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. ]