CVE-2021-47145 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: do not BUG_ON in link_to_fixup_dir
While doing error injection testing I got the following panic
kernel BUG at fs/btrfs/tree-log.c:1862! invalid opcode: 0000 [#1] SMP NOPTI
CPU: 1 PID: 7836 Comm: mount Not tainted 5.13.0-rc1+ #305 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014 RIP: 0010:link_to_fixup_dir+0xd5/0xe0 RSP: 0018:ffffb5800180fa30 EFLAGS: 00010216 RAX: fffffffffffffffb RBX: 00000000fffffffb RCX: ffff8f595287faf0 RDX: ffffb5800180fa37 RSI: ffff8f5954978800 RDI: 0000000000000000 RBP: ffff8f5953af9450 R08: 0000000000000019 R09: 0000000000000001 R10: 000151f408682970 R11: 0000000120021001 R12: ffff8f5954978800 R13: ffff8f595287faf0 R14: ffff8f5953c77dd0 R15: 0000000000000065 FS: 00007fc5284c8c40(0000) GS:ffff8f59bbd00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fc5287f47c0 CR3: 000000011275e002 CR4: 0000000000370ee0 Call Trace: replay_one_buffer+0x409/0x470 ? btree_read_extent_buffer_pages+0xd0/0x110 walk_up_log_tree+0x157/0x1e0 walk_log_tree+0xa6/0x1d0 btrfs_recover_log_trees+0x1da/0x360 ? replay_one_extent+0x7b0/0x7b0 open_ctree+0x1486/0x1720 btrfs_mount_root.cold+0x12/0xea ? __kmalloc_track_caller+0x12f/0x240 legacy_get_tree+0x24/0x40 vfs_get_tree+0x22/0xb0 vfs_kern_mount.part.0+0x71/0xb0 btrfs_mount+0x10d/0x380 ? vfs_parse_fs_string+0x4d/0x90 legacy_get_tree+0x24/0x40 vfs_get_tree+0x22/0xb0 path_mount+0x433/0xa10 __x64_sys_mount+0xe3/0x120 do_syscall_64+0x3d/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae
We can get -EIO or any number of legitimate errors from btrfs_search_slot(), panicing here is not the appropriate response. The error path for this code handles errors properly, simply return the error.
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Analysis
by VulDB Data Team • 08/04/2025
The vulnerability described in CVE-2021-47145 represents a critical flaw in the Linux kernel's btrfs file system implementation that manifests as an improper use of BUG_ON macro during error handling. This issue occurs within the link_to_fixup_dir function located in fs/btrfs/tree-log.c at line 1862, where the kernel encounters an error condition that should be gracefully handled but instead triggers an immediate system panic. The vulnerability was identified during error injection testing, a common security assessment technique that deliberately introduces faults to evaluate system resilience. When btrfs_search_slot() returns error codes such as -EIO or other legitimate error conditions, the current implementation incorrectly uses BUG_ON which terminates the kernel execution rather than properly managing the error through established error handling pathways.
The technical flaw stems from a violation of proper error handling protocols within the kernel's btrfs subsystem. The link_to_fixup_dir function contains a BUG_ON statement that should only be used for unrecoverable internal consistency checks, not for legitimate error conditions that can occur during normal system operation. This misclassification of error types directly contravenes established kernel development practices and security principles. When legitimate errors occur during btrfs log tree recovery operations, particularly during mount processes, the kernel should return appropriate error codes to allow higher-level components to handle the failure gracefully rather than causing an immediate system crash. The specific error injection testing scenario that exposed this vulnerability demonstrates how a well-intentioned but improperly implemented error handling path can lead to denial of service conditions that compromise system stability.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable denial of service attacks against systems running btrfs file systems. When exploited, this vulnerability can cause kernel panics during mount operations, effectively preventing file system access and potentially disrupting critical system services. The vulnerability affects systems using the btrfs file system during the mount process, particularly when recovering log trees after system crashes or unclean shutdowns. Attackers could potentially leverage this weakness to repeatedly trigger mount operations that cause kernel panics, leading to sustained service disruption. The vulnerability also represents a broader concern about error handling quality within kernel subsystems, as improper error management can create cascading failures that impact overall system reliability and security posture.
Mitigation strategies for this vulnerability focus on proper error handling implementation and kernel updates to address the specific code path. The fix involves replacing the BUG_ON statement with appropriate error return handling that allows the system to gracefully process legitimate error conditions. This aligns with the principle of defensive programming and proper error propagation as outlined in various kernel security guidelines and standards. System administrators should prioritize applying kernel updates that contain the patched version of the btrfs implementation, which properly handles error conditions without triggering kernel panics. Additionally, monitoring systems should be configured to detect kernel panics and restart mechanisms should be implemented to minimize service disruption. The fix also emphasizes the importance of thorough testing of error injection scenarios during kernel development, particularly for critical subsystems like file systems where improper error handling can lead to complete system failures rather than just application-level errors. This vulnerability serves as a reminder of the critical importance of proper error handling in kernel space code, where the difference between a graceful error recovery and system crash can be the distinction between maintaining service availability and complete system compromise.