KVM: x86: Bypass register cache when querying CPL from kvm_sched_out()

When querying guest CPL to determine if a vCPU was preempted while in
kernel mode, bypass the register cache, i.e. always read SS.AR_BYTES from
the VMCS on Intel CPUs.  If the kernel is running with full preemption
enabled, using the register cache in the preemption path can result in
stale and/or uninitialized data being cached in the segment cache.

In particular the following scenario is currently possible:

 - vCPU is just created, and the vCPU thread is preempted before
   SS.AR_BYTES is written in vmx_vcpu_reset().

 - When scheduling out the vCPU task, kvm_arch_vcpu_in_kernel() =>
   vmx_get_cpl() reads and caches '0' for SS.AR_BYTES.

 - vmx_vcpu_reset() => seg_setup() configures SS.AR_BYTES, but doesn't
   invoke vmx_segment_cache_clear() to invalidate the cache.

As a result, KVM retains a stale value in the cache, which can be read,
e.g. via KVM_GET_SREGS.  Usually this is not a problem because the VMX
segment cache is reset on each VM-Exit, but if the userspace VMM (e.g KVM
selftests) reads and writes system registers just after the vCPU was
created, _without_ modifying SS.AR_BYTES, userspace will write back the
stale '0' value and ultimately will trigger a VM-Entry failure due to
incorrect SS segment type.

Note, the VM-Enter failure can also be avoided by moving the call to
vmx_segment_cache_clear() until after the vmx_vcpu_reset() initializes all
segments.  However, while that change is correct and desirable (and will
come along shortly), it does not address the underlying problem that
accessing KVM's register caches from !task context is generally unsafe.

In addition to fixing the immediate bug, bypassing the cache for this
particular case will allow hardening KVM register caching log to assert
that the caches are accessed only when KVM _knows_ it is safe to do so.

Fixes: de63ad4cf497 ("KVM: X86: implement the logic for spinlock optimization")
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Closes: https://lore.kernel.org/all/20240716022014.240960-3-mlevitsk@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20241009175002.1118178-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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