powerpc: Fix stack protector crashes on CPU hotplug

Recently in commit 7241d26e ("powerpc/64: properly initialise
the stackprotector canary on SMP.") we fixed a crash with stack
protector on SMP by initialising the stack canary in
cpu_idle_thread_init().

But this can also causes crashes, when a CPU comes back online after
being offline:

  Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: pnv_smp_cpu_kill_self+0x2a0/0x2b0
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.0-rc3-gcc-7.3.1-00168-g4ffe713b #94
  Call Trace:
    dump_stack+0xb0/0xf4 (unreliable)
    panic+0x144/0x328
    __stack_chk_fail+0x2c/0x30
    pnv_smp_cpu_kill_self+0x2a0/0x2b0
    cpu_die+0x48/0x70
    arch_cpu_idle_dead+0x20/0x40
    do_idle+0x274/0x390
    cpu_startup_entry+0x38/0x50
    start_secondary+0x5e4/0x600
    start_secondary_prolog+0x10/0x14

Looking at the stack we see that the canary value in the stack frame
doesn't match the canary in the task/paca. That is because we have
reinitialised the task/paca value, but then the CPU coming online has
returned into a function using the old canary value. That causes the
comparison to fail.

Instead we can call boot_init_stack_canary() from start_secondary()
which never returns. This is essentially what the generic code does in
cpu_startup_entry() under #ifdef X86, we should make that non-x86
specific in a future patch.

Fixes: 7241d26e ("powerpc/64: properly initialise the stackprotector canary on SMP.")
Reported-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>

arch/powerpc/kernel/smp.c

@@ -35,6 +35,7 @@
 #include <linux/profile.h>
 #include <linux/processor.h>
 #include <linux/random.h>
+#include <linux/stackprotector.h>
 
 #include <asm/ptrace.h>
 #include <linux/atomic.h>
@@ -1014,16 +1015,9 @@ static void cpu_idle_thread_init(unsigned int cpu, struct task_struct *idle)
 {
 	struct thread_info *ti = task_thread_info(idle);
 
-#ifdef CONFIG_STACKPROTECTOR
-	idle->stack_canary = get_random_canary();
-#endif
-
 #ifdef CONFIG_PPC64
 	paca_ptrs[cpu]->__current = idle;
 	paca_ptrs[cpu]->kstack = (unsigned long)ti + THREAD_SIZE - STACK_FRAME_OVERHEAD;
-#ifdef CONFIG_STACKPROTECTOR
-	paca_ptrs[cpu]->canary = idle->stack_canary;
-#endif
 #endif
 	ti->cpu = cpu;
 	secondary_ti = current_set[cpu] = ti;
@@ -1316,6 +1310,8 @@ void start_secondary(void *unused)
 	notify_cpu_starting(cpu);
 	set_cpu_online(cpu, true);
 
+	boot_init_stack_canary();
+
 	local_irq_enable();
 
 	/* We can enable ftrace for secondary cpus now */