CVE-2024-37354 in Linux
Summary
by MITRE • 06/25/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix crash on racing fsync and size-extending write into prealloc
We have been seeing crashes on duplicate keys in btrfs_set_item_key_safe():
BTRFS critical (device vdb): slot 4 key (450 108 8192) new key (450 108 8192) ------------[ cut here ]------------
kernel BUG at fs/btrfs/ctree.c:2620! invalid opcode: 0000 [#1] PREEMPT SMP PTI
CPU: 0 PID: 3139 Comm: xfs_io Kdump: loaded Not tainted 6.9.0 #6 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40 04/01/2014 RIP: 0010:btrfs_set_item_key_safe+0x11f/0x290 [btrfs]
With the following stack trace:
#0 btrfs_set_item_key_safe (fs/btrfs/ctree.c:2620:4) #1 btrfs_drop_extents (fs/btrfs/file.c:411:4) #2 log_one_extent (fs/btrfs/tree-log.c:4732:9) #3 btrfs_log_changed_extents (fs/btrfs/tree-log.c:4955:9) #4 btrfs_log_inode (fs/btrfs/tree-log.c:6626:9) #5 btrfs_log_inode_parent (fs/btrfs/tree-log.c:7070:8) #6 btrfs_log_dentry_safe (fs/btrfs/tree-log.c:7171:8) #7 btrfs_sync_file (fs/btrfs/file.c:1933:8) #8 vfs_fsync_range (fs/sync.c:188:9) #9 vfs_fsync (fs/sync.c:202:9) #10 do_fsync (fs/sync.c:212:9) #11 __do_sys_fdatasync (fs/sync.c:225:9) #12 __se_sys_fdatasync (fs/sync.c:223:1) #13 __x64_sys_fdatasync (fs/sync.c:223:1) #14 do_syscall_x64 (arch/x86/entry/common.c:52:14) #15 do_syscall_64 (arch/x86/entry/common.c:83:7) #16 entry_SYSCALL_64+0xaf/0x14c (arch/x86/entry/entry_64.S:121)
So we're logging a changed extent from fsync, which is splitting an extent in the log tree. But this split part already exists in the tree, triggering the BUG().
This is the state of the log tree at the time of the crash, dumped with drgn (https://github.com/osandov/drgn/blob/main/contrib/btrfs_tree.py) to get more details than btrfs_print_leaf() gives us:
>>> print_extent_buffer(prog.crashed_thread().stack_trace()[0]["eb"])
leaf 33439744 level 0 items 72 generation 9 owner 18446744073709551610 leaf 33439744 flags 0x100000000000000 fs uuid e5bd3946-400c-4223-8923-190ef1f18677 chunk uuid d58cb17e-6d02-494a-829a-18b7d8a399da item 0 key (450 INODE_ITEM 0) itemoff 16123 itemsize 160 generation 7 transid 9 size 8192 nbytes 8473563889606862198 block group 0 mode 100600 links 1 uid 0 gid 0 rdev 0 sequence 204 flags 0x10(PREALLOC) atime 1716417703.220000000 (2024-05-22 15:41:43) ctime 1716417704.983333333 (2024-05-22 15:41:44) mtime 1716417704.983333333 (2024-05-22 15:41:44) otime 17592186044416.000000000 (559444-03-08 01:40:16) item 1 key (450 INODE_REF 256) itemoff 16110 itemsize 13 index 195 namelen 3 name: 193 item 2 key (450 XATTR_ITEM 1640047104) itemoff 16073 itemsize 37 location key (0 UNKNOWN.0 0) type XATTR transid 7 data_len 1 name_len 6 name: user.a data a item 3 key (450 EXTENT_DATA 0) itemoff 16020 itemsize 53 generation 9 type 1 (regular) extent data disk byte 303144960 nr 12288 extent data offset 0 nr 4096 ram 12288 extent compression 0 (none) item 4 key (450 EXTENT_DATA 4096) itemoff 15967 itemsize 53 generation 9 type 2 (prealloc) prealloc data disk byte 303144960 nr 12288 prealloc data offset 4096 nr 8192 item 5 key (450 EXTENT_DATA 8192) itemoff 15914 itemsize 53 generation 9 type 2 (prealloc) prealloc data disk byte 303144960 nr 12288 prealloc data offset 8192 nr 4096 ...
So the real problem happened earlier: notice that items 4 (4k-12k) and 5 (8k-12k) overlap. Both are prealloc extents. Item 4 straddles i_size and item 5 starts at i_size.
Here is the state of ---truncated---
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Analysis
by VulDB Data Team • 04/06/2026
The vulnerability described in CVE-2024-37354 affects the Linux kernel's Btrfs file system implementation, specifically within the handling of file synchronization operations combined with size-extending writes into preallocated extents. This flaw manifests as a kernel crash triggered by a race condition between fsync operations and writes that extend file sizes into preallocated regions. The root cause lies in the btrfs_set_item_key_safe() function located in fs/btrfs/ctree.c at line 2620, where a BUG() macro is invoked due to duplicate keys being detected during extent logging. The crash occurs when the system attempts to log a changed extent from an fsync operation, which splits an extent in the log tree, but this split part already exists in the tree, causing an invalid operation to be executed. This condition is classified under CWE-119 as an out-of-bounds read or write, and can be mapped to ATT&CK technique T1059.001 for privilege escalation through kernel exploitation.
The technical execution path begins with a fsync operation initiated through the vfs_fsync_range function, leading to btrfs_sync_file and subsequently btrfs_log_inode_parent. The sequence involves logging changed extents using btrfs_log_changed_extents, which calls log_one_extent and eventually btrfs_set_item_key_safe. During this process, the system encounters overlapping preallocated extents in the inode's extent tree, specifically items 4 and 5 in the extent buffer, where item 4 spans from 4k to 12k and item 5 starts at 8k to 12k, both marked as preallocated. This overlap creates a scenario where duplicate keys are present in the tree structure, which the current implementation does not properly handle, leading to the kernel BUG() and subsequent system crash. The problem is exacerbated by concurrent access patterns where the racing conditions between fsync and size-extending writes create an inconsistent state in the Btrfs metadata structures.
The operational impact of this vulnerability is significant as it can cause system instability and potential data loss in environments using Btrfs file systems. The crash occurs during normal file system operations, particularly when applications perform fsync calls while simultaneously writing to files that are subject to size extensions. This vulnerability affects systems running Linux kernel versions that include the affected Btrfs implementation, particularly those using kernel versions 6.9.0 and potentially earlier versions. The issue is particularly concerning in high-availability environments or systems where file integrity and system stability are paramount, as a single fsync operation could bring down the entire system. The vulnerability is categorized under ATT&CK technique T1490 for data destruction, as it can lead to system crashes and potentially data corruption during critical file system operations. Additionally, this vulnerability can be exploited to cause denial of service conditions, impacting system availability.
Mitigation strategies for CVE-2024-37354 should focus on applying the latest kernel patches that address the race condition in Btrfs extent handling. System administrators should ensure that all systems using Btrfs file systems are updated to kernel versions that include the fix for this vulnerability. Monitoring for fsync operations combined with size-extending writes should be implemented to identify potential risk scenarios. The recommended approach includes updating to kernel versions that contain the specific fix for btrfs_set_item_key_safe() function, which resolves the duplicate key handling issue during extent logging operations. Additionally, implementing proper system hardening measures and avoiding concurrent fsync operations with size-extending writes can help reduce the risk exposure. Organizations should also consider implementing automated patch management systems to ensure timely deployment of security updates and maintain system integrity. The fix typically involves modifying the extent logging logic to properly handle overlapping preallocated extents and ensure that duplicate keys are detected and handled gracefully rather than causing kernel crashes.