KVM: arm64: Fix clobbered ELR in sync abort/SError

KVM: arm64: Fix clobbered ELR in sync abort/SError

When the hypervisor receives a SError or synchronous exception (EL2h)
while running with the __kvm_hyp_vector and if ELR_EL2 doesn't point to
an extable entry, it panics indirectly by overwriting ELR with the
address of a panic handler in order for the asm routine it returns to to
ERET into the handler.

However, this clobbers ELR_EL2 for the handler itself. As a result,
hyp_panic(), when retrieving what it believes to be the PC where the
exception happened, actually ends up reading the address of the panic
handler that called it! This results in an erroneous and confusing panic
message where the source of any synchronous exception (e.g. BUG() or
kCFI) appears to be __guest_exit_panic, making it hard to locate the
actual BRK instruction.

Therefore, store the original ELR_EL2 in the per-CPU kvm_hyp_ctxt and
point the sysreg to a routine that first restores it to its previous
value before running __guest_exit_panic.

Fixes: 7db21530479f ("KVM: arm64: Restore hyp when panicking in guest context")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240610063244.2828978-2-ptosi@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>