CVE-2020-36784 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
i2c: cadence: fix reference leak when pm_runtime_get_sync fails
The PM reference count is not expected to be incremented on return in functions cdns_i2c_master_xfer and cdns_reg_slave.
However, pm_runtime_get_sync will increment pm usage counter even failed. Forgetting to putting operation will result in a reference leak here.
Replace it with pm_runtime_resume_and_get to keep usage counter balanced.
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Analysis
by VulDB Data Team • 12/06/2024
The vulnerability CVE-2020-36784 resides within the Linux kernel's I2C subsystem, specifically in the Cadence I2C controller driver implementation. This issue represents a reference leak that occurs during power management operations, fundamentally compromising the system's resource management integrity. The flaw manifests in the handling of power management runtime references within two critical functions: cdns_i2c_master_xfer and cdns_reg_slave, where the expected behavior of reference count management becomes inconsistent with actual implementation.
The technical root cause stems from an improper interaction between the power management subsystem and the I2C driver's resource handling mechanisms. When pm_runtime_get_sync fails, it still increments the power management usage counter despite the operation's unsuccessful completion. This behavior creates a scenario where the reference count becomes unbalanced, as the system fails to decrement the counter during error conditions. The function pm_runtime_get_sync is designed to acquire a runtime power management reference, but when it encounters failure, it does not automatically release the acquired reference, leading to a persistent resource leak that accumulates over time.
The operational impact of this vulnerability extends beyond simple resource consumption issues, potentially leading to system instability and degraded performance in embedded systems utilizing Cadence I2C controllers. The reference leak creates a gradual accumulation of unreleased power management references, which can eventually exhaust available power management resources or cause the system to enter an inconsistent power state. This vulnerability is particularly concerning in embedded environments where power management is critical for device operation and battery life optimization, as it can lead to unexpected device shutdowns or failure to enter low power states when required.
The fix implemented addresses this issue by replacing pm_runtime_get_sync with pm_runtime_resume_and_get, which properly maintains the usage counter balance regardless of operation success or failure. This change ensures that the power management reference count remains consistent by automatically handling the reference decrement when operations fail. The solution aligns with established power management best practices and follows the principle of balanced reference counting, where every increment must have a corresponding decrement to maintain system resource integrity. This remediation approach directly addresses the underlying CWE-170 issue related to improper handling of reference counts and prevents the accumulation of leaked references that could compromise system stability.
From a cybersecurity perspective, this vulnerability represents a potential denial of service vector that could be exploited in embedded systems or IoT devices where power management is critical for operational reliability. The reference leak could be leveraged to gradually consume system resources, leading to service unavailability or device malfunction. The ATT&CK framework would categorize this under privilege escalation or denial of service techniques, as it affects the fundamental power management capabilities of the system. Organizations deploying Linux-based systems with Cadence I2C controllers should prioritize this patch to prevent potential exploitation that could lead to system instability or unauthorized access through compromised power management states. The vulnerability demonstrates the importance of proper resource management in kernel space, where even seemingly minor reference counting errors can have significant operational consequences across embedded and IoT deployments.