CVE-2024-49855 in Linux
Summary
by MITRE • 10/21/2024
In the Linux kernel, the following vulnerability has been resolved:
nbd: fix race between timeout and normal completion
If request timetout is handled by nbd_requeue_cmd(), normal completion has to be stopped for avoiding to complete this requeued request, other use-after-free can be triggered.
Fix the race by clearing NBD_CMD_INFLIGHT in nbd_requeue_cmd(), meantime make sure that cmd->lock is grabbed for clearing the flag and the requeue.
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Analysis
by VulDB Data Team • 01/19/2026
The vulnerability identified as CVE-2024-49855 affects the Linux kernel's network block device implementation, specifically within the nbd subsystem responsible for handling network block device requests. This issue represents a critical race condition that can lead to severe memory corruption and potential system instability. The vulnerability manifests when the nbd_requeue_cmd() function processes request timeouts while normal completion operations are simultaneously occurring, creating a dangerous overlap that can result in use-after-free conditions.
The technical flaw stems from improper synchronization between timeout handling and normal request completion mechanisms within the nbd subsystem. When a request times out and triggers the nbd_requeue_cmd() function, the system fails to properly prevent concurrent normal completion operations from executing against the same request. This race condition occurs because the NBD_CMD_INFLIGHT flag, which indicates that a command is currently in flight and being processed, is not consistently cleared before requeuing operations. The lack of proper locking mechanisms means that multiple threads or processes can simultaneously attempt to manipulate the same command structure, leading to unpredictable behavior and memory corruption.
The operational impact of this vulnerability extends beyond simple system instability to potentially enable privilege escalation and denial of service attacks. Attackers could exploit this race condition to trigger use-after-free scenarios that might allow them to execute arbitrary code with kernel privileges, effectively compromising the entire system. The vulnerability affects systems running Linux kernels that implement the nbd subsystem, particularly those utilizing network block devices for storage operations. Given that nbd is commonly used in virtualization environments, cloud computing infrastructures, and distributed storage systems, the potential attack surface is significant.
The fix implemented addresses the core synchronization issue by ensuring that the NBD_CMD_INFLIGHT flag is properly cleared within the nbd_requeue_cmd() function while maintaining appropriate locking mechanisms. This approach follows established security practices for concurrent programming and memory management within kernel space operations. The solution requires that cmd->lock is acquired before clearing the flag and performing the requeue operation, preventing the race condition from occurring. This mitigation aligns with common security patterns recommended for preventing use-after-free vulnerabilities and race conditions in kernel-level code. The fix also corresponds to CWE-362, which specifically addresses race conditions in concurrent programming, and supports the ATT&CK framework's system compromise tactics by preventing unauthorized privilege escalation through memory corruption vulnerabilities.
Security practitioners should prioritize patching systems running affected Linux kernels to prevent exploitation of this vulnerability. The fix demonstrates proper kernel security practices and emphasizes the importance of maintaining proper synchronization primitives in high-concurrency kernel subsystems. Organizations utilizing network block devices should conduct thorough testing of the patched kernel versions to ensure compatibility with existing storage configurations while verifying that the race condition has been properly resolved. This vulnerability serves as a reminder of the critical importance of proper locking mechanisms and synchronization in kernel space programming, where even subtle race conditions can lead to severe security implications.