CVE-2021-4441 in Linux
Summary
by MITRE • 08/22/2024
In the Linux kernel, the following vulnerability has been resolved:
spi: spi-zynq-qspi: Fix a NULL pointer dereference in zynq_qspi_exec_mem_op()
In zynq_qspi_exec_mem_op(), kzalloc() is directly used in memset(), which could lead to a NULL pointer dereference on failure of kzalloc().
Fix this bug by adding a check of tmpbuf.
This bug was found by a static analyzer. The analysis employs differential checking to identify inconsistent security operations (e.g., checks or kfrees) between two code paths and confirms that the inconsistent operations are not recovered in the current function or the callers, so they constitute bugs.
Note that, as a bug found by static analysis, it can be a false positive or hard to trigger. Multiple researchers have cross-reviewed the bug.
Builds with CONFIG_SPI_ZYNQ_QSPI=m show no new warnings, and our static analyzer no longer warns about this code.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/12/2024
The vulnerability identified as CVE-2021-4441 resides within the Linux kernel's SPI subsystem, specifically affecting the zynq-qspi driver implementation. This issue manifests as a NULL pointer dereference condition that occurs during memory operation execution within the zynq_qspi_exec_mem_op() function. The flaw represents a classic memory management error where the kernel attempts to use a pointer that has not been properly validated after allocation, creating a potential system crash scenario that could be exploited to cause denial of service conditions.
The technical root cause stems from improper error handling in the memory allocation sequence where kzalloc() is used directly within memset() operations without subsequent validation of the allocation result. When kzalloc() fails to allocate memory, it returns a NULL pointer which then gets dereferenced in the subsequent memory operations. This pattern violates standard kernel programming practices and creates a direct pathway for system instability. The vulnerability was identified through automated static analysis techniques that employ differential checking methodologies to detect inconsistent security operations across different code execution paths, demonstrating the sophisticated nature of modern vulnerability discovery tools.
From an operational perspective, this vulnerability affects systems utilizing Xilinx Zynq SoC platforms that rely on the quad SPI interface for memory operations. The impact extends to embedded systems and IoT devices that depend on the zynq-qspi driver for external memory access, potentially creating widespread disruption across automotive, industrial control, and networking equipment. The vulnerability's trigger mechanism makes it particularly concerning as it can occur during normal operation when memory allocation fails, though the static analysis approach suggests it may be difficult to reproduce in practice. The fact that multiple researchers cross-verified the finding adds credibility to its validity while the absence of new warnings in builds with CONFIG_SPI_ZYNQ_QSPI=m indicates the fix has been properly implemented.
The mitigation strategy involves adding proper validation checks for the tmpbuf pointer before proceeding with memory operations, ensuring that all allocation results are verified before use. This fix aligns with established security practices and follows the principle of defensive programming where all external inputs and system resources are validated before use. The vulnerability demonstrates the importance of comprehensive static analysis in kernel development and highlights the need for consistent error handling patterns throughout the codebase. The resolution addresses the underlying CWE-476 principle of NULL pointer dereference while maintaining the driver's functionality and system stability. The fix also reflects the broader ATT&CK framework consideration for kernel-level vulnerabilities, where such issues can serve as initial access points for more sophisticated attacks targeting embedded system security.