CVE-2024-50241

What it actually does

Failure to initialize refcount and async_copies fields in struct nfsd4_copy before an error path invokes cleanup_async_copy(), resulting in refcount underflow and potential memory corruption in the Linux kernel NFSD subsystem.

Why it matters to the business

NFS server processes may crash or become unstable, disrupting file sharing services and requiring system restarts; low exploit likelihood limits widespread impact but still poses availability risk for affected kernels.

How an attack unfolds

An attacker sends a crafted NFSv4 COPY operation that triggers an error inside nfsd4_copy(); because the structure fields are uninitialized, cleanup_async_copy() dereferences them and causes a refcount underflow that can be leveraged for memory corruption or denial of service.

How to fix it

Apply the kernel patch that moves initialization of nfsd4_copy fields earlier; for the listed flashplugin-nonfree package on Ubuntu 14.04 LTS, upgrade to version 11.2.202.535ubuntu0.14.04.1 or later equivalent kernel update containing the fix.

How to confirm the fix worked

Confirm the updated kernel or package version is installed, then exercise NFS COPY operations that previously hit the error path while monitoring for refcount warnings or crashes.

Typical attack path

1. Establish NFSv4 mount to target server
2. Issue COPY compound request crafted to hit error path
3. Trigger cleanup_async_copy() on uninitialized fields
4. Induce refcount underflow leading to kernel instability

How to detect it

Review kernel logs for messages referencing nfsd4_copy, cleanup_async_copy, or refcount underflow; inspect NFSv4 traffic for unusual COPY operations around the time of any kernel oops or panic.

What your security team should do

• Prioritize patching systems exposing NFSv4 services
• Monitor kernel logs for refcount underflow messages related to nfsd4_copy
• Restrict NFSv4 access to trusted clients until patched