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.

Weakness

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• If possible, choose a language or compiler that performs automatic bounds checking.
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
• Use libraries or frameworks that make it easier to handle numbers without unexpected consequences.
• Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++). [REF-106]
• Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.
• Use unsigned integers where possible. This makes it easier to perform validation for integer overflows. When signed integers are required, ensure that the range check includes minimum values as well as maximum values.
• Understand the programming language’s underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, “not-a-number” calculations, and how the language handles numbers that are too large or too small for its underlying representation. [REF-7]
• Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/92.html

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