# CVE-2024-56581: btrfs: ref-verify: fix use-after-free after invalid ref action

> A use-after-free vulnerability in the Linux kernel's Btrfs filesystem allows local attackers to potentially execute arbitrary code or escalate privileges.

- **Severity:** High 7.0 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Dec 2024
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-55524, AZL-55617, BELL-CVE-2024-56581, DEBIAN-CVE-2024-56581, GHSA-9xj5-r9g9-xvp9, UBUNTU-CVE-2024-56581

## What is CVE-2024-56581?

CVE-2024-56581 is a high-severity use after free vulnerability in Linux kernel, published on 27 Dec 2024. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

btrfs: ref-verify: fix use-after-free after invalid ref action

At btrfs_ref_tree_mod() after we successfully inserted the new ref entry
(local variable 'ref') into the respective block entry's rbtree (local
variable 'be'), if we find an unexpected action of BTRFS_DROP_DELAYED_REF,
we error out and free the ref entry without removing it from the block
entry's rbtree. Then in the error path of btrfs_ref_tree_mod() we call
btrfs_free_ref_cache(), which iterates over all block entries and then
calls free_block_entry() for each one, and there we will trigger a
use-after-free when we are called against the block entry to which we
added the freed ref entry to its rbtree, since the rbtree still points
to the block entry, as we didn't remove it from the rbtree before freeing
it in the error path at btrfs_ref_tree_mod(). Fix this by removing the
new ref entry from the rbtree before freeing it.

Syzbot report this with the following stack traces:

   BTRFS error (device loop0 state EA):   Ref action 2, root 5, ref_root 0, parent 8564736, owner 0, offset 0, num_refs 18446744073709551615
      __btrfs_mod_ref+0x7dd/0xac0 fs/btrfs/extent-tree.c:2523
      update_ref_for_cow+0x9cd/0x11f0 fs/btrfs/ctree.c:512
      btrfs_force_cow_block+0x9f6/0x1da0 fs/btrfs/ctree.c:594
      btrfs_cow_block+0x35e/0xa40 fs/btrfs/ctree.c:754
      btrfs_search_slot+0xbdd/0x30d0 fs/btrfs/ctree.c:2116
      btrfs_insert_empty_items+0x9c/0x1a0 fs/btrfs/ctree.c:4314
      btrfs_insert_empty_item fs/btrfs/ctree.h:669 [inline]
      btrfs_insert_orphan_item+0x1f1/0x320 fs/btrfs/orphan.c:23
      btrfs_orphan_add+0x6d/0x1a0 fs/btrfs/inode.c:3482
      btrfs_unlink+0x267/0x350 fs/btrfs/inode.c:4293
      vfs_unlink+0x365/0x650 fs/namei.c:4469
      do_unlinkat+0x4ae/0x830 fs/namei.c:4533
      __do_sys_unlinkat fs/namei.c:4576 [inline]
      __se_sys_unlinkat fs/namei.c:4569 [inline]
      __x64_sys_unlinkat+0xcc/0xf0 fs/namei.c:4569
      do_syscall_x64 arch/x86/entry/common.c:52 [inline]
      do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
      entry_SYSCALL_64_after_hwframe+0x77/0x7f
   BTRFS error (device loop0 state EA):   Ref action 1, root 5, ref_root 5, parent 0, owner 260, offset 0, num_refs 1
      __btrfs_mod_ref+0x76b/0xac0 fs/btrfs/extent-tree.c:2521
      update_ref_for_cow+0x96a/0x11f0
      btrfs_force_cow_block+0x9f6/0x1da0 fs/btrfs/ctree.c:594
      btrfs_cow_block+0x35e/0xa40 fs/btrfs/ctree.c:754
      btrfs_search_slot+0xbdd/0x30d0 fs/btrfs/ctree.c:2116
      btrfs_lookup_inode+0xdc/0x480 fs/btrfs/inode-item.c:411
      __btrfs_update_delayed_inode+0x1e7/0xb90 fs/btrfs/delayed-inode.c:1030
      btrfs_update_delayed_inode fs/btrfs/delayed-inode.c:1114 [inline]
      __btrfs_commit_inode_delayed_items+0x2318/0x24a0 fs/btrfs/delayed-inode.c:1137
      __btrfs_run_delayed_items+0x213/0x490 fs/btrfs/delayed-inode.c:1171
      btrfs_commit_transaction+0x8a8/0x3740 fs/btrfs/transaction.c:2313
      prepare_to_relocate+0x3c4/0x4c0 fs/btrfs/relocation.c:3586
      relocate_block_group+0x16c/0xd40 fs/btrfs/relocation.c:3611
      btrfs_relocate_block_group+0x77d/0xd90 fs/btrfs/relocation.c:4081
      btrfs_relocate_chunk+0x12c/0x3b0 fs/btrfs/volumes.c:3377
      __btrfs_balance+0x1b0f/0x26b0 fs/btrfs/volumes.c:4161
      btrfs_balance+0xbdc/0x10c0 fs/btrfs/volumes.c:4538
   BTRFS error (device loop0 state EA):   Ref action 2, root 5, ref_root 0, parent 8564736, owner 0, offset 0, num_refs 18446744073709551615
      __btrfs_mod_ref+0x7dd/0xac0 fs/btrfs/extent-tree.c:2523
      update_ref_for_cow+0x9cd/0x11f0 fs/btrfs/ctree.c:512
      btrfs_force_cow_block+0x9f6/0x1da0 fs/btrfs/ctree.c:594
      btrfs_cow_block+0x35e/0xa40 fs/btrfs/ctree.c:754
      btrfs_search_slot+0xbdd/0x30d0 fs/btrfs/ctree.c:2116
      btrfs_lookup_inode+0xdc/0x480 fs/btrfs/inode-item.c:411
      __btrfs_update_delayed_inode+0x1e7/0xb90 fs/btrfs/delayed-inode.c:1030
      btrfs_update_delayed_i
---truncated---

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2024-56581?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-56581?

Apply the vendor's patch.

## Is CVE-2024-56581 being exploited?

There is no known exploitation of CVE-2024-56581 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2024-56581 and fix it if it is affected.

What it is: CVE-2024-56581 is a high-severity use after free vulnerability in Linux kernel, published on 27 Dec 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2024-56581 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2024-56581/ (Markdown: https://insights.sbomify.com/CVE-2024-56581/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-56581
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-56581
- OSV: https://osv.dev/vulnerability/CVE-2024-56581

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-56581/
Summaries are generated and may contain mistakes; the official description above is the record.