CVE-2021-47601 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
tee: amdtee: fix an IS_ERR() vs NULL bug
The __get_free_pages() function does not return error pointers it returns NULL so fix this condition to avoid a NULL dereference.
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Analysis
by VulDB Data Team • 10/04/2025
The vulnerability identified as CVE-2021-47601 resides within the Linux kernel's trusted execution environment subsystem, specifically affecting the amdtee driver implementation. This flaw demonstrates a critical programming error that could lead to system instability and potential security implications within the kernel's memory management and error handling mechanisms. The vulnerability manifests in the interaction between kernel memory allocation functions and error checking logic, creating a condition where improper error handling could result in system crashes or unexpected behavior.
The technical root cause of this vulnerability stems from a fundamental misunderstanding of the return values from the __get_free_pages() kernel function. This function is designed to allocate contiguous pages of memory for kernel use and traditionally returns NULL upon allocation failure rather than error pointers. However, the amdtee driver implementation incorrectly assumed that __get_free_pages() would return error pointers similar to other kernel allocation functions. This erroneous assumption led to the use of IS_ERR() macro checks against a return value that would never contain error pointers, creating a logical flaw in the error handling path.
The operational impact of this vulnerability extends beyond simple memory allocation failures, potentially enabling a NULL dereference condition that could crash the kernel or allow for privilege escalation attacks. When the amdtee driver attempts to process memory allocation results, the incorrect error checking logic means that NULL values from __get_free_pages() are not properly handled, leading to attempts to dereference null pointers. This condition violates the fundamental principles of kernel memory safety and can result in system panics or unpredictable behavior that undermines the integrity of the trusted execution environment.
This vulnerability aligns with CWE-476, which specifically addresses NULL pointer dereference conditions in software implementations, and represents a classic example of improper error handling in kernel space code. The flaw demonstrates how seemingly minor coding errors in kernel subsystems can have significant security implications, particularly within trusted execution environments where memory management and error handling are critical for system security. From an ATT&CK perspective, this vulnerability could potentially be leveraged as part of a privilege escalation technique, though the specific attack vectors would depend on how the error condition manifests within the broader kernel execution context.
The mitigation strategy for CVE-2021-47601 involves correcting the error handling logic within the amdtee driver to properly account for the actual return values of __get_free_pages(). This requires replacing IS_ERR() checks with appropriate NULL pointer validation and ensuring that all memory allocation results are properly validated before subsequent operations. The fix demonstrates the importance of understanding kernel API contracts and the specific return value semantics of memory allocation functions. System administrators should ensure that kernel updates containing this fix are applied promptly, as the vulnerability exists in the core kernel memory management subsystem and affects all systems running affected kernel versions. The resolution emphasizes the critical need for thorough code review processes and adherence to kernel development best practices, particularly when dealing with error handling in security-sensitive subsystems.