arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors

arm64: Fix panic() when Spectre-v2 causes Spectre-BHB to re-allocate KVM vectors

Sami reports that linux panic()s when resuming from suspend to RAM. This
is because when CPUs are brought back online, they re-enable any
necessary mitigations.

The Spectre-v2 and Spectre-BHB mitigations interact as both need to
done by KVM when exiting a guest. Slots KVM can use as vectors are
allocated, and templates for the mitigation are patched into the vector.

This fails if a new slot needs to be allocated once the kernel has finished
booting as it is no-longer possible to modify KVM's vectors:
| root@adam:/sys/devices/system/cpu/cpu1# echo 1 > online
| Unable to handle kernel write to read-only memory at virtual add>
| Mem abort info:
|   ESR = 0x9600004e
|   Exception class = DABT (current EL), IL = 32 bits
|   SET = 0, FnV = 0
|   EA = 0, S1PTW = 0
| Data abort info:
|   ISV = 0, ISS = 0x0000004e
|   CM = 0, WnR = 1
| swapper pgtable: 4k pages, 48-bit VAs, pgdp = 000000000f07a71c
| [ffff800000b4b800] pgd=00000009ffff8803, pud=00000009ffff7803, p>
| Internal error: Oops: 9600004e [#1] PREEMPT SMP
| Modules linked in:
| Process swapper/1 (pid: 0, stack limit = 0x0000000063153c53)
| CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.252-dirty #14
| Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno De>
| pstate: 000001c5 (nzcv dAIF -PAN -UAO)
| pc : __memcpy+0x48/0x180
| lr : __copy_hyp_vect_bpi+0x64/0x90

| Call trace:
|  __memcpy+0x48/0x180
|  kvm_setup_bhb_slot+0x204/0x2a8
|  spectre_bhb_enable_mitigation+0x1b8/0x1d0
|  __verify_local_cpu_caps+0x54/0xf0
|  check_local_cpu_capabilities+0xc4/0x184
|  secondary_start_kernel+0xb0/0x170
| Code: b8404423 b80044c3 36180064 f8408423 (f80084c3)
| ---[ end trace 859bcacb09555348 ]---
| Kernel panic - not syncing: Attempted to kill the idle task!
| SMP: stopping secondary CPUs
| Kernel Offset: disabled
| CPU features: 0x10,25806086
| Memory Limit: none
| ---[ end Kernel panic - not syncing: Attempted to kill the idle ]

This is only a problem on platforms where there is only one CPU that is
vulnerable to both Spectre-v2 and Spectre-BHB.

The Spectre-v2 mitigation identifies the slot it can re-use by the CPU's
'fn'. It unconditionally writes the slot number and 'template_start'
pointer. The Spectre-BHB mitigation identifies slots it can re-use by
the CPU's template_start pointer, which was previously clobbered by the
Spectre-v2 mitigation.

When there is only one CPU that is vulnerable to both issues, this causes
Spectre-v2 to try to allocate a new slot, which fails.

Change both mitigations to check whether they are changing the slot this
CPU uses before writing the percpu variables again.

This issue only exists in the stable backports for Spectre-BHB which have
to use totally different infrastructure to mainline.

Reported-by: Sami Lee <sami.lee@mediatek.com>
Fixes: 3e3904125fcc ("arm64: Mitigate spectre style branch history side channels")
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>