Linux Kernel CVE-2025-40205

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.

The affected component is the Btrfs filesystem within the mainline and stable Linux kernel. The exact vulnerable version ranges are not enumerated in the input (CWE and per-CPE version data were not supplied), but the presence of eight separate git.kernel.org stable-branch commits (e.g., 0276c8582488022f057b4cec21975a5edf079f47, 361d67276eb8ec6be8f27f4ad6c6090459438fee, 43143776b0a7604d873d1a6f3e552a00aa930224, 60de2f55d2aca53e81b4ef2a67d7cc9e1eb677db, 742b44342204e5dfe3926433823623c1a0c581df, d3a9a8e1275eb9b87f006b5562a287aea3f6885f, d91f6626133698362bba08fbc04bd72c466806d3, dff4f9ff5d7f289e4545cc936362e01ed3252742) indicates the fix was backported across multiple stable series. Distribution exposure is confirmed for Ubuntu, which shipped kernel fixes in advisories USN-8126-1 (https://ubuntu.com/security/notices/USN-8126-1) and USN-8125-1 (https://ubuntu.com/security/notices/USN-8125-1). Only kernels built with Btrfs support and using file-handle encoding (NFS export or name_to_handle_at) exercise the vulnerable path.