CVE-2021-47132 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
mptcp: fix sk_forward_memory corruption on retransmission
MPTCP sk_forward_memory handling is a bit special, as such field is protected by the msk socket spin_lock, instead of the plain socket lock.
Currently we have a code path updating such field without handling the relevant lock:
__mptcp_retrans() -> __mptcp_clean_una_wakeup()
Several helpers in __mptcp_clean_una_wakeup() will update sk_forward_alloc, possibly causing such field corruption, as reported by Matthieu.
Address the issue providing and using a new variant of blamed function which explicitly acquires the msk spin lock.
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Analysis
by VulDB Data Team • 03/13/2025
The vulnerability CVE-2021-47132 represents a critical memory corruption issue within the Linux kernel's Multipath TCP implementation, specifically affecting the mptcp sk_forward_memory field handling. This flaw exists in the kernel's networking stack where the Multipath TCP protocol manages multiple concurrent TCP connections across different network paths. The vulnerability stems from improper locking mechanisms during retransmission operations, creating a race condition that can lead to memory corruption and potential system instability. The issue is particularly significant because it affects the core functionality of multipath TCP connections, which are used in high-performance networking environments where reliability and data integrity are paramount.
The technical flaw manifests in the specific code path involving __mptcp_retrans() function that calls __mptcp_clean_una_wakeup() without proper synchronization. Unlike standard socket operations that use the conventional socket lock, the mptcp implementation employs a specialized msk socket spin_lock for protecting the sk_forward_memory field. This specialized locking mechanism is designed to handle the unique requirements of multipath TCP's connection management, but the vulnerability occurs when the code path fails to acquire this specific lock before modifying the forward memory allocation field. The __mptcp_clean_una_wakeup() function contains several helper routines that directly update sk_forward_alloc, which can cause corruption when accessed concurrently without proper lock acquisition.
The operational impact of this vulnerability extends beyond simple memory corruption to potentially compromise the entire networking stack of affected systems. When the sk_forward_memory field becomes corrupted during retransmission operations, it can lead to incorrect buffer management decisions, packet loss, or even system crashes. This vulnerability affects systems running Linux kernel versions that include the multipath TCP implementation, particularly those in enterprise environments where high-throughput networking is critical. The race condition can be triggered during normal network operations involving retransmissions, making it difficult to predict when the corruption might occur and potentially leading to denial of service conditions or data integrity issues in applications relying on multipath TCP connections.
The fix for this vulnerability involves implementing a new variant of the affected function that explicitly acquires the msk spin lock before performing any modifications to the sk_forward_memory field. This approach aligns with the established security principle of proper resource synchronization and follows the CWE-362 pattern for concurrent access violations. The solution ensures that all modifications to the mptcp-specific memory allocation fields occur under proper locking mechanisms, preventing the race conditions that led to corruption. This fix demonstrates the importance of maintaining consistent locking strategies throughout kernel code, particularly in high-performance networking implementations where timing and synchronization are critical. The mitigation approach also reflects best practices from the ATT&CK framework's defense evasion techniques, where maintaining system integrity through proper synchronization mechanisms prevents potential exploitation of concurrency vulnerabilities. Organizations should prioritize applying this kernel patch to prevent potential exploitation of this memory corruption vulnerability in production environments.