CVE-2021-47051 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
spi: fsl-lpspi: Fix PM reference leak in lpspi_prepare_xfer_hardware()
pm_runtime_get_sync will increment pm usage counter even it failed. Forgetting to putting operation will result in reference leak here. Fix it by replacing it with pm_runtime_resume_and_get to keep usage counter balanced.
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Analysis
by VulDB Data Team • 12/09/2024
The vulnerability identified as CVE-2021-47051 resides within the Linux kernel's spi subsystem, specifically affecting the fsl-lpspi driver implementation. This issue represents a power management reference counting flaw that can lead to resource exhaustion and system instability. The vulnerability occurs in the lpspi_prepare_xfer_hardware() function where improper handling of power management runtime references creates a scenario where the power management usage counter becomes unbalanced. The flaw stems from the incorrect usage of pm_runtime_get_sync() which increments the power management reference counter regardless of whether the operation succeeds or fails. When this function encounters an error condition, it fails to decrement the reference counter through the corresponding pm_runtime_put_sync() call, resulting in a persistent reference leak that can accumulate over time.
The technical implementation of this vulnerability demonstrates a classic resource management error pattern where the power management subsystem's reference counting mechanism is not properly maintained. The pm_runtime_get_sync() function is designed to acquire a power management reference and increment the usage counter, but when this operation fails, the subsequent cleanup path is not properly executed. This creates a situation where the reference counter remains incremented indefinitely, preventing the hardware from entering low power states and potentially causing resource exhaustion. The flaw operates at the kernel level where power management operations are critical for system efficiency and hardware lifecycle management.
The operational impact of this vulnerability extends beyond simple resource leakage to potentially affect system stability and power consumption patterns. When the power management reference counter becomes unbalanced, the affected SPI hardware controller cannot properly transition to low power states, leading to increased power consumption and potential thermal issues. The cumulative effect of multiple reference leaks can eventually cause the system to exhaust available power management resources, potentially leading to system hangs or forced reboots. This vulnerability affects embedded systems and devices that rely on the Freescale LPSPI (Low Power SPI) controller implementation, particularly those using the fsl-lpspi driver for SPI communication operations.
The mitigation strategy for CVE-2021-47051 involves replacing the problematic pm_runtime_get_sync() call with pm_runtime_resume_and_get() in the lpspi_prepare_xfer_hardware() function. This change ensures that the power management reference counter remains balanced regardless of operation success or failure conditions. The pm_runtime_resume_and_get() function provides the appropriate behavior by both resuming the device and acquiring the reference while maintaining proper error handling. This fix aligns with established power management best practices and follows the principle of balanced reference counting. The solution addresses the root cause by ensuring that all power management operations maintain proper reference balance through the use of appropriate API calls that handle both success and failure scenarios correctly. This vulnerability demonstrates the importance of proper resource management in kernel space and aligns with CWE-474 standards for improper handling of reference counting mechanisms. The fix also supports ATT&CK technique T1547.001 related to system service manipulation by ensuring proper device power state management and preventing potential system instability caused by resource exhaustion.