CVE-2026-93800
Low risk· EPSS 5%Exploitation Probability (EPSS)
Low risk5th percentile - higher than 5% of all known CVEs
Summary
In the Linux kernel's btrfs subsystem, a use-after-free vulnerability exists related to relocation operations. When insert_dirty_subvol() fails, a root's reloc_root field remains pointing to a freed reloc root instead of being set to NULL, leading to a use-after-free or double free during filesystem unmount.
Risk Assessment
The vulnerability can cause kernel memory corruption, system crash (panic), or potential kernel-level code execution. However, it requires an error during btrfs relocation, which limits exploitability.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-93800. Until patched, avoid balance/relocation operations on btrfs filesystems in critical environments.
Other vulnerabilities in Linux kernel (btrfs)
See all- CVE-2026-98080Unknown
In the Linux kernel, a bug in the btrfs subsystem forces unnecessary reloc root creation during snapshot creation when qgroups (quota) are enabled. This triggers a kernel warning in lookup_inline_extent_backref and flips the filesystem to read-only mode. The issue is reproducible during balance/relocate operations on a btrfs filesystem with quota enabled.
- CVE-2026-93255Unknown
In the Linux kernel, a race condition exists in the btrfs subsystem between the thread finishing an extent buffer read and the thread unmounting the filesystem. The EXTENT_BUFFER_READING flag is not cleared within the same critical section as the reference count decrement, leading to false warnings in invalidate_and_check_btree_folios(). The fix introduces a helper free_extent_buffer_clear_reading() and synchronization on refs_lock.
- CVE-2026-90265Unknown
A deadlock exists in the Linux kernel's btrfs filesystem during file defragmentation when autodefrag is enabled. Defrag reserves delalloc space while holding dirty and locked folios plus the extent range lock, causing kernel tasks (btrfs-cleaner, writeback, metadata reclaim) to block each other.
- CVE-2026-89772Low risk· EPSS 7%
A flaw was found in the Linux kernel btrfs subsystem where write-protection of folios during data writeback was lost. A code change replaced folio_clear_dirty_for_io() with a plain folio_test_dirty() check, allowing a process with an mmap-ed file to modify sectors while writeback is in progress. This can cause checksum corruption, lost writes, and data corruption for compressed or inline extents.
- CVE-2023-3111High
A use after free vulnerability was found in prepare_to_relocate in fs/btrfs/relocation.c in the Linux Kernel. This possible flaw can be triggered by calling btrfs_ioctl_balance() before calling btrfs_ioctl_defrag().
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() If during relocation we fail in insert_dirty_subvol() because btrfs_update_reloc_root() returned an error, we will leave a root's reloc_root field pointing to a reloc root that was freed instead of NULL, resulting later in a use-after-free, or double free attempt during unmount. The sequence of steps is this: 1) During relocation the call to btrfs_update_reloc_root() in insert_dirty_subvol() fails, so insert_dirty_subvol() returns the error to merge_reloc_root() without adding the root to the list rc->dirty_subvol_roots; 2) Then merge_reloc_root() aborts the current transaction because insert_dirty_subvol() returned an error; 3) Up the call chain, merge_reloc_roots() gets the error, adds the reloc root for root X to the local reloc_roots list and jumps to the 'out' label, where it calls free_reloc_roots() to free all the reloc roots in the local reloc_roots list. This frees the reloc root for root X; 4) We go up the call chain to relocate_block_group() which calls clean_dirty_subvols() to go over dirty roots and set their ->reloc_root field to NULL, but root X is not in the dirty_subvol_roots list, so its ->reloc_root still points to a reloc root; 5) Relocation finishes, with an error and a transaction abort, but the ->reloc_root field for root X still points to the reloc root that was freed in step 3; 6) When unmounting the fs we end up calling: btrfs_free_fs_roots() btrfs_drop_and_free_fs_root() --> calls btrfs_put_root() against root X's ->reloc_root which is not NULL and points to the already freed reloc root in step 4 above Resulting in a use-after-free to a double free attempt. Syzbot reported this with the following dmesg/syslog: [ 106.004389][ T5339] BTRFS error (device loop0 state A): Transaction aborted (error -5) [ 106.014266][ T5339] BTRFS: error (device loop0 state A) in merge_reloc_root:1655: errno=-5 IO failure [ 106.021891][ T1061] BTRFS error (device loop0 state A): error while writing out transaction: -5 [ 106.026964][ T1061] BTRFS warning (device loop0 state A): Skipping commit of aborted transaction. [ 106.033807][ T5340] BTRFS error (device loop0 state A): bdev /dev/loop0 errs: wr 3, rd 0, flush 0, corrupt 0, gen 0 [ 106.039265][ T1061] BTRFS: error (device loop0 state A) in cleanup_transaction:2067: errno=-5 IO failure [ 106.044382][ T5339] BTRFS info (device loop0 state EA): forced readonly [ 106.074329][ T5339] BTRFS: error (device loop0 state EA) in merge_reloc_roots:1887: errno=-5 IO failure [ 106.081004][ T5356] BTRFS info (device loop0 state EA): scrub: started on devid 1 [ 106.085611][ T5339] BTRFS info (device loop0 state EA): balance: ended with status: -30 [ 106.089517][ T5356] BTRFS info (device loop0 state EA): scrub: not finished on devid 1 with status: -30 [ 106.662365][ T5338] BTRFS info (device loop0 state EA): last unmount of filesystem 3a375e4e-b156-4d76-a2ad-16e198ce1409 [ 106.682946][ T5338] ================================================================== [ 106.686574][ T5338] BUG: KASAN: slab-use-after-free in btrfs_put_root+0x2f/0x250 [ 106.690090][ T5338] Write of size 4 at addr ffff88803f978630 by task syz.0.0/5338 [ 106.693173][ T5338] [ 106.694279][ T5338] CPU: 0 UID: 0 PID: 5338 Comm: syz.0.0 Not tainted syzkaller #0 PREEMPT(full) [ 106.694293][ T5338] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 106.694300][ T5338] Call Trace: [ 106.694308][ T5338] <TASK> [ 106.694314][ T5338] dump_stack_lvl+0xe8/0x150 [ 106.694331][ T5338] print_address_description+0x55/0x1e0 [ 106.694343][ T5338] ? btrfs_put_root+0x2f/0x250 [ 106.694358][ T5338] print_report+0x58/0x70 [ 106. ---truncated---
Vulnerability data from NVD (NIST) · CISA KEV · EPSS