CVE-2020-27171
Description
A flaw was found in the Linux kernels eBPF verification code. By default accessing the eBPF verifier is only accessible to privileged users with CAP_SYS_ADMIN. A flaw that triggers Integer underflow when restricting speculative pointer arithmetic allows unprivileged local users to leak the content of kernel memory. The highest threat from this vulnerability is to data confidentiality.
Statement
The default Red Hat Enterprise Linux kernel differs from the upstream, that prevents unprivileged users from being able to use eBPF by the kernel.unprivileged_bpf_disabled sysctl.
The setting of 1 would mean that unprivileged users can not use eBPF (which is a default setting), and this mitigating the flaw.
This would require a privileged user with CAP_SYS_ADMIN or root to be able to abuse this flaw reducing its attack space, and may even cause a denial-of-service problem.
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.
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).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6 | 6 | N/A |
| Attack Vector | Local | Local | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | High | High | 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 | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:H
NVD: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Instability
This weakness will generally lead to undefined behavior and therefore crashes. In the case of overflows involving loop index variables, the likelihood of infinite loops is also high.
Integrity
Technical Impact: Modify Memory
If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the wrap around results in other conditions such as buffer overflows, further memory corruption may occur.
Confidentiality,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism
This weakness can sometimes trigger buffer overflows which can be used to execute arbitrary code. This is usually outside the scope of a program's implicit security policy.
Acknowledgements
Red Hat would like to thank Piotr Krysiuk <piotras@gmail.com> for reporting this issue.
Frequently Asked Questions
Not sure what something means? Check out our Security Glossary.
Want to get errata notifications? Sign up here.
