CVE-2020-28974
Description
An out-of-bounds (OOB) SLAB memory access flaw was found in the Linux kernel's fbcon driver module. A bounds check failure allows a local attacker with special user privileges to gain access to out-of-bounds memory, leading to a system crash or leaking of internal kernel information. The highest threat from this vulnerability is to system availability.
Mitigation
Add 'nomodeset' option as kernel boot parameter to disable frame buffering in /etc/default/grub, and run 'grub2-mkconfig -o /boot/grub2/grub.cfg' and reboot.
# cat /proc/cmdline
BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-147.el8.x86_64 root=/dev/mapper/rhel_rhel8u2--1-root ro crashkernel=auto resume=/dev/mapper/rhel_rhel8u2--1-swap rd.lvm.lv=rhel_rhel8u2-1/root rd.lvm.lv=rhel_rhel8u2-1/swap nomodeset
# ls -l /dev/fb*
ls: cannot access '/dev/fb*': No such file or directory
The above said mitigation does not so effect for rhel6 kernel , and is only applicable for rhel7/8.
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 | N/A |
| Attack Vector | Physical | Physical | 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 | Low | Low | N/A |
| Integrity Impact | Low | Low | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:P/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H
NVD: CVSS:3.1/AV:P/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H
Red Hat CVSS v3 Score Explanation
- Attacker will need a physical access to the tty (AV:P),
- With privilege required is only to the root or users in tty group to access it. (PR:H)
- This flaw may have an OOB read of about 32 bytes (C:L)
- There can be a copy of data prior to display on the destination ttyX and this may even lead to a oob write causing an Integrity (I:L)
Acknowledgements
Red Hat would like to thank Yuan Ming <yuanmingbuaa@gmail com> (Tsinghua University) for reporting this issue.
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