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Introduction

In mid-2024, the Linux kernel team patched a bug affecting the drm/tegra graphics driver—one that might go unnoticed by most but could cause subtle failures or even security problems. This bug, tracked as CVE-2024-53078, revolved around how error pointers returned by a low-level function were mishandled due to a common programming mistake: mixing up NULL and error-pointer checks with IS_ERR().

In this post, we'll break down what happened, see how such bugs are dangerous, learn how to write code that avoids this, and show how an exploit might work in theory. Finally, you'll find references and links to the upstream fix.

Where?

drm/tegra subsystem in the Linux Kernel (used on NVIDIA Tegra SoCs).

What?

Incorrect error handling of return value from iommu_paging_domain_alloc(): checked for NULL, but it returns error pointers (ERR_PTR()) instead.

Status?

Patched in Linux.

Here’s the kind of pattern that shows up a lot in kernel code, but sometimes gets botched

// Bad!
domain = iommu_paging_domain_alloc(...);
if (!domain) {
    // error handling, maybe log, maybe bail out
}

The problem here: iommu_paging_domain_alloc() doesn’t return NULL for errors. It returns an encoded error pointer, using the macros ERR_PTR() and friends.

The correct mechanism is to use the IS_ERR() macro

// Good!
domain = iommu_paging_domain_alloc(...);
if (IS_ERR(domain)) {
    // Error handling
}

The Patch

Here’s how the Linux maintainers fixed CVE-2024-53078 (upstream diff):

-	domain = iommu_paging_domain_alloc(dev, iommu);
-	if (!domain)
+	domain = iommu_paging_domain_alloc(dev, iommu);
+	if (IS_ERR(domain))
 		return -ENOMEM;

So simple, but so important!

Kernel panics: A crash if you de-reference an error pointer by mistake.

- Logic errors: Failing to handle an error properly—leading to hardware not being initialized, or earlier allocated resources leaking.
- Opportunity for exploitation: In edge cases, it’s possible to trick the kernel into making false assumptions about success; with enough creativity, adversaries can chain this to escalate privileges or break out of sandboxes.

Rule: _Always use IS_ERR()/PTR_ERR() to check error returns from functions that use error pointers._

Proof of Concept & Exploit

This bug by itself generally leads to denial-of-service (crash), but creative attackers can sometimes make use of it, especially in combination with other bugs.

How an Attacker Might Leverage This

Suppose a device fails in a way that causes iommu_paging_domain_alloc() to return an error pointer. The old code just checks for NULL, so it happily passes an error pointer as if it's a valid object to later functions. When those functions call through it, they might dereference the error, leading to a crash (oops or panic), or even worse, undefined kernel behavior.

Example sploit chain

1. Malicious user triggers device hotplug/init with parameters that force iommu_paging_domain_alloc() to fail.

Triggering With Userspace

While most users can’t directly hit drm/tegra code paths, if you’ve got CAP_SYS_RAWIO or can trigger device removal/addition, you could try the following pseudo-code:

int fd = open("/dev/dri/card", O_RDWR);  // Or the correct device
// trigger device modesetting or reconfiguration
// send bad parameters to cause resource exhaustion
// observe dmesg for kernel warnings or oopses

NB: Crashing the kernel is generally not subtle or directly exploitable for local privilege escalation, but such bugs are valuable for chaining with other vulnerabilities.

- Patch in Kernel.org
- CVE-2024-53078 at Mitre (when assigned)
- Linux DRM Tegra Maintainers
- Understanding IS_ERR() and PTR_ERR()

Conclusion

CVE-2024-53078 is a great example of how tiny errors in pointer error-handling paradigms can cause major headaches. Even if you’re not hacking on the Linux kernel, understanding this kind of bug will help you write safer code—especially in C where API contracts matter. Always be explicit in your error checks, and never assume you know what a return value means without reading the docs.

Timeline

Published on: 11/19/2024 18:15:27 UTC
Last modified on: 11/25/2024 13:31:57 UTC