CVE-2021-4155
Description
A data leak flaw was found in the way XFS_IOC_ALLOCSP IOCTL in the XFS filesystem allowed for size increase of files with unaligned size. A local attacker could use this flaw to leak data on the XFS filesystem otherwise not accessible to them.
Mitigation
This issue can be mitigated by ensuring xfs_alloc_file_space is not called with "0" as an argument.
This can be done with a SystemTap script (which resets "0" with XFS_BMAPI_PREALLOC), below are the steps:
1) Save the following script in a 'CVE-2021-4155.stp' file
--- On Red Hat Enterprise Linux 6 ---
probe module("xfs").function("xfs_alloc_file_space") {
if ($alloc_type == 0)
$alloc_type = 0x40; # XFS_BMAPI_PREALLOC
}
--- On Red Hat Enterprise Linux 6 ---
--- On Red Hat Enterprise Linux 7 onwards ---
probe module("xfs").function("xfs_alloc_file_space") {
if ($alloc_type == 0)
$alloc_type = 0x8; # XFS_BMAPI_PREALLOC
}
--- On Red Hat Enterprise Linux 7 onwards ---
2) Install systemtap package and its dependencies
# yum install -y systemtap systemtap-runtime
# yum install -y kernel-devel kernel-debuginfo
3) Build the mitigation kernel module as root.
# stap -r `uname -r` -m cve_2021_4155.ko -g CVE-2021-4155.stp -p4
4) Load the mitigation module as root
# staprun -L cve_2021_4155.ko
What is SystemTap and how to use it?
https://access.redhat.com/solutions/5441
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.5 | 5.5 | N/A |
| Attack Vector | Local | Local | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | High | High | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
NVD: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Understanding the Weakness (CWE)
Integrity,Availability,Confidentiality
Technical Impact: DoS: Crash, Exit, or Restart; Execute Unauthorized Code or Commands; Read Memory; Modify Memory
If the incorrect calculation is used in the context of memory allocation, then the software may create a buffer that is smaller or larger than expected. If the allocated buffer is smaller than expected, this could lead to an out-of-bounds read or write (CWE-119), possibly causing a crash, allowing arbitrary code execution, or exposing sensitive data.
Acknowledgements
Red Hat would like to thank Kirill Tkhai (Virtuozzo Kernel team) for reporting this issue.
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