CVE-2021-4218

What it actually does

A flaw in the Linux kernel SVC RDMA counters implementation causes a kernel panic when the associated sysctl counter is read.

Why it matters to the business

Local users can force repeated system crashes and reboots, resulting in unplanned downtime and loss of availability on affected CentOS/RHEL systems.

How an attack unfolds

An attacker with local shell access reads the vulnerable SVC RDMA sysctl counter, triggering an immediate kernel panic that forces the system to reboot.

How to fix it

Apply the latest vendor kernel updates that address the SVC RDMA counters flaw; until patches are available, restrict unprivileged local access to sysctl interfaces via SELinux, AppArmor, or namespace isolation.

How to confirm the fix worked

Verify that the vulnerable sysctl entry no longer triggers a panic after applying updates; confirm kernel version matches or exceeds the vendor-supplied fix and test read access under normal privilege levels.

Typical attack path

1. Obtain a local shell on the target system
2. Locate and read the SVC RDMA counter sysctl entry
3. Trigger kernel panic and observe forced reboot

How to detect it

Review kernel logs (dmesg, /var/log/messages, journalctl) for panic entries referencing SVC RDMA counters; correlate with user login events immediately preceding the crash.

• kernel: Unable to handle kernel NULL pointer dereference
• sysctl_svc_rdma
• panic: reading SVC RDMA counters

What your security team should do

• Prioritize deployment of updated kernels on all CentOS/RHEL systems exposing RDMA functionality
• Harden local access controls to limit sysctl exposure to trusted users only
• Enable kernel panic logging and monitoring to detect repeated crashes quickly