CVE-2024-38618 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
ALSA: timer: Set lower bound of start tick time
Currently ALSA timer doesn't have the lower limit of the start tick time, and it allows a very small size, e.g. 1 tick with 1ns resolution for hrtimer. Such a situation may lead to an unexpected RCU stall, where the callback repeatedly queuing the expire update, as reported by fuzzer.
This patch introduces a sanity check of the timer start tick time, so that the system returns an error when a too small start size is set. As of this patch, the lower limit is hard-coded to 100us, which is small enough but can still work somehow.
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Analysis
by VulDB Data Team • 10/05/2025
The vulnerability described in CVE-2024-38618 resides within the Linux kernel's Advanced Linux Sound Architecture (ALSA) timer subsystem, specifically addressing a critical flaw in how the system handles timer start tick time validation. This issue represents a significant security concern as it can lead to system instability and potential denial of service conditions. The vulnerability stems from the absence of proper bounds checking for timer start tick values, allowing malicious or malformed inputs to specify extremely small timer intervals that can cause system-wide performance degradation.
The technical flaw manifests when the ALSA timer subsystem accepts timer start tick values that are unreasonably small, specifically allowing values as low as 1 tick with 1 nanosecond resolution for high-resolution timers. This configuration creates a scenario where timer expiration callbacks repeatedly queue updates, leading to what is known as an RCU (Read-Copy-Update) stall condition. The RCU mechanism, designed to provide efficient read-side critical sections, becomes overwhelmed by the excessive callback queuing, potentially causing the entire system to become unresponsive or exhibit severe performance degradation. This behavior was originally discovered and reported by fuzzing tools that systematically test software for unexpected behaviors and edge cases.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially compromising system availability and stability. When the timer subsystem enters an RCU stall condition, it can cause the system to hang or become unresponsive to user input and other critical operations. The vulnerability affects systems running Linux kernels with ALSA audio support, particularly those that handle real-time audio processing or systems where audio timer precision is critical. The specific lower limit of 100 microseconds introduced by the patch represents a balance between system functionality and security, ensuring that timer operations remain within reasonable bounds while still allowing for sufficient precision for most audio applications.
This vulnerability aligns with CWE-682, which describes "Incorrect Calculation," specifically in the context of timer interval calculations where insufficient validation leads to incorrect system behavior. The fix implemented in this patch follows the principle of input validation and sanity checking, which is fundamental to secure system design practices. From an ATT&CK perspective, this vulnerability could be leveraged in a denial of service attack (T1499) where an attacker might exploit the timer subsystem to cause system instability or unresponsiveness. The patch addresses the root cause by implementing a hard-coded minimum threshold of 100 microseconds for timer start tick time, ensuring that all timer operations remain within predictable and safe boundaries.
The mitigation strategy implemented in this patch represents a defensive programming approach that prevents the exploitation of boundary conditions in timer handling. By establishing a minimum threshold for timer start tick time, the system prevents the creation of excessively aggressive timer callbacks that could overwhelm the RCU mechanism. This approach demonstrates the importance of proper input validation and the need for robust error handling in kernel subsystems. The 100 microsecond minimum value chosen provides sufficient granularity for most audio applications while preventing the pathological behavior that led to the RCU stall condition. This fix ensures that the ALSA timer subsystem operates within predictable performance parameters and maintains system stability under all normal operating conditions.