arm64/entry: Mask DAIF in cpu_switch_to(), call_on_irq_stack()

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

arm64/entry: Mask DAIF in cpu_switch_to(), call_on_irq_stack()

`cpu_switch_to()` and `call_on_irq_stack()` manipulate SP to change

to different stacks along with the Shadow Call Stack if it is enabled.

Those two stack changes cannot be done atomically and both functions

can be interrupted by SErrors or Debug Exceptions which, though unlikely,

is very much broken : if interrupted, we can end up with mismatched stacks

and Shadow Call Stack leading to clobbered stacks.

In `cpu_switch_to()`, it can happen when SP_EL0 points to the new task,

but x18 stills points to the old task's SCS. When the interrupt handler

tries to save the task's SCS pointer, it will save the old task

SCS pointer (x18) into the new task struct (pointed to by SP_EL0),

clobbering it.

In `call_on_irq_stack()`, it can happen when switching from the task stack

to the IRQ stack and when switching back. In both cases, we can be

interrupted when the SCS pointer points to the IRQ SCS, but SP points to

the task stack. The nested interrupt handler pushes its return addresses

on the IRQ SCS. It then detects that SP points to the task stack,

calls `call_on_irq_stack()` and clobbers the task SCS pointer with

the IRQ SCS pointer, which it will also use !

This leads to tasks returning to addresses on the wrong SCS,

or even on the IRQ SCS, triggering kernel panics via CONFIG_VMAP_STACK

or FPAC if enabled.

This is possible on a default config, but unlikely.

However, when enabling CONFIG_ARM64_PSEUDO_NMI, DAIF is unmasked and

instead the GIC is responsible for filtering what interrupts the CPU

should receive based on priority.

Given the goal of emulating NMIs, pseudo-NMIs can be received by the CPU

even in `cpu_switch_to()` and `call_on_irq_stack()`, possibly *very*

frequently depending on the system configuration and workload, leading

to unpredictable kernel panics.

Completely mask DAIF in `cpu_switch_to()` and restore it when returning.

Do the same in `call_on_irq_stack()`, but restore and mask around

the branch.

Mask DAIF even if CONFIG_SHADOW_CALL_STACK is not enabled for consistency

of behaviour between all configurations.

Introduce and use an assembly macro for saving and masking DAIF,

as the existing one saves but only masks IF.