# CVE-2024-50118: btrfs: reject ro->rw reconfiguration if there are hard ro requirements

> A null pointer dereference in the Linux kernel's btrfs filesystem allows local attackers to cause a system crash during specific remount operations.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 5 Nov 2024
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2024-50118, DEBIAN-CVE-2024-50118, GHSA-5mc8-q53p-h4pr, UBUNTU-CVE-2024-50118

## What is CVE-2024-50118?

CVE-2024-50118 is a medium-severity vulnerability in Linux kernel, published on 5 Nov 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: reject ro->rw reconfiguration if there are hard ro requirements

[BUG]
Syzbot reports the following crash:

  BTRFS info (device loop0 state MCS): disabling free space tree
  BTRFS info (device loop0 state MCS): clearing compat-ro feature flag for FREE_SPACE_TREE (0x1)
  BTRFS info (device loop0 state MCS): clearing compat-ro feature flag for FREE_SPACE_TREE_VALID (0x2)
  Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
  RIP: 0010:backup_super_roots fs/btrfs/disk-io.c:1691 [inline]
  RIP: 0010:write_all_supers+0x97a/0x40f0 fs/btrfs/disk-io.c:4041
  Call Trace:
   <TASK>
   btrfs_commit_transaction+0x1eae/0x3740 fs/btrfs/transaction.c:2530
   btrfs_delete_free_space_tree+0x383/0x730 fs/btrfs/free-space-tree.c:1312
   btrfs_start_pre_rw_mount+0xf28/0x1300 fs/btrfs/disk-io.c:3012
   btrfs_remount_rw fs/btrfs/super.c:1309 [inline]
   btrfs_reconfigure+0xae6/0x2d40 fs/btrfs/super.c:1534
   btrfs_reconfigure_for_mount fs/btrfs/super.c:2020 [inline]
   btrfs_get_tree_subvol fs/btrfs/super.c:2079 [inline]
   btrfs_get_tree+0x918/0x1920 fs/btrfs/super.c:2115
   vfs_get_tree+0x90/0x2b0 fs/super.c:1800
   do_new_mount+0x2be/0xb40 fs/namespace.c:3472
   do_mount fs/namespace.c:3812 [inline]
   __do_sys_mount fs/namespace.c:4020 [inline]
   __se_sys_mount+0x2d6/0x3c0 fs/namespace.c:3997
   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

[CAUSE]
To support mounting different subvolume with different RO/RW flags for
the new mount APIs, btrfs introduced two workaround to support this feature:

- Skip mount option/feature checks if we are mounting a different
  subvolume

- Reconfigure the fs to RW if the initial mount is RO

Combining these two, we can have the following sequence:

- Mount the fs ro,rescue=all,clear_cache,space_cache=v1
  rescue=all will mark the fs as hard read-only, so no v2 cache clearing
  will happen.

- Mount a subvolume rw of the same fs.
  We go into btrfs_get_tree_subvol(), but fc_mount() returns EBUSY
  because our new fc is RW, different from the original fs.

  Now we enter btrfs_reconfigure_for_mount(), which switches the RO flag
  first so that we can grab the existing fs_info.
  Then we reconfigure the fs to RW.

- During reconfiguration, option/features check is skipped
  This means we will restart the v2 cache clearing, and convert back to
  v1 cache.
  This will trigger fs writes, and since the original fs has "rescue=all"
  option, it skips the csum tree read.

  And eventually causing NULL pointer dereference in super block
  writeback.

[FIX]
For reconfiguration caused by different subvolume RO/RW flags, ensure we
always run btrfs_check_options() to ensure we have proper hard RO
requirements met.

In fact the function btrfs_check_options() doesn't really do many
complex checks, but hard RO requirement and some feature dependency
checks, thus there is no special reason not to do the check for mount
reconfiguration.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2024-50118?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-50118?

Apply the vendor's patch.

## Is CVE-2024-50118 being exploited?

There is no known exploitation of CVE-2024-50118 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-2024-50118 and fix it if it is affected.

What it is: CVE-2024-50118 is a medium-severity vulnerability in Linux kernel, published on 5 Nov 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-2024-50118 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-2024-50118/ (Markdown: https://insights.sbomify.com/CVE-2024-50118/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-50118
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-50118
- OSV: https://osv.dev/vulnerability/CVE-2024-50118

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-50118/
Summaries are generated and may contain mistakes; the official description above is the record.