CVE-2022-49075

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix qgroup reserve overflow the qgroup limit

We use extent_changeset->bytes_changed in qgroup_reserve_data() to record

how many bytes we set for EXTENT_QGROUP_RESERVED state. Currently the

bytes_changed is set as "unsigned int", and it will overflow if we try to

fallocate a range larger than 4GiB. The result is we reserve less bytes

and eventually break the qgroup limit.

Unlike regular buffered/direct write, which we use one changeset for

each ordered extent, which can never be larger than 256M. For

fallocate, we use one changeset for the whole range, thus it no longer

respects the 256M per extent limit, and caused the problem.

The following example test script reproduces the problem:

$ cat qgroup-overflow.sh

#!/bin/bash

DEV=/dev/sdj

MNT=/mnt/sdj

mkfs.btrfs -f $DEV

mount $DEV $MNT

# Set qgroup limit to 2GiB.

btrfs quota enable $MNT

btrfs qgroup limit 2G $MNT

# Try to fallocate a 3Gi

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References