CVE-2020-36779 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
i2c: stm32f7: fix reference leak when pm_runtime_get_sync fails
The PM reference count is not expected to be incremented on return in these stm32f7_i2c_xx serious functions.
However, pm_runtime_get_sync will increment the PM reference count 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 identified as CVE-2020-36779 resides within the Linux kernel's i2c subsystem, specifically affecting the stm32f7 I2C driver implementation. This issue represents a subtle but significant resource management flaw that can lead to system instability and potential denial of service conditions. The vulnerability manifests in the stm32f7_i2c_xx functions where power management runtime operations are improperly handled, creating a scenario where reference counts become unbalanced and resources remain allocated indefinitely.
The technical root cause stems from the improper handling of power management runtime operations within the I2C driver code. When pm_runtime_get_sync fails during its execution, it still increments the power management reference count as part of its internal implementation logic. This behavior contradicts the expected operational pattern where reference count modifications should be symmetrical - meaning if a reference count is incremented, it must be decremented elsewhere. The failure to account for this behavior results in a reference leak where the power management subsystem maintains an inflated reference count that never gets properly reduced.
This vulnerability operates at the kernel level within the Linux kernel's power management framework and specifically impacts devices utilizing the stm32f7 I2C controller implementation. The operational impact extends beyond simple resource leakage to potentially compromise system stability and resource availability. When multiple I2C operations fail in this manner, the accumulated reference leaks can lead to system resource exhaustion, preventing proper power management transitions and potentially causing the device to become unresponsive or enter an unstable state. The vulnerability is particularly concerning in embedded systems where memory and power management resources are constrained and critical for system operation.
The fix implemented for this vulnerability addresses the specific reference counting issue by replacing pm_runtime_get_sync with pm_runtime_resume_and_get. This change ensures proper balancing of the usage counter through symmetric operations, where the reference count is incremented only when the operation succeeds and properly managed throughout the function execution lifecycle. The solution aligns with established power management best practices and maintains the expected behavior of the power management subsystem while preventing the accumulation of reference counts that would otherwise lead to resource leaks. This remediation follows the principles outlined in CWE-404, which addresses improper resource management through improper handling of reference counts in system resources. The fix also demonstrates adherence to ATT&CK techniques related to system resource exhaustion and privilege escalation through improper resource management, ensuring that the system maintains proper operational integrity and prevents potential attackers from exploiting the reference leak to gain unauthorized access or cause system instability.
The vulnerability represents a classic example of improper resource management within kernel space operations, where the failure to properly balance reference counting operations creates lasting system impacts. The resolution ensures that the power management subsystem operates correctly and maintains proper resource accounting, preventing the accumulation of reference counts that could lead to system resource exhaustion. This fix is critical for maintaining the reliability and stability of embedded systems that depend on proper power management and resource handling within the Linux kernel's I2C subsystem.