# CVE-2021-46958: btrfs: fix race between transaction aborts and fsyncs leading to use-after-free

> A race condition in the Linux kernel's Btrfs filesystem can lead to a use-after-free, potentially causing system crashes or arbitrary code execution.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/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 Feb 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-46958, GHSA-gmfg-g4f3-5fg9, UBUNTU-CVE-2021-46958

## What is CVE-2021-46958?

CVE-2021-46958 is a high-severity use after free vulnerability in Linux kernel, published on 27 Feb 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: fix race between transaction aborts and fsyncs leading to use-after-free

There is a race between a task aborting a transaction during a commit,
a task doing an fsync and the transaction kthread, which leads to an
use-after-free of the log root tree. When this happens, it results in a
stack trace like the following:

  BTRFS info (device dm-0): forced readonly
  BTRFS warning (device dm-0): Skipping commit of aborted transaction.
  BTRFS: error (device dm-0) in cleanup_transaction:1958: errno=-5 IO failure
  BTRFS warning (device dm-0): lost page write due to IO error on /dev/mapper/error-test (-5)
  BTRFS warning (device dm-0): Skipping commit of aborted transaction.
  BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0xa4e8 len 4096 err no 10
  BTRFS error (device dm-0): error writing primary super block to device 1
  BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e000 len 4096 err no 10
  BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e008 len 4096 err no 10
  BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e010 len 4096 err no 10
  BTRFS: error (device dm-0) in write_all_supers:4110: errno=-5 IO failure (1 errors while writing supers)
  BTRFS: error (device dm-0) in btrfs_sync_log:3308: errno=-5 IO failure
  general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b68: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
  CPU: 2 PID: 2458471 Comm: fsstress Not tainted 5.12.0-rc5-btrfs-next-84 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
  RIP: 0010:__mutex_lock+0x139/0xa40
  Code: c0 74 19 (...)
  RSP: 0018:ffff9f18830d7b00 EFLAGS: 00010202
  RAX: 6b6b6b6b6b6b6b68 RBX: 0000000000000001 RCX: 0000000000000002
  RDX: ffffffffb9c54d13 RSI: 0000000000000000 RDI: 0000000000000000
  RBP: ffff9f18830d7bc0 R08: 0000000000000000 R09: 0000000000000000
  R10: ffff9f18830d7be0 R11: 0000000000000001 R12: ffff8c6cd199c040
  R13: ffff8c6c95821358 R14: 00000000fffffffb R15: ffff8c6cbcf01358
  FS:  00007fa9140c2b80(0000) GS:ffff8c6fac600000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007fa913d52000 CR3: 000000013d2b4003 CR4: 0000000000370ee0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   ? __btrfs_handle_fs_error+0xde/0x146 [btrfs]
   ? btrfs_sync_log+0x7c1/0xf20 [btrfs]
   ? btrfs_sync_log+0x7c1/0xf20 [btrfs]
   btrfs_sync_log+0x7c1/0xf20 [btrfs]
   btrfs_sync_file+0x40c/0x580 [btrfs]
   do_fsync+0x38/0x70
   __x64_sys_fsync+0x10/0x20
   do_syscall_64+0x33/0x80
   entry_SYSCALL_64_after_hwframe+0x44/0xae
  RIP: 0033:0x7fa9142a55c3
  Code: 8b 15 09 (...)
  RSP: 002b:00007fff26278d48 EFLAGS: 00000246 ORIG_RAX: 000000000000004a
  RAX: ffffffffffffffda RBX: 0000563c83cb4560 RCX: 00007fa9142a55c3
  RDX: 00007fff26278cb0 RSI: 00007fff26278cb0 RDI: 0000000000000005
  RBP: 0000000000000005 R08: 0000000000000001 R09: 00007fff26278d5c
  R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000340
  R13: 00007fff26278de0 R14: 00007fff26278d96 R15: 0000563c83ca57c0
  Modules linked in: btrfs dm_zero dm_snapshot dm_thin_pool (...)
  ---[ end trace ee2f1b19327d791d ]---

The steps that lead to this crash are the following:

1) We are at transaction N;

2) We have two tasks with a transaction handle attached to transaction N.
   Task A and Task B. Task B is doing an fsync;

3) Task B is at btrfs_sync_log(), and has saved fs_info->log_root_tree
   into a local variable named 'log_root_tree' at the top of
   btrfs_sync_log(). Task B is about to call write_all_supers(), but
   before that...

4) Task A calls btrfs_commit_transaction(), and after it sets the
   transaction state to TRANS_STATE_COMMIT_START, an error happens before
   it w
---truncated---

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2021-46958?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-46958?

Apply the vendor's patch.

## Is CVE-2021-46958 being exploited?

There is no known exploitation of CVE-2021-46958 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-2021-46958 and fix it if it is affected.

What it is: CVE-2021-46958 is a high-severity use after free vulnerability in Linux kernel, published on 27 Feb 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-2021-46958 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-2021-46958/ (Markdown: https://insights.sbomify.com/CVE-2021-46958/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-46958
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-46958
- OSV: https://osv.dev/vulnerability/CVE-2021-46958

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-46958/
Summaries are generated and may contain mistakes; the official description above is the record.