powerpc/64: properly initialise the stackprotector canary on SMP.

commit 06ec27aea9fc ("powerpc/64: add stack protector support")
doesn't initialise the stack canary on SMP secondary CPU's paca,
leading to the following false positive report from the
stack protector.

smp: Bringing up secondary CPUs ...
Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: __schedule+0x978/0xa80
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.0-rc7-next-20181010-autotest-autotest #1
Call Trace:
[c000001fed5b3bf0] [c000000000a0ef3c] dump_stack+0xb0/0xf4 (unreliable)
[c000001fed5b3c30] [c0000000000f9d68] panic+0x140/0x308
[c000001fed5b3cc0] [c0000000000f9844] __stack_chk_fail+0x24/0x30
[c000001fed5b3d20] [c000000000a2c3a8] __schedule+0x978/0xa80
[c000001fed5b3e00] [c000000000a2c9b4] schedule_idle+0x34/0x60
[c000001fed5b3e30] [c00000000013d344] do_idle+0x224/0x3d0
[c000001fed5b3ec0] [c00000000013d6e0] cpu_startup_entry+0x30/0x50
[c000001fed5b3ef0] [c000000000047f34] start_secondary+0x4d4/0x520
[c000001fed5b3f90] [c00000000000b370] start_secondary_prolog+0x10/0x14

This patch properly initialises the stack_canary of the secondary
idle tasks.

Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Fixes: 06ec27aea9fc ("powerpc/64: add stack protector support")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/smp.c

@@ -34,6 +34,7 @@
 #include <linux/topology.h>
 #include <linux/profile.h>
 #include <linux/processor.h>
+#include <linux/random.h>
 
 #include <asm/ptrace.h>
 #include <linux/atomic.h>
@@ -810,9 +811,16 @@ 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;