xen/arm: vgic-v2: Don't crash the hypervisor if the SGI target mode is invalid

The GICv2 spec reserved the value 0b11 for GICD_SGIR.TargetListFilter.

Even if it's an invalid value, a malicious guest could write this value
and threfore crash the hypervisor.

Replace the BUG() by logging the error and inject a data abort to the guest.

This was introduced by commit ea37fd2111
"xen/arm: split vgic driver into generic and vgic-v2 driver".

This is CVE-2015-0268 / XSA-117.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/vgic-v2.c

@@ -257,7 +257,10 @@ static int vgic_v2_to_sgi(struct vcpu *v, register_t sgir)
         sgi_mode = SGI_TARGET_SELF;
         break;
     default:
-        BUG();
+        printk(XENLOG_G_DEBUG
+               "%pv: vGICD: unhandled GICD_SGIR write %"PRIregister" with wrong mode\n",
+               v, sgir);
+        return 0;
     }
 
     return vgic_to_sgi(v, sgir, sgi_mode, virq, vcpu_mask);