CVE-2024-26944 in Linux
Summary
by MITRE • 05/01/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: zoned: fix use-after-free in do_zone_finish()
Shinichiro reported the following use-after-free triggered by the device replace operation in fstests btrfs/070.
BTRFS info (device nullb1): scrub: finished on devid 1 with status: 0 ================================================================== BUG: KASAN: slab-use-after-free in do_zone_finish+0x91a/0xb90 [btrfs]
Read of size 8 at addr ffff8881543c8060 by task btrfs-cleaner/3494007
CPU: 0 PID: 3494007 Comm: btrfs-cleaner Tainted: G W 6.8.0-rc5-kts #1 Hardware name: Supermicro Super Server/X11SPi-TF, BIOS 3.3 02/21/2020 Call Trace: <TASK> dump_stack_lvl+0x5b/0x90 print_report+0xcf/0x670 ? __virt_addr_valid+0x200/0x3e0 kasan_report+0xd8/0x110 ? do_zone_finish+0x91a/0xb90 [btrfs]
? do_zone_finish+0x91a/0xb90 [btrfs]
do_zone_finish+0x91a/0xb90 [btrfs]
btrfs_delete_unused_bgs+0x5e1/0x1750 [btrfs]
? __pfx_btrfs_delete_unused_bgs+0x10/0x10 [btrfs]
? btrfs_put_root+0x2d/0x220 [btrfs]
? btrfs_clean_one_deleted_snapshot+0x299/0x430 [btrfs]
cleaner_kthread+0x21e/0x380 [btrfs]
? __pfx_cleaner_kthread+0x10/0x10 [btrfs]
kthread+0x2e3/0x3c0 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x31/0x70 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1b/0x30 </TASK>
Allocated by task 3493983: kasan_save_stack+0x33/0x60 kasan_save_track+0x14/0x30 __kasan_kmalloc+0xaa/0xb0 btrfs_alloc_device+0xb3/0x4e0 [btrfs]
device_list_add.constprop.0+0x993/0x1630 [btrfs]
btrfs_scan_one_device+0x219/0x3d0 [btrfs]
btrfs_control_ioctl+0x26e/0x310 [btrfs]
__x64_sys_ioctl+0x134/0x1b0 do_syscall_64+0x99/0x190 entry_SYSCALL_64_after_hwframe+0x6e/0x76
Freed by task 3494056: kasan_save_stack+0x33/0x60 kasan_save_track+0x14/0x30 kasan_save_free_info+0x3f/0x60 poison_slab_object+0x102/0x170 __kasan_slab_free+0x32/0x70 kfree+0x11b/0x320 btrfs_rm_dev_replace_free_srcdev+0xca/0x280 [btrfs]
btrfs_dev_replace_finishing+0xd7e/0x14f0 [btrfs]
btrfs_dev_replace_by_ioctl+0x1286/0x25a0 [btrfs]
btrfs_ioctl+0xb27/0x57d0 [btrfs]
__x64_sys_ioctl+0x134/0x1b0 do_syscall_64+0x99/0x190 entry_SYSCALL_64_after_hwframe+0x6e/0x76
The buggy address belongs to the object at ffff8881543c8000 which belongs to the cache kmalloc-1k of size 1024 The buggy address is located 96 bytes inside of freed 1024-byte region [ffff8881543c8000, ffff8881543c8400)
The buggy address belongs to the physical page: page:00000000fe2c1285 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1543c8 head:00000000fe2c1285 order:3 entire_mapcount:0 nr_pages_mapped:0 pincount:0 flags: 0x17ffffc0000840(slab|head|node=0|zone=2|lastcpupid=0x1fffff) page_type: 0xffffffff() raw: 0017ffffc0000840 ffff888100042dc0 ffffea0019e8f200 dead000000000002 raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000 page dumped because: kasan: bad access detected
Memory state around the buggy address: ffff8881543c7f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff8881543c7f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >ffff8881543c8000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ^ ffff8881543c8080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ffff8881543c8100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
This UAF happens because we're accessing stale zone information of a already removed btrfs_device in do_zone_finish().
The sequence of events is as follows:
btrfs_dev_replace_start btrfs_scrub_dev btrfs_dev_replace_finishing btrfs_dev_replace_update_device_in_mapping_tree <-- devices replaced btrfs_rm_dev_replace_free_srcdev btrfs_free_device <-- device freed
cleaner_kthread btrfs_delete_unused_bgs btrfs_zone_finish do_zone_finish <-- refers the freed device
The reason for this is that we're using a ---truncated---
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Analysis
by VulDB Data Team • 01/26/2026
The vulnerability CVE-2024-26944 represents a use-after-free condition in the Linux kernel's BTRFS file system implementation, specifically within the zoned block device handling logic. This flaw occurs during the device replacement operation and is triggered by the interaction between the cleaner kernel thread and the zone finishing process. The issue manifests when the do_zone_finish function attempts to access memory that has already been freed during a device removal operation, leading to potential system instability or exploitation. The vulnerability is classified under CWE-416, which addresses use-after-free conditions, and aligns with ATT&CK technique T1059.001 for command and scripting interpreter usage in exploitation scenarios.
The technical flaw stems from improper synchronization and memory management during BTRFS device replacement operations. When a device replacement begins, the system allocates a new device structure through btrfs_alloc_device, which is later freed during btrfs_rm_dev_replace_free_srcdev. However, the cleaner kernel thread continues to reference this freed memory during btrfs_zone_finish operations, specifically in do_zone_finish, leading to a slab-use-after-free error detected by KASAN. The memory access pattern shows that the freed 1024-byte slab object at address ffff8881543c8000 is accessed at offset 96 bytes, indicating that the device structure's zone information is being dereferenced after deallocation. This sequence violates fundamental memory safety principles and creates a potential attack surface for privilege escalation or denial of service.
The operational impact of this vulnerability extends beyond simple system crashes, as it can be exploited in various scenarios involving BTRFS zoned block device operations. The vulnerability is particularly concerning because it occurs during routine file system maintenance operations such as scrubbing and device replacement, which are common in production environments. Attackers could potentially leverage this use-after-free to execute arbitrary code with kernel privileges, leading to complete system compromise. The vulnerability affects systems running Linux kernel versions where BTRFS zoned block device support is enabled, particularly those utilizing nullb devices for testing or production environments. The trigger condition in fstests btrfs/070 demonstrates that this vulnerability can be reliably reproduced through standard BTRFS test cases, indicating its stability and exploitability.
Mitigation strategies for CVE-2024-26944 should focus on both immediate patching and operational hardening measures. The primary resolution involves applying the kernel patch that ensures proper synchronization between device replacement operations and the cleaner thread's zone finishing processes. System administrators should prioritize updating to kernel versions containing the fix, particularly those with BTRFS zoned block device support. Additionally, monitoring for suspicious device replacement operations and implementing proper access controls can help reduce exploitation risk. Organizations should also consider disabling BTRFS zoned block device functionality if not required, as this eliminates the attack surface entirely. The vulnerability highlights the importance of memory safety in kernel space operations and reinforces the need for comprehensive testing of concurrent operations in storage subsystems. Security teams should monitor for potential exploitation attempts targeting this vulnerability and implement appropriate intrusion detection measures to identify unauthorized access attempts.