arm64: smp: Don't enter kernel with NULL stack pointer or task struct

Although SMP bringup is inherently racy, we can significantly reduce
the window during which secondary CPUs can unexpectedly enter the
kernel by sanity checking the 'stack' and 'task' fields of the
'secondary_data' structure. If the booting CPU gave up waiting for us,
then they will have been cleared to NULL and we should spin in a WFE; WFI
loop instead.
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/head.S

@@ -724,14 +724,22 @@ __secondary_switched:
 
 	adr_l	x0, secondary_data
 	ldr	x1, [x0, #CPU_BOOT_STACK]	// get secondary_data.stack
+	cbz	x1, __secondary_too_slow
 	mov	sp, x1
 	ldr	x2, [x0, #CPU_BOOT_TASK]
+	cbz	x2, __secondary_too_slow
 	msr	sp_el0, x2
 	mov	x29, #0
 	mov	x30, #0
 	b	secondary_start_kernel
 ENDPROC(__secondary_switched)
 
+__secondary_too_slow:
+	wfe
+	wfi
+	b	__secondary_too_slow
+ENDPROC(__secondary_too_slow)
+
 /*
  * The booting CPU updates the failed status @__early_cpu_boot_status,
  * with MMU turned off.

arch/arm64/kernel/smp.c

@@ -136,6 +136,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 
 	secondary_data.task = NULL;
 	secondary_data.stack = NULL;
+	__flush_dcache_area(&secondary_data, sizeof(secondary_data));
 	status = READ_ONCE(secondary_data.status);
 	if (ret && status) {