CVE-2021-31440
Description
An out-of-bounds access flaw was found in the Linux kernel’s implementation of the eBPF code verifier, where an incorrect register bounds calculation while checking unsigned 32-bit instructions in an eBPF program occurs.. By default accessing the eBPF verifier is only accessible to privileged users with CAP_SYS_ADMIN. The issue results from the lack of proper validation of user-supplied eBPF programs prior to executing them. A local user could use this flaw to crash the system or possibly escalate their privileges on the system.
Mitigation
The default Red Hat Enterprise Linux kernel prevents unprivileged users from being able to use eBPF by the kernel.unprivileged_bpf_disabled sysctl. This would require a privileged user with CAP_SYS_ADMIN or root to be able to abuse this flaw reducing its attack space.
For the Red Hat Enterprise Linux 7 the eBPF for unprivileged users is always disabled.
For the Red Hat Enterprise Linux 8 to confirm the current state, inspect the sysctl with the command:
# cat /proc/sys/kernel/unprivileged_bpf_disabled
The setting of 1 would mean that unprivileged users can not use eBPF, mitigating the flaw.
A kernel update will be required to mitigate the flaw for the root or users with CAP_SYS_ADMIN capabilities.
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 | 7 | 7 | 8.8 |
| Attack Vector | Local | Local | Local |
| Attack Complexity | High | High | Low |
| Privileges Required | Low | Low | Low |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Changed |
| Confidentiality | High | High | High |
| Integrity Impact | High | High | High |
| Availability Impact | High | High | High |
Vector
Red Hat: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
NVD: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
cve.org: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Red Hat CVSS v3 Score Explanation
The vulnerability is similar like CVE-2020-8835, so CVSS should be similar (AC:H and S:U): the vulnerability itself allows privileges escalation in the scope of the OS being used (so without changing the scope and the BPF should be considered as part of the host machine, but not as separated virtual machine) and the usage of the vulnerability is complex enough (first need access to BPF execution and then apart of running bypass code itself then need to bypass some of the problems for actual running of the exploit code, ex. https://www.blackhat.com/eu-16/briefings/schedule/#randomization-cant-stop-bpf-jit-spray-4516 ).
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart
If the incorrect calculation causes the program to move into an unexpected state, it may lead to a crash or impairment of service.
Integrity,Confidentiality,Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (Other); Execute Unauthorized Code or Commands
If the incorrect calculation is used in the context of resource allocation, it could lead to an out-of-bounds operation (CWE-119) leading to a crash or even arbitrary code execution. Alternatively, it may result in an integer overflow (CWE-190) and / or a resource consumption problem (CWE-400).
Access Control
Technical Impact: Gain Privileges or Assume Identity
In the context of privilege or permissions assignment, an incorrect calculation can provide an attacker with access to sensitive resources.
Access Control
Technical Impact: Bypass Protection Mechanism
If the incorrect calculation leads to an insufficient comparison (CWE-697), it may compromise a protection mechanism such as a validation routine and allow an attacker to bypass the security-critical code.
Acknowledgements
Red Hat would like to thank Manfred Paul (RedRocket CTF team) for reporting this issue.
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