomic contexts, the kernel risks deadlocks if other parts of the system hold related resources while attempting console output during a crash dump or emergency log emission. Furthermore, because write_atomic can be invoked from Non-Maskable Interrupt (NMI) context, which is documented to avoid any form of locking entirely, the presence of clk_enable() introduces an unsafe acquisition of the common-clock enable_lock in NMI handlers. This not only compromises system stability but also undermines the predictability required for forensic analysis during catastrophic failures. The vulnerability specifically impacts Arm32 and arm64 System-on-Chips described via Device Tree that rely on gateable clocks, whereas ARM SBSA/ACPI systems are unaffected because they do not use clk_enable in this manner, allowing the call to short-circuit safely when no clock is present.
From a standards perspective, this issue aligns with CWE-829, which covers Inclusion of Functionality from Untrusted Control Sources, though more accurately it reflects CWE-674: Unnecessary Complexity or Redundancy in Code that leads to improper resource management and context violations. It also relates to ATT&CK technique T1053, Scheduled Task/Job, if one considers the timing aspect of when these atomic writes occur during system events, although the primary classification is a kernel-level concurrency violation rather than an external attack vector. The root cause is fundamentally a design flaw where power management optimizations were incorrectly applied to code paths that must remain strictly non-blocking and lock-free under all execution contexts, including interrupt and NMI levels.
The remediation strategy involves restructuring the clock management lifecycle for the pl011 console driver to decouple hardware availability from individual write operations. Instead of enabling and disabling the clock on every atomic or threaded write attempt, which is both inefficient and dangerous in atomic contexts, the solution mandates keeping the clock enabled for the entire duration that the console is available for output. This is achieved by calling clk_prepare_enable() during the pl011_console_setup phase when the driver initializes and binding it to the device's availability. Conversely, the clock reference is released via clk_disable_unprepare() in the console exit callback when the subsystem shuts down. This approach eliminates all per-write clk_enable and clk_disable pairs from both write_atomic and write_thread functions, thereby removing the problematic locking calls from atomic and interrupt contexts entirely.
Additionally, the fix addresses power management interactions during system suspend and resume cycles to ensure that clock gating does not interfere with console functionality when required by administrative policies. When printk suspends consoles, the driver must carefully manage its reference count relative to uart_suspend_port operations. The reference is dropped only after uart_suspend_port has explicitly stopped console access, ensuring no race conditions occur where a write might attempt to use a disabled clock. Similarly, upon resume, the reference is restored before uart_resume_port begins operation. Crucially, this logic respects wake-capable tty states that must remain active and honors the no_console_suspend kernel parameter by keeping the clock enabled when console suspension is explicitly disabled. While maintaining the clock continuously incurs a platform-dependent power cost—ranging from negligible for fixed always-on oscillators to significant for gateable clock branches—the trade-off is necessary to preserve system stability and debuggability in real-time preemptible environments, with the assurance that resources are properly released when the port is genuinely suspended by the serial core.