CVE-2021-4149 in Linux
Summary
by MITRE • 03/24/2022
A vulnerability was found in btrfs_alloc_tree_b in fs/btrfs/extent-tree.c in the Linux kernel due to an improper lock operation in btrfs. In this flaw, a user with a local privilege may cause a denial of service (DOS) due to a deadlock problem.
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Analysis
by VulDB Data Team • 03/25/2022
The vulnerability identified as CVE-2021-4149 represents a critical flaw in the Linux kernel's btrfs file system implementation that specifically targets the btrfs_alloc_tree_b function located in fs/btrfs/extent-tree.c. This issue manifests as an improper lock operation that creates a potential deadlock condition within the kernel's memory management subsystem. The vulnerability exists in the way the btrfs file system handles concurrent access to shared resources during tree allocation operations, creating a scenario where multiple kernel threads can become indefinitely blocked waiting for each other's resources.
The technical nature of this vulnerability stems from inadequate synchronization mechanisms within the btrfs implementation where the locking strategy fails to properly handle nested or recursive lock acquisition patterns. When the btrfs_alloc_tree_b function attempts to allocate memory for tree structures, it encounters a scenario where it must acquire multiple locks in a specific order, but the locking mechanism does not prevent circular wait conditions that lead to system-wide deadlocks. This improper lock operation creates a condition where kernel threads attempting to perform btrfs operations become stuck in an indefinite waiting state, effectively rendering the system unresponsive to further file system operations.
The operational impact of this vulnerability is significant as it allows a local user with minimal privileges to trigger a denial of service condition that can bring the entire system to a halt. The deadlock condition affects the kernel's ability to manage memory allocations and file system operations, causing the system to become unresponsive to legitimate user requests and potentially leading to complete system crashes or reboots. This vulnerability is particularly concerning in multi-user environments or systems where btrfs file systems are actively used, as it can be exploited to disrupt system availability without requiring elevated privileges.
From a cybersecurity perspective, this vulnerability aligns with CWE-362, which addresses concurrent execution using shared resources, and demonstrates characteristics similar to those described in ATT&CK technique T1499.001 for network denial of service attacks. The flaw represents a classic race condition or deadlock scenario that can be exploited through legitimate system usage patterns. Organizations should prioritize patching this vulnerability as it provides a direct path for local users to cause system-wide disruption, potentially leading to unauthorized service disruption or availability attacks against critical infrastructure.
Mitigation strategies should focus on applying the latest kernel updates from the Linux kernel maintainers, which include proper lock ordering and synchronization fixes. System administrators should also consider implementing monitoring solutions to detect unusual system behavior that may indicate deadlock conditions, though the primary defense remains timely patch management. Additional operational security measures could include restricting local user access to file system operations when possible and implementing proper system resource limits to prevent exploitation from causing complete system failure. The vulnerability underscores the importance of thorough testing of kernel-level synchronization mechanisms and proper lock ordering protocols in preventing deadlock conditions that can compromise system availability.