lib: cpus: sanity check pointers before use

The cpu_ops structure contains a lot of function pointers. It
is a good idea to verify that the function pointer is not NULL
before executing it.

This patch sanity checks each pointer before use to prevent any
unforeseen crashes. These checks have been enabled for debug
builds only.

Change-Id: Ib208331c20e60f0c7c582a20eb3d8cc40fb99d21
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

lib/cpus/aarch64/cpu_helpers.S

@@ -78,6 +78,10 @@ func prepare_cpu_pwr_dwn
 	mov	x1, #CPU_PWR_DWN_OPS
 	add	x1, x1, x2, lsl #3
 	ldr	x1, [x0, x1]
+#if ENABLE_ASSERTIONS
+	cmp	x1, #0
+	ASM_ASSERT(ne)
+#endif
 	br	x1
 endfunc prepare_cpu_pwr_dwn
 
@@ -171,6 +175,10 @@ func get_cpu_ops_ptr
 
 	/* Subtract the increment and offset to get the cpu-ops pointer */
 	sub	x0, x4, #(CPU_OPS_SIZE + CPU_MIDR)
+#if ENABLE_ASSERTIONS
+	cmp	x0, #0
+	ASM_ASSERT(ne)
+#endif
 error_exit:
 	ret
 endfunc get_cpu_ops_ptr
@@ -276,7 +284,15 @@ func print_errata_status
 	 * turn.
 	 */
 	mrs	x0, tpidr_el3
+#if ENABLE_ASSERTIONS
+	cmp	x0, #0
+	ASM_ASSERT(ne)
+#endif
 	ldr	x1, [x0, #CPU_DATA_CPU_OPS_PTR]
+#if ENABLE_ASSERTIONS
+	cmp	x1, #0
+	ASM_ASSERT(ne)
+#endif
 	ldr	x0, [x1, #CPU_ERRATA_FUNC]
 	cbz	x0, .Lnoprint
 
@@ -326,6 +342,10 @@ func check_wa_cve_2017_5715
 	ASM_ASSERT(ne)
 #endif
 	ldr	x0, [x0, #CPU_DATA_CPU_OPS_PTR]
+#if ENABLE_ASSERTIONS
+	cmp	x0, #0
+	ASM_ASSERT(ne)
+#endif
 	ldr	x0, [x0, #CPU_EXTRA1_FUNC]
 	/*
 	 * If the reserved function pointer is NULL, this CPU
@@ -359,6 +379,10 @@ func wa_cve_2018_3639_get_disable_ptr
 	ASM_ASSERT(ne)
 #endif
 	ldr	x0, [x0, #CPU_DATA_CPU_OPS_PTR]
+#if ENABLE_ASSERTIONS
+	cmp	x0, #0
+	ASM_ASSERT(ne)
+#endif
 	ldr	x0, [x0, #CPU_EXTRA2_FUNC]
 	ret
 endfunc wa_cve_2018_3639_get_disable_ptr