powerpc/kernel: Avoid memory corruption at early stage

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: 6f4441ef7009 ("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>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index d6e195e8cd4c..5a23b69f8129 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/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));