CVE-2024-53084 - Breaking Reference Loops in Linux DRM/Imagination GPU Driver

A new vulnerability has been patched in the Linux kernel affecting the DRM (Direct Rendering Manager) for Imagination GPUs. This flaw, tracked as CVE-2024-53084, could cause memory or resource leaks due to a reference loop when cleaning up graphics driver resources. This post will walk through the issue, how it was fixed, and show example code—using simple language for anyone interested in Linux kernel security.

The Core Problem: Reference Loops

When a user-space program closes its connection to the graphics driver, the driver needs to clean up all resources linked to that process. In the imagination GPU DRM code, these resources form a chain that accidentally turns into a loop:

PVR GEM Object
    ↓
GPU scheduler "finished" fence
    ↓
GPU scheduler "scheduled" fence
    ↓
PVR driver "done" fence
    ↓
PVR Context
    ↓
PVR VM Context
    ↓
PVR VM Mappings
    ↓
(PVR GEM Object - back to beginning!)

Most of these connections are protected using Linux's internal *reference counting*. But the link *from* the PVR VM Context *to* the PVR VM Mappings is a "soft" relationship—it's just a pointer, not a refcount. That means freeing these mappings only happens when the context is destroyed.

Why is this a problem?

If you just start destroying resources from the top, you'll find that each object has a reference to the next object, which means none of them ever hit zero references—they stay alive, using up memory and other resources... potentially forever!

The Fix: Break the Loop!

The solution applied in the patch is simple but crucial: Clean up the VM mappings BEFORE tearing down the PVR Context.

By freeing the mappings at the right time, you break the cycle, and the rest of the objects can naturally be released by their refcounts.

Patch Example

Here's a simplified patch that shows the essential change. In the driver's cleanup code, the order of freeing has been changed:

Before (Vulnerable code)

void pvr_destroy_vm_context(struct pvr_vm_context *vm_ctx)
{
    // ...
    pvr_destroy_context(vm_ctx->context);
    pvr_free_vm_mappings(vm_ctx);
    // ...
}

After (Patched code)

void pvr_destroy_vm_context(struct pvr_vm_context *vm_ctx)
{
    // Free VM mappings first to break any loops
    pvr_free_vm_mappings(vm_ctx);
    pvr_destroy_context(vm_ctx->context);
    // ...
}

This small change ensures that all VM mappings are destroyed—and any soft reference is broken—so the rest of the objects can be cleaned up safely.

Exploit Details

While this bug doesn't allow direct code execution or privilege escalation, it *can* be abused to create a denial-of-service (DoS) situation:

- How: By repeatedly opening and closing the GPU device node, processes can create dozens/hundreds/thousands of driver contexts.

Over time, the kernel could run out of memory or hit resource limits, causing slowdowns or crashes.

Note: This bug does not allow reading or writing kernel memory, only leaking resources.

Proof-of-Concept Approach

// This is a mockup, not a direct exploit!
int main() {
    for (int i = ; i < 10000; ++i) {
        int fd = open("/dev/dri/card", O_RDWR);
        if (fd == -1) continue;
        // Perform minimal ioctl(s) to allocate VM/context
        close(fd);
        // Leaked resources accumulate each time!
    }
    return ;
}

After running such a loop, system memory or driver resources will not be properly released until reboot.

References

- Linux kernel commit resolving CVE-2024-53084

(Replace <FIXED_COMMIT_HASH> with the real commit hash when available.)

- Imagination DRM Source

RedHat Security Advisory:
https://access.redhat.com/security/cve/CVE-2024-53084

Conclusion

CVE-2024-53084 is a classic example of the pitfalls of complex object lifecycles in C code—especially in the Linux kernel, where reference counting reigns supreme but *soft* references still creep in. Thanks to rapid patching, this resource leak bug is now closed. But it reminds all kernel developers: watch out for cycles, and always clean up in the right order!

Be sure your kernel is up-to-date to avoid these subtle but dangerous resource leaks.

Timeline

Published on: 11/19/2024 18:15:27 UTC
Last modified on: 11/27/2024 19:41:38 UTC