# CVE-2026-99174: ext4: free the sbi after a grace period


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** AZL-111326, BELL-CVE-2026-99174, DEBIAN-CVE-2026-99174, GHSA-h9fx-4jcw-948x, UBUNTU-CVE-2026-99174

## What is CVE-2026-99174?

CVE-2026-99174 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

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

ext4: free the sbi after a grace period

ext4_get_link() follows a symlink that keeps its target in a data block
in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT).
That reads s_mount_state in the ASSERT() of ext4_getblk() and again in
ext4_es_lookup_extent(), and the lookup bumps the extent status hit or
miss counter, all through sb->s_fs_info.

ext4_put_super() destroys those counters, clears sb->s_fs_info and frees
the sbi without waiting for a grace period. A pathwalk that is still in
ext4_get_link() when the last reference to the mount goes away then
dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu
counters.

With a pathwalk held up in ext4_get_link() while the last reference
goes away:

  KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af]
  RIP: 0010:ext4_getblk+0xf0/0x640
  Call Trace:
   <TASK>
   ext4_get_link+0x26c/0x410
   pick_link+0xf96/0x1780
   step_into_slowpath+0x9b8/0x1250
   path_lookupat+0x484/0xb00
   filename_lookup+0x1f1/0x5b0
   vfs_statx+0xd7/0x3b0
   vfs_fstatat+0x4d/0x90
   __do_sys_newfstatat+0xa0/0x100
   do_syscall_64+0x10e/0x5e0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

So far that has been hard to hit. umount(2) waits for a grace period in
namespace_unlock() before it drops the mount, so only a mount whose last
reference goes away elsewhere, like the close of a file or a chdir out of
a lazily unmounted directory, can race a walker.

Keep sb->s_fs_info and free the sbi together with the extent status
counters after a grace period. Split the counters out of
ext4_es_unregister_shrinker() for that. The shrinker still gets wasted
in ext4_put_super(), the counters go with the sbi.

## Which versions are affected by CVE-2026-99174?


## How do I fix CVE-2026-99174?

Upgrade to a fixed version.

## Is CVE-2026-99174 being exploited?

There is no known exploitation of CVE-2026-99174 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99174 and fix it if it is affected.

What it is: CVE-2026-99174 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99174 in the wild and no public exploit code.

Affected product: Linux. 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-2026-99174/ (Markdown: https://insights.sbomify.com/CVE-2026-99174/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99174
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99174
- OSV: https://osv.dev/vulnerability/CVE-2026-99174

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99174/
Summaries are generated and may contain mistakes; the official description above is the record.