Description

In the Linux kernel, the following vulnerability has been resolved: iommu/vt-d: Force requesting ACS when tboot is enabled Currently the conditions of requesting ACS in detect_intel_iommu() don't include tboot, leading to a possible misconfiguration with ACS disabled (e.g. due to user opts) while iommu is later forced on by tboot_force_iommu(). Fix it by checking tboot in detect_intel_iommu().

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.3",
  "pubDate": "2026-09-11T20:19:25.347Z",
  "pubdate": "2026-09-11T20:19:25.347Z",
  "executiveSummary": "This vulnerability involves an improper configuration of the Intel IOMMU (vt-d) subsystem in the Linux kernel when operating in conjunction with Trusted Boot (tboot).\nThe core issue lies in an incomplete conditional check within the 'detect_intel_iommu()' function that fails to enforce Access Control Services (ACS) when tboot is active.\nIf ACS is disabled by user-space configuration, the system may inadvertently allow DMA requests that should be restricted, even if tboot subsequently forces the IOMMU to enable.\nThe primary impact is the potential for compromised device isolation, which could allow malicious hardware or compromised device drivers to perform unauthorized memory access via DMA.\nExploitation requires an attacker to have the ability to influence boot parameters or reside within a system where device isolation is misconfigured, effectively bypassing the security guarantees provided by the IOMMU.\nRisk is elevated in environments relying on tboot for measured launch, as the inconsistency between expected security posture and actual hardware isolation state creates an exploitable window for privilege escalation or unauthorized data access.",
  "technicalDetails": "The vulnerability originates in the 'drivers/iommu/intel/iommu.c' file, specifically within the 'detect_intel_iommu()' function. This function is responsible for evaluating hardware capabilities and system settings to determine whether to enable Intel IOMMU (vt-d) and associated security features like ACS.\nIn the Linux kernel's initialization sequence, the kernel checks whether the IOMMU should be enabled. If tboot is active, the system calls 'tboot_force_iommu()', which mandates the activation of the IOMMU regardless of certain user configurations.\nHowever, the logic governing the request for ACS—a PCIe feature that ensures peer-to-peer transactions remain within the IOMMU's visibility—did not account for the state of tboot. Consequently, if a user provided boot arguments that disabled ACS, the system would honor that preference during the initial detection phase, even if tboot later forces the IOMMU to enable.\nThe attack flow proceeds as follows: An attacker with local access or the ability to manipulate boot parameters ensures that ACS is explicitly disabled (e.g., via kernel command-line parameters). Upon system boot, 'detect_intel_iommu()' evaluates the configuration; because it does not recognize the pending tboot requirement for forced IOMMU enablement, it proceeds with the user-defined, insecure ACS configuration.\nWhen tboot initializes and invokes 'tboot_force_iommu()', the IOMMU is enabled, but the ACS features required for comprehensive isolation remain disabled due to the earlier logic path. This leaves the system in a state where the IOMMU is active, but individual PCIe device transactions are not correctly routed or restricted by ACS, creating a vulnerability to DMA-based attacks.\nA malicious actor could leverage this misconfiguration by exploiting a peripheral device to perform DMA read/write operations to memory regions that should have been protected. Since ACS is bypassed, the IOMMU cannot effectively mediate traffic that does not transit through the root complex in the expected manner. This facilitates arbitrary memory read/write capabilities, potentially leading to kernel memory corruption, exfiltration of cryptographic keys, or escalation of privileges to Ring 0.\nThis issue is strictly related to the order of operations and the logic gaps in the initialization flow of the Intel IOMMU driver and is particularly relevant in platforms utilizing Trusted Execution Technology (TXT) and tboot for platform integrity."
}