KVM: arm64: Don't retire aborted MMIO instruction

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

KVM: arm64: Don't retire aborted MMIO instruction

Returning an abort to the guest for an unsupported MMIO access is a

documented feature of the KVM UAPI. Nevertheless, it's clear that this

plumbing has seen limited testing, since userspace can trivially cause a

WARN in the MMIO return:

WARNING: CPU: 0 PID: 30558 at arch/arm64/include/asm/kvm_emulate.h:536 kvm_handle_mmio_return+0x46c/0x5c4 arch/arm64/include/asm/kvm_emulate.h:536

Call trace:

kvm_handle_mmio_return+0x46c/0x5c4 arch/arm64/include/asm/kvm_emulate.h:536

kvm_arch_vcpu_ioctl_run+0x98/0x15b4 arch/arm64/kvm/arm.c:1133

kvm_vcpu_ioctl+0x75c/0xa78 virt/kvm/kvm_main.c:4487

__do_sys_ioctl fs/ioctl.c:51 [inline]

__se_sys_ioctl fs/ioctl.c:893 [inline]

__arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:893

__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]

invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49

el0_svc_common+0x1e0/0x23c arch/arm64/kernel/syscall.c:132

do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151

el0_svc+0x38/0x68 arch/arm64/kernel/entry-common.c:712

el0t_64_sync_handler+0x90/0xfc arch/arm64/kernel/entry-common.c:730

el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

The splat is complaining that KVM is advancing PC while an exception is

pending, i.e. that KVM is retiring the MMIO instruction despite a

pending synchronous external abort. Womp womp.

Fix the glaring UAPI bug by skipping over all the MMIO emulation in

case there is a pending synchronous exception. Note that while userspace

is capable of pending an asynchronous exception (SError, IRQ, or FIQ),

it is still safe to retire the MMIO instruction in this case as (1) they

are by definition asynchronous, and (2) KVM relies on hardware support

for pending/delivering these exceptions instead of the software state

machine for advancing PC.