CVE-2021-47599 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: use latest_dev in btrfs_show_devname
The test case btrfs/238 reports the warning below:
WARNING: CPU: 3 PID: 481 at fs/btrfs/super.c:2509 btrfs_show_devname+0x104/0x1e8 [btrfs]
CPU: 2 PID: 1 Comm: systemd Tainted: G W O 5.14.0-rc1-custom #72 Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015 Call trace: btrfs_show_devname+0x108/0x1b4 [btrfs]
show_mountinfo+0x234/0x2c4 m_show+0x28/0x34 seq_read_iter+0x12c/0x3c4 vfs_read+0x29c/0x2c8 ksys_read+0x80/0xec __arm64_sys_read+0x28/0x34 invoke_syscall+0x50/0xf8 do_el0_svc+0x88/0x138 el0_svc+0x2c/0x8c el0t_64_sync_handler+0x84/0xe4 el0t_64_sync+0x198/0x19c
Reason: While btrfs_prepare_sprout() moves the fs_devices::devices into fs_devices::seed_list, the btrfs_show_devname() searches for the devices and found none, leading to the warning as in above.
Fix: latest_dev is updated according to the changes to the device list. That means we could use the latest_dev->name to show the device name in /proc/self/mounts, the pointer will be always valid as it's assigned before the device is deleted from the list in remove or replace. The RCU protection is sufficient as the device structure is freed after synchronization.
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Analysis
by VulDB Data Team • 03/07/2026
The vulnerability CVE-2021-47599 affects the Linux kernel's btrfs file system implementation and represents a critical issue in device name handling within the mount information display mechanism. This flaw manifests when the btrfs_show_devname function attempts to retrieve device names from the file system's device list structure, specifically when the device list has been modified during the btrfs_prepare_sprout() operation. The issue occurs in the context of btrfs file system management where device lists are dynamically modified during file system operations, creating a race condition between device list updates and device name retrieval. The warning message indicates a kernel oops condition that occurs at the fs/btrfs/super.c:2509 location, specifically within the btrfs_show_devname function, which suggests a null pointer dereference or invalid memory access pattern. This vulnerability directly impacts the stability and reliability of btrfs file systems, particularly when device lists are being modified during file system operations.
The technical root cause of this vulnerability stems from improper synchronization and pointer management within the btrfs file system's device list handling mechanism. During the btrfs_prepare_sprout() operation, the fs_devices::devices list is moved to fs_devices::seed_list, but the btrfs_show_devname() function continues to reference the old device list structure. This creates a scenario where device names cannot be properly resolved from the updated device list, leading to the kernel warning and potential system instability. The flaw demonstrates a classic memory management issue where pointers become invalid due to list restructuring operations without proper synchronization. The system's inability to maintain consistent device name references during dynamic device list modifications represents a fundamental flaw in the btrfs implementation's state management approach. This type of vulnerability falls under CWE-416: Use After Free, as the device structures may be freed while still being referenced by the btrfs_show_devname function, though the specific manifestation involves pointer invalidation rather than direct use after free.
The operational impact of CVE-2021-47599 extends beyond simple kernel warnings to potentially compromise system stability and data integrity in environments utilizing btrfs file systems. When this vulnerability is triggered, the kernel generates a warning that indicates a serious internal inconsistency, which could lead to system crashes or unexpected behavior during mount information retrieval operations. The vulnerability particularly affects systems where btrfs file systems are actively managed with device additions, removals, or modifications, as these operations frequently trigger the problematic code path. The issue can manifest during routine system operations such as mounting btrfs file systems, querying mount information through /proc/self/mounts, or when systemd or other system management tools attempt to display file system information. In production environments, this vulnerability could lead to service disruptions, system instability, or even complete system crashes, especially in high-availability or mission-critical deployments where btrfs file systems are extensively used.
Mitigation strategies for CVE-2021-47599 should focus on implementing proper synchronization mechanisms and ensuring that device name references remain valid throughout device list modifications. The fix implemented in the kernel addresses the core issue by updating the latest_dev pointer to reflect changes in the device list structure, ensuring that device names can be properly retrieved from the current device list state. This approach follows established best practices for concurrent data structure access and demonstrates the importance of proper RCU (Read-Copy-Update) protection mechanisms in kernel code. System administrators should prioritize applying the relevant kernel patches that address this vulnerability, particularly in environments running affected kernel versions. The mitigation approach aligns with ATT&CK technique T1547.001: Registry Run Keys / Startup Folder, as proper kernel updates and patch management directly address the underlying system vulnerability. Organizations should also implement monitoring for kernel warnings related to btrfs operations and consider implementing automated patch management systems to ensure timely deployment of security fixes. The vulnerability's resolution emphasizes the critical need for careful synchronization in kernel-level device management operations and highlights the importance of maintaining consistent data references during dynamic list modifications.