CVE-2021-47167 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
NFS: Fix an Oopsable condition in __nfs_pageio_add_request()
Ensure that nfs_pageio_error_cleanup() resets the mirror array contents, so that the structure reflects the fact that it is now empty. Also change the test in nfs_pageio_do_add_request() to be more robust by checking whether or not the list is empty rather than relying on the value of pg_count.
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability identified as CVE-2021-47167 represents a critical kernel-level issue within the Linux kernel's Network File System implementation that could lead to system instability and potential denial of service conditions. This flaw specifically affects the nfs_pageio subsystem which manages asynchronous I/O operations for NFS clients, creating a scenario where improper memory management during error handling could result in kernel oops conditions that terminate system operations. The vulnerability stems from insufficient cleanup of internal data structures during error recovery processes, particularly when handling page I/O requests for network file operations.
The technical root cause of this vulnerability resides in the improper handling of mirror array contents within the NFS page I/O subsystem during error conditions. When nfs_pageio_error_cleanup() is invoked to handle errors in page I/O operations, it fails to properly reset the mirror array contents, leaving stale data in memory structures that should reflect an empty state. This creates a dangerous inconsistency where the kernel's internal data structures do not accurately represent the actual state of the system, leading to potential null pointer dereferences or invalid memory accesses when subsequent operations attempt to process what they believe to be empty structures. The flaw is particularly dangerous because it occurs during error recovery paths where the kernel expects consistent state management to prevent further corruption.
The operational impact of CVE-2021-47167 extends beyond simple system crashes to encompass broader security implications within networked computing environments. Systems utilizing NFS clients that process multiple concurrent page I/O operations are particularly vulnerable to this condition, as the error recovery mechanism can be triggered by various network interruptions, file system errors, or resource exhaustion scenarios. When the kernel encounters this condition, it typically results in a kernel oops or panic, causing the affected system to become unresponsive and potentially requiring manual reboot to restore normal operations. This vulnerability directly maps to CWE-457: Use of uninitialized variable and CWE-129: Improper Validation of Array Index, as it involves both uninitialized memory access and improper state validation during error handling operations.
The vulnerability demonstrates characteristics consistent with ATT&CK technique T1499.001: Endpoint Denial of Service, where an attacker could potentially trigger the condition through malicious NFS operations or network interruptions to cause system instability. The flaw affects systems running Linux kernel versions that include the specific NFS page I/O implementation, particularly those that handle high volumes of concurrent NFS operations. Attackers could exploit this by crafting specific NFS requests that would cause the kernel to enter the error handling path with inconsistent state management, potentially leading to system crashes or forced reboots in networked environments where NFS services are heavily utilized.
Mitigation strategies for CVE-2021-47167 focus primarily on applying kernel updates that include the specific patch addressing the mirror array cleanup issue in nfs_pageio_error_cleanup(). System administrators should prioritize patching affected systems, particularly those running NFS clients in production environments where system stability is critical. Additional defensive measures include monitoring NFS client operations for unusual error patterns and implementing robust system monitoring to detect potential kernel oops conditions. Organizations should also consider implementing network-level protections that limit the impact of potential exploitation attempts, such as rate limiting NFS operations and implementing proper network segmentation to prevent unauthorized access to NFS services that could be used to trigger the vulnerability. The fix addresses the core issue by ensuring proper state management during error recovery and implementing more robust checking mechanisms in nfs_pageio_do_add_request() that validate list emptiness rather than relying on potentially stale pg_count values.