Linux is the backbone of countless servers, desktops, and embedded devices. Its reliability and flexibility, in part, come from continuous improvements—and sometimes, that means patching up security holes as they're discovered. One such hole is CVE-2021-46918, affecting the Linux kernel’s dmaengine subsystem, specifically within the Intel Data Streaming Accelerator (idxd) driver. This vulnerability is subtle but demonstrates how even advanced hardware security features like MSIX permissions can become vectors for attacks if not managed carefully.

This article breaks down what the vulnerability is, why it matters, how it can be exploited, and what the patch looks like. All in simple, straightforward terms.

What Is CVE-2021-46918?

CVE-2021-46918 is a vulnerability in the Linux kernel’s dmaengine idxd driver that failed to properly clear MSIX (Message Signaled Interrupt Extension) permission entries and PASID (Process Address Space ID) programming during device shutdown. The issue was:

- MSIX entries (which control device interrupts for modern PCIe devices) remained enabled after the idxd driver was shutdown.
- PASID entries (which are associated with assigning device accesses to specific process address spaces) also remained active.
- This inconsistency between device startup (probe) and shutdown could allow unintended access or privilege escalation on affected systems.

Why Is This Important?

- Uncleared permissions mean that when the idxd device is restarted or even used by another process, the previous permissions are still active.
- Potential privilege escalation: Attackers could trick the kernel or a process into reusing stale MSIX/PASID permissions, possibly elevating their privileges or causing undefined behavior.
- Kernel security best practice: Always remove permissions and reset hardware state on shutdown. Leaving things "half-baked" creates room for races and other attacks.

How the Vulnerability Works

Let's look concretely at what was missing and what was fixed.

When a driver is loaded (probe), permissions are assigned and entries are programmed so the device can function. However, when the driver is shut down, *these permissions were not revoked*. Here’s a simplified version:

Device Plugged or Booted

- MSIX entries and PASID entries are *programmed/enabled*.

Pseudo-code (Before Patch)

// During device probe (initialization)
setup_msix_permissions();
setup_pasid_entries();

// During device removal
// (Was empty! Should have cleared permissions here.)

The Fixed Version

To close this security hole, the maintainers added code to actively clear the MSIX permission entries and PASID programming during shutdown.

Pseudo-code (After Patch)

// During device probe
setup_msix_permissions();
setup_pasid_entries();

// During device removal/shutdown
clear_msix_permissions();
clear_pasid_entries();

So, when the device goes away, everything is reset and ready for a safe new start, with no dangling permissions.

Threat Model

Imagine you are a malicious user on a shared server with access to the DMA engine (through some means, like a user-space driver). When the device is shut down, the MSIX and PASID permissions are not cleared. When the device is restarted, it might use *your* process's PASID again, or even your interrupt permissions.

Prepare malicious PASID context or MSIX mapping.

2. Cause shutdown/removal of the idxd device (or unload/reload the module).

If successful, you might interfere with another process, crash the system, or gain privileges.

While a working exploit would require hardware-specific sequence and some (not trivial) understanding of DMA/PASID operations, the risk of compromise is very real in sensitive deployments.

Code Snippet: The Patch

Here’s a simplified version of what the patch looks like, according to the original commit:

static void idxd_shutdown(struct device *dev)
{
    struct idxd_dev *idxd = dev_get_drvdata(dev);

    // Existing shutdown code...

    // BEFORE PATCH: missing cleanup for MSIX and PASID

    // AFTER PATCH: added these lines
    idxd_clear_msix_perm(isxd);
    idxd_clear_pasid_entries(isxd);
}

Now, when the driver shuts down, both MSIX permission entries and PASID entries are cleared.

- Linux Kernel Commit Fixing CVE-2021-46918
- CVE entry on Mitre
- Linux dmaengine idxd Documentation

Who needs to worry?

- System administrators running Linux on servers with Intel DSA/idxd hardware.

Summary

- CVE-2021-46918 is a subtle but important privilege escalation vulnerability in the Linux dmaengine idxd driver.

Vulnerabilities in low-level drivers matter. Always audit for permission leaks on shutdown!

If you want to experiment or verify, check the commit log for exact code and apply it to your build.

Timeline

Published on: 02/27/2024 07:15:08 UTC
Last modified on: 04/10/2024 14:47:16 UTC