KVM: PPC: Book3S HV: Prohibit setting illegal transaction state in MSR

Currently it is possible for userspace (e.g. QEMU) to set a value
for the MSR for a guest VCPU which has both of the TS bits set,
which is an illegal combination.  The result of this is that when
we execute a hrfid (hypervisor return from interrupt doubleword)
instruction to enter the guest, the CPU will take a TM Bad Thing
type of program interrupt (vector 0x700).

Now, if PR KVM is configured in the kernel along with HV KVM, we
actually handle this without crashing the host or giving hypervisor
privilege to the guest; instead what happens is that we deliver a
program interrupt to the guest, with SRR0 reflecting the address
of the hrfid instruction and SRR1 containing the MSR value at that
point.  If PR KVM is not configured in the kernel, then we try to
run the host's program interrupt handler with the MMU set to the
guest context, which almost certainly causes a host crash.

This closes the hole by making kvmppc_set_msr_hv() check for the
illegal combination and force the TS field to a safe value (00,
meaning non-transactional).

Cc: stable@vger.kernel.org # v3.9+
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv.c

@@ -224,6 +224,12 @@ static void kvmppc_core_vcpu_put_hv(struct kvm_vcpu *vcpu)
 
 static void kvmppc_set_msr_hv(struct kvm_vcpu *vcpu, u64 msr)
 {
+	/*
+	 * Check for illegal transactional state bit combination
+	 * and if we find it, force the TS field to a safe state.
+	 */
+	if ((msr & MSR_TS_MASK) == MSR_TS_MASK)
+		msr &= ~MSR_TS_MASK;
 	vcpu->arch.shregs.msr = msr;
 	kvmppc_end_cede(vcpu);
 }