Date: June 2024
Author: [Your Name Here]
Quick Summary
A memory leak was found and fixed in the SUNRPC (Secure Unix Network RPC) subsystem of the Linux kernel, specifically related to how GSS-API (Generic Security Service Application Program Interface) contexts were handled. Known as CVE-2023-52653, this bug could cause kernel memory to be wasted, particularly when importing Kerberos security contexts. Let’s break down what happened, see some code, and explore the practical details — including why this matters and how it can be exploited.
SUNRPC: Standard protocol for remote procedure calls; used across many Unix-like systems.
- GSS (Generic Security Service): Provides a way for applications to use secure channels (like for NFS with Kerberos).
- gss_import_v2_context: Part of the Linux kernel logic that imports version 2 GSS contexts — basically, brings in authentication/security information for SUNRPC.
What Was the Problem? (in Easy Terms)
When a kernel function called gss_import_v2_context duplicated some memory (using kmemdup) to store information (mech_used.data), it never actually freed that memory in some error situations. So, on certain failures, the kernel would just “lose track” of memory, causing a leak.
Not freed by its only caller, gss_krb5_import_sec_context
- So, if the GSS context import fails due to bad data, the kernel eats up RAM, and repeated calls multiply the leak
Even small leaks in kernel code are a big deal — the kernel has no garbage collector, so every leak cumulates forever!
Here’s the bad code (stripped down)
ctx->mech_used.data = kmemdup(some_data, len, GFP_KERNEL);
if (!ctx->mech_used.data)
goto out_error; // on error, does not free previous ctx allocations
...
// error exit
out_error:
kfree(ctx);
return NULL;
Problem:
If allocation of mech_used.data fails, earlier memory (ctx->mech_used.data) is never freed.
The main fix is to change the call chain so that freeing happens correctly
- The last call of gss_import_v2_context becomes a call to gss_krb5_import_ctx_v2, making sure everything is freed.
A key snippet from the patch:
// Now, at the end, they free everything properly:
if (!ctx->mech_used.data) {
kfree(ctx);
return NULL;
}
...
// Or, more generally, use a proper free function for the entire context
gss_free_ctx_v2(ctx);
This is *not* a remote code execution bug, but a crafty local user (or misbehaving NFS client) could
- Repeatedly trigger the buggy import (e.g., by sending bad/malformed GSS tokens)
Cause the kernel to leak a chunk of memory each time
- Over time, exhaust kernel memory (“kernel memory DoS”), leading to system instability or complete failure
*Proof of Concept (PoC):*
A simplified example in userland using the Linux NFS tools or manually crafted GSS tokens sent through an NFS mount could repeatedly hit this import code, particularly if using unsupported, malformed Kerberos tickets. As a simple sketch in pseudo-C:
while (1) {
// Send bogus GSS context to kernel RPC interface via mounted NFS share
// Each failure leaks memory!
}
If you maintain your own kernel: Apply the patch from kernel mainline.
- All modern Linux distributions have backported this fix (as of late 2023/early 2024).
- Check your distro’s advisories
Original References
- Linux Kernel Patch: dc12be7c95b5d715cca84e96007fd375c75d14b
- CVE entry at Mitre
- NVD Details
TL;DR
CVE-2023-52653 was a classic memory leak in the Linux kernel’s SUNRPC GSS/Kerberos code — now fixed, but worth knowing about, especially if you run file servers (like NFS) with strong-auth enabled.
Patch your kernel and remember: Memory leaks are more than just a nuisance in the kernel — any leak can crash the system, one little bug at a time!
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Timeline
Published on: 05/01/2024 13:15:48 UTC
Last modified on: 11/05/2024 17:35:05 UTC