CVE-2024-45817 - A Deep Dive into the Recursive Deadlock Vulnerability in x86's APIC Architecture

In computing, the x86's APIC (Advanced Programmable Interrupt Controller) plays a crucial role in handling multiple sources of interrupts, allowing for efficient communication between the hardware and the operating system. However, an unforeseen vulnerability has been discovered in this architecture, identified by the CVE (Common Vulnerabilities and Exposures) reference number CVE-2024-45817, which leads to a recursive deadlock situation.

In this extensive post, we will explore the details of this vulnerability, examine the code snippet, and discuss the exploitation and possible mitigation strategies for the issue.

The Vulnerability

This vulnerability occurs when the error conditions within the x86's APIC architecture are reported using a status register. The operating system is able to opt for receiving an interrupt when a new error occurs. The issue arises from the ability to configure the error interrupt with an illegal vector, which causes an error to be generated when the error interrupt is activated.

The main problem is that this case leads to a recursion through the vlapic_error() function in Xen, an open-source hypervisor. Although the recursion is bounded and only generates an interrupt when a new status bit is set, the lock that protects the state within Xen becomes recursively taken, causing a deadlock situation.

Let's take a look at the code snippet below, which demonstrates the call to vlapic_error()

static void vlapic_error(struct vlapic *vlapic)
{
    unsigned int lvt1, lvt2;
    unsigned long flags;
    struct domain *d = vlapic->domain;

    spin_lock_irqsave(&vlapic->apic_lock, flags);

    lvt1 = vlapic_get_reg(vlapic, APIC_LVTERR);
    lvt2 = vlapic->lvt[APIC_LVTERR];

    if ( /* error conditions */ )
    {
        /* ... */
        if ( lvt1 != lvt2 )
        {
            gdprintk(XENLOG_WARNING,
                     "vlapic_error: Different LVT Error values: x%x/x%x\n",
                     lvt1, lvt2);
            vlapic_set_reg(vlapic, APIC_LVTERR, lvt2);
        }

        if ( lvt1 & APIC_LVT_MASKED )
            goto out;

        if ( lvt1 & APIC_MODE_MASK )
            vlapic_error(vlapic); /* recursive call */
        /* ... */
    }

out:
    spin_unlock_irqrestore(&vlapic->apic_lock, flags);
}

In this code snippet, the vlapic_error() function is recursively called under certain error conditions. The deadlock situation occurs because the spin_lock_irqsave() function is called before entering the recursion.

Exploit Details

An attacker can exploit this vulnerability by inducing a specific error condition within the x86 APIC architecture, which triggers the recursion and subsequently, the deadlock situation. By causing the system to re-enter the vlapic_error() function and attempting to take the lock again, the system becomes unusable.

Original References

1. CVE Reference: CVE-2024-45817
2. Mailing List Post: Xen-devel mailing list post
3. Patch Submission: Xen-devel Patch Submission

Mitigation Strategies

To address this vulnerability, a patch has been proposed that prevents the recursive call to the vlapic_error() function. The patch ensures that the recursion does not occur, and the lock is appropriately released, avoiding the deadlock situation.

In conclusion, the CVE-2024-45817 vulnerability within the x86's APIC architecture and Xen demonstrates the potential for exploitation by malicious actors. While the patch proposed helps mitigate the issue, users and developers should remain aware of this and similar vulnerabilities in the future to protect their systems and software.

Timeline

Published on: 09/25/2024 11:15:12 UTC
Last modified on: 11/21/2024 09:38:08 UTC