CVE-2024-26985: Resolving the Linux Kernel Vulnerability, drm/xe: Fix bo Leak in intel_fb_bo_framebuffer_init

Recently, the Linux kernel experienced a significant vulnerability (CVE-2024-26985) that needed to be addressed. This post aims to provide an exclusive, long read on this vulnerability, its impact, and how the issue has been resolved. We will dive deep into the code snippet responsible for the fix, the links to the original references, and the details of the exploit.

Background

The CVE-2024-26985 vulnerability pertains to the "Direct Rendering Manager" (DRM) component in the Linux kernel, specifically in the drm/xe module. This module is responsible for handling the framebuffer memory objects (bo) for the Intel Xe GPU. The issue lies in the "intel_fb_bo_framebuffer_init" function that causes a bo leak. Essentially, a bo reference is unintentionally left out which can lead to memory leak and resource exhaustion issues.

Exploit Details

The issue that CVE-2024-26985 addresses is that the intel_fb_bo_framebuffer_init function fails to unreference the bo object in the error path, causing the bo reference to leak. This can lead to considerable performance degradation and instability issues for systems that utilize the Intel Xe GPU.

The Fix:

The fix for this vulnerability involves adding a proper unreference operation for the bo object in the error path to prevent the aforementioned leak. In addition to resolving the leak, the return value on successful execution has been clarified to zero, making the process more transparent and easier to understand.

Here is the code snippet that demonstrates the implementation of the fix

static int intel_fb_bo_framebuffer_init(struct drm_device *dev,
				        struct drm_framebuffer *fb)
{
	struct drm_i915_gem_object *bo;
	int err;

	bo = intel_fb_obj_get_bo(fb);
	if (!bo)
		return -ENODEV;

	/* Prevent the bo from being released while the fb is active */
	err = i915_gem_object_pin_to_display_plane(bo); 
	if (err)
		goto err;

	fb->obj[] = &bo->base;

	return ;

err:
	i915_gem_object_unpin_from_display_plane(bo);
	return err;
}

This code snippet is essentially an updated version of the drm/xe module's intel_fb_bo_framebuffer_init function, incorporating the fix for the bo leak. The most notable inclusion is the addition of the unreference operation in the error path, as shown in this line:

i915_gem_object_unpin_from_display_plane(bo);

Furthermore, the return value for successful execution is explicitly set to , which makes it clear what the expected behavior should be upon the successful completion of the function.

return ;

For those who wish to examine the original commit where this fix was made, you can find it here: a2f3d731be3893e730417ae319076fcaffdf549

This particular update to the codebase was "cherry-picked" from the aforementioned commit, ensuring that the fix performed there was cleanly integrated into the mainline code for the Linux kernel.

Conclusion

The CVE-2024-26985 vulnerability is a serious issue for the Linux kernel, as it causes potential performance degradation and resource exhaustion in systems using Intel Xe GPUs. Thankfully, the problem has been addressed through the addition of the bo unreference operation and clear return value in the code snippet provided above. With the fix now merged into the mainline Linux kernel codebase, users can breathe a sigh of relief knowing that their systems will no longer be plagued by the vulnerability.

Timeline

Published on: 05/01/2024 06:15:16 UTC
Last modified on: 12/19/2024 08:51:45 UTC