# CVE-2026-63892: thunderbolt: property: Reject dir\_len \< 4 to prevent size\_t underflow

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-07-19 · Updated: 2026-07-19

## Affected

- Linux: version range not reported
- Linux: 4.15

## Description

In the Linux kernel, the following vulnerability has been resolved: thunderbolt: property: Reject dir_len < 4 to prevent size_t underflow On the non-root path, __tb_property_parse_dir() takes dir_len from entry->length (u16 widened to size_t). Two distinct OOB conditions follow when entry->length < 4: 1. The non-root path begins with kmemdup(&block[dir_offset], sizeof(*dir->uuid), ...) which always reads 4 dwords from dir_offset. tb_property_entry_valid() only enforces dir_offset + entry->length <= block_len, so a crafted entry with dir_offset close to the end of the property block and entry->length in 0..3 passes that gate but lets the UUID copy run off the block (e.g. dir_offset = 497, dir_len = 3 in a 500-dword block reads block[497..501]). 2. After the kmemdup, content_len = dir_len - 4 underflows size_t to ~SIZE_MAX, nentries becomes SIZE_MAX / 4, and the entry walk runs OOB on each iteration until an entry fails validation or the kernel oopses on an unmapped page. Reject dir_len < 4 on the non-root path *before* the UUID kmemdup, which closes both holes. Also move INIT_LIST_HEAD(&dir->properties) up to immediately after the dir allocation so the new error-return path (and the existing uuid-alloc failure path) calling tb_property_free_dir() sees a walkable list rather than the zero-initialized NULL next/prev that list_for_each_entry_safe() would oops on.

## 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.
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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/37abc4504fa19d8f9f1e87792e8a2b8fdb308e40
- https://git.kernel.org/stable/c/e2d4d51cf5785815fa4e91e0c019e3eb2506a84c
- https://git.kernel.org/stable/c/de618299190b418291609e6921557253bd417e25
- https://git.kernel.org/stable/c/5506c825f14d810f0690b1f4367cb7249ebb387a
- https://git.kernel.org/stable/c/542a13890b742099c461d70920e97b14e568f6ec
- https://git.kernel.org/stable/c/d548179adcc87e1bc66b17e00352a1f536e76065
- https://git.kernel.org/stable/c/3bec49ca55e08fb085cc4318f24b1b37eaab28cb
- https://git.kernel.org/stable/c/de21b59c29e31c5108ddc04210631bbfab81b997

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