CVE-2022-49075

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 3GiB file. This should fail.

echo

echo "Try to fallocate a 3GiB file..."

fallocate -l 3G $MNT/3G.file

# Try to fallocate a 5GiB file.

echo

echo "Try to fallocate a 5GiB file..."

fallocate -l 5G $MNT/5G.file

# See we break the qgroup limit.

echo

sync

btrfs qgroup show -r $MNT

umount $MNT

When running the test:

$ ./qgroup-overflow.sh

(...)

Try to fallocate a 3GiB file...

fallocate: fallocate failed: Disk quota exceeded

Try to fallocate a 5GiB file...

qgroupid         rfer         excl     max_rfer

--------         ----         ----     --------

0/5           5.00GiB      5.00GiB      2.00GiB

Since we have no control of how bytes_changed is used, it's better to

set it to u64.