Bug 2414715 (CVE-2025-40205)

Summary: CVE-2025-40205 kernel: btrfs: avoid potential out-of-bounds in btrfs_encode_fh()
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A buffer overflow vulnerability was found in the Linux kernel's Btrfs filesystem when encoding file handles. A local user can trigger this issue when the filesystem needs to encode a file handle where the parent directory exists in a different subvolume than the file itself. The function tells callers it will write 32 bytes but actually writes 40 bytes when encoding the parent root ID, resulting in an 8-byte buffer overflow that can corrupt adjacent memory and cause crashes or denial of service.
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Description OSIDB Bzimport 2025-11-12 23:01:59 UTC 
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.
Comment 1 Mauro Matteo Cascella 2025-11-13 08:33:10 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025111247-CVE-2025-40205-ad43@gregkh/T