x86/ioapic: Handle allocation failures gracefully

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

x86/ioapic: Handle allocation failures gracefully

Breno observed panics when using failslab under certain conditions during

runtime:

can not alloc irq_pin_list (-1,0,20)

Kernel panic - not syncing: IO-APIC: failed to add irq-pin. Can not proceed

panic+0x4e9/0x590

mp_irqdomain_alloc+0x9ab/0xa80

irq_domain_alloc_irqs_locked+0x25d/0x8d0

__irq_domain_alloc_irqs+0x80/0x110

mp_map_pin_to_irq+0x645/0x890

acpi_register_gsi_ioapic+0xe6/0x150

hpet_open+0x313/0x480

That's a pointless panic which is a leftover of the historic IO/APIC code

which panic'ed during early boot when the interrupt allocation failed.

The only place which might justify panic is the PIT/HPET timer_check() code

which tries to figure out whether the timer interrupt is delivered through

the IO/APIC. But that code does not require to handle interrupt allocation

failures. If the interrupt cannot be allocated then timer delivery fails

and it either panics due to that or falls back to legacy mode.

Cure this by removing the panic wrapper around __add_pin_to_irq_node() and

making mp_irqdomain_alloc() aware of the failure condition and handle it as

any other failure in this function gracefully.