CVE-2026-97481 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
serial: 8250: fix possible ISR soft lockup
There are rare cases in which the host gets stuck in the ISR because it is flooded with messages during the startup phase.
The reason for the soft lockup in the ISR is the missing FIFO error IRQ (FIFOE) handling. Not handling it and reporting IRQ_HANDLED triggers the IRQ immediately again.
Fix this by adding a check for the FIFOE status and clearing the FIFO if no data is ready (DR).
This behavior was observed on an AM62L device which uses the OMAP 8250 driver. Fix it for all 8250 drivers, since the OMAP driver's special IRQ setup handling may trigger this behavior more frequently, but it is not ensured that other 8250 drivers aren't affected.
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Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel serial subsystem contains a critical flaw within the 8250 UART driver family that can lead to system instability through an interrupt service routine soft lockup. This vulnerability manifests during the initialization or startup phase of serial communication, where the host processor becomes trapped in a tight loop handling interrupts. The root cause is identified as missing logic for handling FIFO error conditions, specifically the absence of proper processing for the FIFO Error (FIFOE) status bit. When this condition occurs and the driver fails to address it correctly by reporting IRQ_HANDLED without clearing the underlying hardware state, the interrupt controller immediately re-triggers the same interrupt. This creates a recursive loop where the CPU remains permanently occupied servicing an unresolvable or continuously recurring error signal, effectively halting other system operations that rely on scheduler time slices.
From a technical perspective, this issue represents a failure in proper resource management and hardware state synchronization within the kernel's device driver layer. The 8250 architecture relies heavily on interrupt-driven data transfer to maintain throughput without consuming excessive CPU cycles during idle periods. However, when the FIFO error flag is set due to conditions such as buffer overflow or framing errors during startup, the lack of explicit handling means the hardware continues to assert its interrupt line. By not checking for this status and clearing the associated FIFO if no valid data is ready via the Data Ready (DR) check, the driver inadvertently signals successful completion of an interrupted task that was actually in a failed state. This misclassification forces the kernel's generic IRQ handler to invoke the same ISR again immediately, resulting in a soft lockup where the system appears frozen or unresponsive despite being technically alive at the hardware level.
The operational impact of this vulnerability is significant for systems relying on serial console access or critical UART-based communication channels. A soft lockup typically results in the freezing of the affected CPU core and can potentially lead to broader system instability depending on kernel configuration, such as whether watchdog timers are active. In embedded environments using devices like the AM62L with OMAP 8250 drivers, this behavior is observed more frequently due to specific IRQ setup handling characteristics inherent to that platform's architecture. However, because the underlying logic flaw exists in the common 8250 driver code shared across multiple architectures, other platforms utilizing similar UART controllers are also at risk of experiencing these lockups under rare but plausible startup conditions involving noisy lines or misconfigured baud rates that trigger FIFO errors early in the connection process.
This vulnerability aligns with CWE-764: Multiple Locks of a Critical Resource and CWE-835: Loop with Unreachable Exit Condition, as it involves entering an infinite loop due to improper state management within a critical execution path protected by interrupt masking. In terms of MITRE ATT&CK mapping, this scenario reflects aspects of T1059 Command and Scripting Interpreter if the lockup prevents security monitoring tools from executing, or more broadly relates to availability impacts similar to Denial of Service (DoS) vectors where system resources are exhausted through inefficient processing loops rather than malicious payload execution. The fix involves implementing a robust check for the FIFOE status bit within the interrupt handler and ensuring that the hardware FIFO is cleared if no data is ready, thereby breaking the recursive interrupt trigger cycle and allowing normal kernel scheduling to resume.
To mitigate this risk, system administrators should apply the latest kernel updates provided by their distribution vendors which include this specific patch for the 8250 serial driver. For environments where immediate patching is not feasible, configuring hardware flow control or adjusting line discipline settings may reduce the likelihood of triggering FIFO errors during startup phases. Additionally, enabling kernel watchdog timers can help detect and recover from such soft lockups by resetting the affected CPU core if it remains unresponsive for a defined period, thereby preserving overall system availability even when individual drivers encounter this flaw.