CVE-2026-80861 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: xhci: bail out of setup if the controller is inaccessible
xhci_gen_setup() locates the operational registers using the capability length read from the very first register:
xhci->op_regs = hcd->regs + HC_LENGTH(readl(&xhci->cap_regs->hc_capbase));
If the controller is dead or has dropped off the bus, that read returns ~0, HC_LENGTH() truncates it to 0xff, and op_regs ends up 0xff bytes past the page-aligned MMIO base, i.e. unaligned. The first access through it, xhci_halt() -> xhci_handshake() reading op_regs->status, is then an unaligned readl() on device memory. arm64 faults on unaligned device accesses, so instead of xhci_handshake() catching the all-ones value and returning -ENODEV, setup oopses:
xhci-pci-renesas 0005:08:00.0: Unable to change power state from D3cold to D0, device inaccessible xhci-pci-renesas 0005:08:00.0: xHCI Host Controller xhci-pci-renesas 0005:08:00.0: new USB bus registered, assigned bus number 1 Unable to handle kernel paging request at virtual address ffff80030a770103 ESR = 0x0000000096000021 FSC = 0x21: alignment fault Internal error: Oops: 0000000096000021 [#1] SMP
pc : xhci_halt [xhci_hcd]
Call trace: xhci_halt xhci_gen_setup xhci_pci_setup usb_add_hcd usb_hcd_pci_probe xhci_pci_common_probe xhci_pci_renesas_probe
This was hit with a Renesas uPD720201 that failed to power up ("Unable to change power state from D3cold to D0, device inaccessible") yet still reached the HCD probe path.
Read the capability register once, and if it reads back the all-ones value (as xhci_handshake() and xhci_reset() already test for), abort setup with -ENODEV before op_regs is derived from it. Reading it once also avoids re-reading a register that may change under a concurrent hot-removal.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified in the Linux kernel's USB xHCI driver involves an improper handling of inaccessible hardware controllers during initialization, specifically within the xhci_gen_setup function. This flaw allows for unaligned memory access on architectures such as arm64 that strictly enforce alignment rules for device memory operations, leading to a kernel panic or system crash when attempting to probe certain faulty or disconnected USB host controller devices. The root cause lies in how the driver calculates the base address of operational registers based on capability data read from hardware without sufficient validation of the physical state of the device itself.
When xhci_gen_setup executes, it attempts to locate the operational register space by reading a value from the first capability register and applying the HC_LENGTH macro. If the USB controller is dead, has dropped off the bus, or fails to power up correctly as seen in cases involving Renesas uPD720201 chips that cannot transition from D3cold to D0 state, this initial read operation returns all ones, represented as ~0. The HC_LENGTH macro then truncates this value to 0xff bytes. Consequently, the calculated address for operational registers becomes unaligned relative to the page-aligned MMIO base because it is offset by an odd number of bytes rather than a multiple aligned with hardware expectations.
The critical failure occurs when subsequent functions like xhci_halt attempt to access these derived addresses through operations such as reading the status register via xhci_handshake. On architectures including arm64, unaligned accesses to device memory are not permitted and trigger an alignment fault exception. Instead of gracefully detecting that the device is inaccessible by recognizing the all-ones value during setup, the kernel proceeds with invalid pointer arithmetic, resulting in a fatal internal error or Oops as documented in the stack trace where xhci_halt calls into xhci_handshake which then faults on the unaligned readl operation. This behavior prevents proper error handling and causes system instability even when the hardware failure is recoverable through standard device removal procedures.
From a security perspective, this issue represents an improper input validation flaw where external or environmental inputs such as hardware state are not adequately checked before being used to compute memory addresses. In terms of industry standards, this aligns with CWE-125 Out-of-bounds Read if the unaligned access leads to reading unintended memory regions, though more accurately it reflects CWE-682 Incorrect Calculation due to improper handling of edge cases in arithmetic operations involving hardware registers. The attack vector is local and requires physical or logical access to trigger a device state that causes the probe path to execute despite hardware failure, potentially leading to denial of service through system crash without privilege escalation since kernel memory integrity remains intact but availability is compromised.
The operational impact includes potential system crashes during USB enumeration processes when encountering faulty peripherals or controllers with power management issues. This affects reliability in environments where hot-plugging occurs frequently or where legacy hardware may exhibit erratic behavior during initialization sequences. Users might experience unexpected reboots or frozen systems specifically tied to USB subsystem interactions, particularly on ARM-based platforms that enforce strict alignment constraints for memory-mapped I/O operations.
Mitigation strategies involve applying the upstream kernel patch which modifies xhci_gen_setup to validate the capability register read immediately upon retrieval. If the value reads back as all ones indicating an inaccessible device, the setup routine should abort early with -ENODEV error code before deriving any operational register addresses from that invalid data. This approach prevents the calculation of unaligned pointers and ensures consistent behavior across different architectures by treating dead devices similarly to how they are handled in handshake routines. Additionally, reading the capability register only once avoids race conditions associated with concurrent hot-removal events where hardware state might change unpredictably between reads.
Administrators should ensure their systems run patched versions of the Linux kernel containing this fix to prevent crashes related to USB controller initialization failures. For organizations managing large fleets of devices including those using Renesas controllers, monitoring for D3cold power transition errors can help identify affected units before they cause systemic issues during boot or runtime enumeration phases. Regular updates and adherence to vendor recommendations regarding hardware compatibility will further reduce exposure to such low-level driver defects that stem from assumptions about stable device presence during initialization sequences.