CVE-2026-98080 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: do not force reloc root creation during qgroup_account_snapshot()
[BUG]
When running btrfs/252 with quota enabled through MKFS_OPTIONS="-O quota", it has a high chance to trigger the following kernel warning and flips the fs RO:
BTRFS info (device dm-2): relocating block group 30408704 flags metadata|dup ------------[ cut here ]------------
WARNING: fs/btrfs/extent-tree.c:879 at lookup_inline_extent_backref+0x74b/0x960 [btrfs], CPU#4: btrfs/2173
CPU: 4 UID: 0 PID: 2173 Comm: btrfs Not tainted 7.2.0-rc6-custom+ #457 PREEMPT(full) 3adc6528fb66f7a55fe1095385818e742f200aab Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022 RIP: 0010:lookup_inline_extent_backref+0x74b/0x960 [btrfs]
Call Trace: <TASK> insert_inline_extent_backref+0x7c/0x160 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
__btrfs_inc_extent_ref+0xa9/0x270 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
__btrfs_run_delayed_refs+0x4af/0x11c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_run_delayed_refs+0x9d/0xf0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
create_pending_snapshot+0x39d/0xf00 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
create_pending_snapshots+0x9b/0xc0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_commit_transaction+0x280/0xeb0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
prepare_to_relocate+0x147/0x200 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
relocate_block_group+0x6b/0x5e0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_relocate_block_group+0x92c/0x2380 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_relocate_chunk+0x3f/0x1a0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_balance+0xa2c/0x19c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
btrfs_ioctl+0x2839/0x2d30 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
__x64_sys_ioctl+0x416/0x9a0 do_syscall_64+0xe1/0x790 entry_SYSCALL_64_after_hwframe+0x4b/0x53 </TASK> ---[ end trace 0000000000000000 ]---
BTRFS info (device dm-2): leaf 4593991680 gen 233 total ptrs 175 free space 5953 owner 2 BTRFS info (device dm-2): refs 3 lock_owner 2173 current 2173 item 0 key (166772736 METADATA_ITEM 1) itemoff 16250 itemsize 33 extent refs 1 gen 222 flags 2 ref#0: tree block backref root 266 [ Skip the tree dump ]
item 174 key (263225344 METADATA_ITEM 0) itemoff 10328 itemsize 33 extent refs 1 gen 162 flags 258 ref#0: tree block backref root 267 BTRFS error (device dm-2): extent item not found for insert, bytenr 179847168 num_bytes 16384 parent 4594335744 root_objectid 273 owner 0 offset 0 BTRFS error (device dm-2): failed to run delayed ref for logical 179847168 num_bytes 16384 type 182 action 1 ref_mod 1: -117
[CAUSE]
The above error is showing that there is a tree reference to a metadata extent that is no longer there.
With "ref_verify" mount option (requires CONFIG_BTRFS_DEBUG), there is some extra debug output:
BTRFS error (device dm-2): dumping block entry [180961280 16384], num_refs 0, metadata 1, from disk 0
BTRFS error (device dm-2): root entry 256, num_refs 18446744073709551615 BTRFS error (device dm-2): root entry 273, num_refs 18446744073709551615 BTRFS error (device dm-2): Ref action 3, root 273, ref_root 273, parent 0, owner 0, offset 0, num_refs 1 btrfs_force_cow_block+0x129/0x7d0 [btrfs]
btrfs_cow_block+0x10a/0x250 [btrfs]
btrfs_search_slot+0x5eb/0xf40 [btrfs]
btrfs_insert_empty_items+0x3a/0x70 [btrfs]
insert_with_overflow+0x53/0x130 [btrfs]
btrfs_insert_dir_item+0x125/0x290 [btrfs]
btrfs_add_link+0xaa/0x410 [btrfs]
btrfs_rename+0x5ea/0xcd0 [btrfs]
btrfs_rename2+0x28/0x60 [btrfs]
vfs_rename+0x5b2/0xe10 filename_renameat2+0x244/0x430 __x64_sys_rename+0x48/0x70 do_syscall_64+0xe1/0x790 entry_SYSCALL_64_after_hwframe+0x4b/0x53 ---truncated---
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel's Btrfs filesystem implementation contains a logic flaw within the quota group accounting mechanism that can lead to file system corruption and subsequent read-only remounting. Specifically, during snapshot operations with quota groups enabled, the function qgroup_account_snapshot incorrectly forces the creation of relocation roots even when such actions are unnecessary or premature. This erroneous behavior disrupts the internal consistency of extent reference tracking, leading to scenarios where tree references point to metadata extents that no longer exist on disk. The vulnerability manifests under specific conditions involving concurrent operations like renaming and block group relocations triggered by balance commands, resulting in kernel warnings and critical errors such as failed delayed reference execution with error code -117.
The root cause lies in the interaction between snapshot creation, quota accounting, and extent tree management. When a pending snapshot is committed, the system attempts to account for space usage changes across quota groups. In this process, the code path invokes logic that assumes certain relocation roots must be created to handle reference updates. However, under high concurrency or specific workload patterns involving btrfs balance operations, these assumptions fail. The result is an inconsistency where a metadata extent item cannot be found during insertion because its references have been incorrectly managed or cleared prematurely. This state violates the integrity of the B-tree structure used by Btrfs for storing file system metadata, causing the kernel to detect invalid states and trigger defensive measures like forcing the file system into read-only mode to prevent further data loss.
From a security perspective, this vulnerability primarily impacts availability rather than confidentiality or integrity in the traditional sense of malicious exploitation. An attacker with local access could potentially trigger this condition by creating rapid snapshots while simultaneously running balance operations on quota-enabled volumes. This would cause denial of service through file system unavailability until manual intervention occurs to remount the volume read-write and repair metadata inconsistencies using fsck tools. The issue aligns with CWE-823, which covers use of outdated or incorrect reference counts, as well as CWE-674 regarding unintended side effects in concurrent operations. In terms of MITRE ATT&CK, this falls under T1529 System Shutdown/Reboot if the resulting corruption leads to system instability requiring restarts, though it is more accurately classified as a reliability flaw rather than an exploit vector for privilege escalation or data theft.
Mitigation strategies involve applying kernel patches that correct the logic in qgroup_account_snapshot to avoid forcing relocation root creation when not strictly required by the current transaction state. System administrators should ensure their Btrfs file systems are updated with kernels containing this fix, particularly those running workloads heavy on snapshots and quota management. Additionally, enabling ref_verify mount options during testing or debugging can help identify similar reference counting issues before they lead to corruption in production environments. Regular backups remain essential as a safeguard against potential data loss resulting from such internal inconsistencies until the underlying code defect is fully resolved through upstream kernel updates.