Pass VM_PROT_EXECUTE to vm_fault for instruction faults.

We need to tell vm_fault the reason for the fault was because we tried to
execute from the memory location. Without this it may return with success
as we only request read-only memory, then we return to the same location
and try to execute from the same memory address. This leads to an infinite
loop raising the same fault and returning to the same invalid location.

MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D18511

sys/arm64/arm64/trap.c

@@ -149,7 +149,7 @@ svc_handler(struct thread *td, struct trapframe *frame)
 
 static void
 data_abort(struct thread *td, struct trapframe *frame, uint64_t esr,
-    uint64_t far, int lower)
+    uint64_t far, int lower, int exec)
 {
 	struct vm_map *map;
 	struct proc *p;
@@ -229,6 +229,8 @@ data_abort(struct thread *td, struct trapframe *frame, uint64_t esr,
 
 	va = trunc_page(far);
 	ftype = ((esr >> 6) & 1) ? VM_PROT_READ | VM_PROT_WRITE : VM_PROT_READ;
+	if (exec)
+		ftype |= VM_PROT_EXECUTE;
 
 	/* Fault in the page. */
 	error = vm_fault(map, va, ftype, VM_FAULT_NORMAL);
@@ -336,7 +338,8 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 	case EXCP_DATA_ABORT:
 		far = READ_SPECIALREG(far_el1);
 		intr_enable();
-		data_abort(td, frame, esr, far, 0);
+		data_abort(td, frame, esr, far, 0,
+		    exception == EXCP_INSN_ABORT);
 		break;
 	case EXCP_BRK:
 #ifdef KDTRACE_HOOKS
@@ -433,7 +436,8 @@ do_el0_sync(struct thread *td, struct trapframe *frame)
 	case EXCP_INSN_ABORT_L:
 	case EXCP_DATA_ABORT_L:
 	case EXCP_DATA_ABORT:
-		data_abort(td, frame, esr, far, 1);
+		data_abort(td, frame, esr, far, 1,
+		    exception == EXCP_INSN_ABORT_L);
 		break;
 	case EXCP_UNKNOWN:
 		if (!undef_insn(0, frame))