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, its better to set it to u64.

Affected Software

References

• https://git.kernel.org/stable/c/0355387ea5b02d353c9415613fab908fac5c52a6
• https://git.kernel.org/stable/c/44277c50fdba5019ca25bfad1b71e2561b0de11b
• https://git.kernel.org/stable/c/4b98799e181b4326a613108cf37acc1f55d21b45
• https://git.kernel.org/stable/c/6bfff81286d4491f02dad7814bae5c77c9ad2320
• https://git.kernel.org/stable/c/7941b74ed49b6db25efbef2256ebef843c11a010
• https://git.kernel.org/stable/c/82ae73ac963cee877ce34f7c31b2b456b516e96c
• https://git.kernel.org/stable/c/b642b52d0b50f4d398cb4293f64992d0eed2e2ce
• https://git.kernel.org/stable/c/f3d97b22a708bf9e3f3ac2ba232bcefd0b0c136b