CVE-2021-47040 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
io_uring: fix overflows checks in provide buffers
Colin reported before possible overflow and sign extension problems in io_provide_buffers_prep(). As Linus pointed out previous attempt did nothing useful, see d81269fecb8ce ("io_uring: fix provide_buffers sign extension").
Do that with help of check__overflow helpers. And fix struct io_provide_buf::len type, as it doesn't make much sense to keep it signed.
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Analysis
by VulDB Data Team • 08/17/2025
The vulnerability CVE-2021-47040 represents a critical security flaw in the Linux kernel's io_uring subsystem, specifically within the io_provide_buffers_prep() function. This issue stems from inadequate overflow and sign extension checks that could potentially allow malicious actors to exploit buffer management operations. The vulnerability was identified through careful code analysis and reported by Colin, who recognized the potential for integer overflows and sign extension problems that could compromise system integrity. The problem manifests in how the kernel handles buffer provisioning operations, where improper validation of buffer parameters could lead to unpredictable behavior and potential privilege escalation.
The technical implementation flaw involves the io_provide_buf::len field which was originally defined with a signed integer type, creating confusion in overflow detection mechanisms. The previous attempt to address this issue through commit d81269fecb8ce proved ineffective and did not provide meaningful protection against the underlying vulnerabilities. This oversight demonstrates a fundamental weakness in the kernel's buffer management validation logic, where the signed nature of the length parameter created ambiguity in overflow detection and allowed for potential exploitation through carefully crafted input parameters. The fix implemented in this CVE addresses the core problem by employing check__overflow helper functions to properly validate buffer parameters and redefines the struct io_provide_buf::len field to use an unsigned type, eliminating the confusion and potential for sign extension attacks.
The operational impact of this vulnerability extends beyond simple buffer management issues and could potentially enable local privilege escalation attacks within the Linux kernel environment. Attackers could exploit the integer overflow conditions to manipulate buffer boundaries, potentially leading to memory corruption and arbitrary code execution with kernel privileges. This represents a significant concern for systems relying heavily on io_uring for high-performance I/O operations, as the vulnerability affects the core kernel subsystem responsible for asynchronous I/O handling. The attack surface is particularly relevant for server environments and systems with high I/O throughput requirements where io_uring is actively utilized for performance optimization.
Mitigation strategies for CVE-2021-47040 require immediate kernel updates to the patched version that implements proper overflow checking using check__overflow helpers and corrects the struct io_provide_buf::len type definition. System administrators should prioritize applying the relevant security patches as soon as possible, particularly in production environments where the io_uring subsystem is actively used. The fix aligns with established security practices for kernel vulnerability remediation and follows the principles outlined in CWE-190, which addresses integer overflow conditions. Organizations should also implement monitoring for unusual I/O patterns that might indicate exploitation attempts and consider implementing additional security controls around kernel module loading and I/O operation management. This vulnerability demonstrates the critical importance of proper integer handling and overflow detection in kernel code, as highlighted by ATT&CK technique T1068 which covers local privilege escalation through kernel vulnerabilities. The remediation process should include comprehensive testing to ensure that the patched kernel maintains compatibility with existing io_uring-based applications while eliminating the identified security risks.