powerpc/kernel: Avoid memory corruption at early stage

commit 6f20e7f2 upstream.

When calling to early_setup(), we pick "boot_paca" up for the master CPU
and initialize that with initialise_paca(). At that point, the SLB
shadow buffer isn't populated yet. Updating the SLB shadow buffer should
corrupt what we had in physical address 0 where the trap instruction is
usually stored.

This hasn't been observed to cause any trouble in practice, but is
obviously fishy.

Fixes: 6f4441ef ("powerpc: Dynamically allocate slb_shadow from memblock")
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/powerpc/kernel/paca.c

@@ -115,6 +115,14 @@ static struct slb_shadow * __init init_slb_shadow(int cpu)
 {
 	struct slb_shadow *s = &slb_shadow[cpu];
 
+	/*
+	 * When we come through here to initialise boot_paca, the slb_shadow
+	 * buffers are not allocated yet. That's OK, we'll get one later in
+	 * boot, but make sure we don't corrupt memory at 0.
+	 */
+	if (!slb_shadow)
+		return NULL;
+
 	s->persistent = cpu_to_be32(SLB_NUM_BOLTED);
 	s->buffer_length = cpu_to_be32(sizeof(*s));