CVE-2026-43363 x86/apic: Disable x2apic on resume if the kernel expects so

What is CVE-2026-43363?

CVE-2026-43363 is a medium-severity denial of service vulnerability in Linux kernel, published on 8 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

x86/apic: Disable x2apic on resume if the kernel expects so

When resuming from s2ram, firmware may re-enable x2apic mode, which may have been disabled by the kernel during boot either because it doesn't support IRQ remapping or for other reasons. This causes the kernel to continue using the xapic interface, while the hardware is in x2apic mode, which causes hangs. This happens on defconfig + bare metal + s2ram.

Fix this in lapic_resume() by disabling x2apic if the kernel expects it to be disabled, i.e. when x2apic_mode = 0.

The ACPI v6.6 spec, Section 16.3 [1] says firmware restores either the pre-sleep configuration or initial boot configuration for each CPU, including MSR state:

When executing from the power-on reset vector as a result of waking from an S2 or S3 sleep state, the platform firmware performs only the hardware initialization required to restore the system to either the state the platform was in prior to the initial operating system boot, or to the pre-sleep configuration state. In multiprocessor systems, non-boot processors should be placed in the same state as prior to the initial operating system boot.

(further ahead)

If this is an S2 or S3 wake, then the platform runtime firmware restores minimum context of the system before jumping to the waking vector. This includes:

CPU configuration. Platform runtime firmware restores the pre-sleep configuration or initial boot configuration of each CPU (MSR, MTRR, firmware update, SMBase, and so on). Interrupts must be disabled (for IA-32 processors, disabled by CLI instruction).

(and other things)

So at least as per the spec, re-enablement of x2apic by the firmware is allowed if "x2apic on" is a part of the initial boot configuration.

[1] https://uefi.org/specs/ACPI/6.6/16_Waking_and_Sleeping.html#initialization

[ bp: Massage. ]

Which versions are affected by CVE-2026-43363?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-43363?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-43363 being exploited?

There is no known exploitation of CVE-2026-43363 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2026-43363, DEBIAN-CVE-2026-43363, GHSA-j96g-282q-pp54, ROOT-OS-DEBIAN-11-CVE-2026-43363, ROOT-OS-UBUNTU-2204-CVE-2026-43363, ROOT-OS-UBUNTU-2404-CVE-2026-43363 and 1 more.