KVM: x86: Fix a 4.14 backport regression related to userspace/guest FPU

Upstream commit:

    f775b13e ("x86,kvm: move qemu/guest FPU switching out to vcpu_run")

introduced a bug, which was later fixed by upstream commit:

    5663d8f9 ("kvm: x86: fix WARN due to uninitialized guest FPU state")

For reasons unknown, both commits were initially passed-over for
inclusion in the 4.14 stable branch despite being tagged for stable.
Eventually, someone noticed that the fixup, commit 5663d8f9, was
missing from stable[1], and so it was queued up for 4.14 and included in
release v4.14.79.

Even later, the original buggy patch, commit f775b13e, was also
applied to the 4.14 stable branch.  Through an unlucky coincidence, the
incorrect ordering did not generate a conflict between the two patches,
and led to v4.14.94 and later releases containing a spurious call to
kvm_load_guest_fpu() in kvm_arch_vcpu_ioctl_run().  As a result, KVM may
reload stale guest FPU state, e.g. after accepting in INIT event.  This
can manifest as crashes during boot, segfaults, failed checksums and so
on and so forth.

Remove the unwanted kvm_{load,put}_guest_fpu() calls, i.e. make
kvm_arch_vcpu_ioctl_run() look like commit 5663d8f9 was backported
after commit f775b13e.

[1] https://www.spinics.net/lists/stable/msg263931.html

Fixes: 4124a4cf ("x86,kvm: move qemu/guest FPU switching out to vcpu_run")
Cc: stable@vger.kernel.org
Cc: Sasha Levin <sashal@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Reported-by: Roman Mamedov
Reported-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kvm/x86.c

@@ -7422,14 +7422,12 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 		}
 	}
 
-	kvm_load_guest_fpu(vcpu);
-
 	if (unlikely(vcpu->arch.complete_userspace_io)) {
 		int (*cui)(struct kvm_vcpu *) = vcpu->arch.complete_userspace_io;
 		vcpu->arch.complete_userspace_io = NULL;
 		r = cui(vcpu);
 		if (r <= 0)
-			goto out_fpu;
+			goto out;
 	} else
 		WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
 
@@ -7438,8 +7436,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	else
 		r = vcpu_run(vcpu);
 
-out_fpu:
-	kvm_put_guest_fpu(vcpu);
 out:
 	kvm_put_guest_fpu(vcpu);
 	post_kvm_run_save(vcpu);