[PATCH] powerpc/64: poison __per_cpu_offset to catch use-before-init

[Nicholas Piggin <npiggin@gmail.com>]

If the boot CPU tries to access per-cpu data of other CPUs before
per cpu areas are set up, it will unexpectedly use offset 0.

Try to catch such accesses by poisoning the __per_cpu_offset array.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
With the early boot machine check handler added to 64s, this might be
worth another try. We did have a bug in mce_init() that was using
cpu_to_node() too early that would be caught by this.

Thanks,
Nick

 arch/powerpc/include/asm/percpu.h | 1 +
 arch/powerpc/kernel/paca.c        | 2 +-
 arch/powerpc/kernel/setup_64.c    | 3 ++-
 3 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/percpu.h b/arch/powerpc/include/asm/percpu.h
index 8e5b7d0b851c..6ca1a9fc5725 100644
--- a/arch/powerpc/include/asm/percpu.h
+++ b/arch/powerpc/include/asm/percpu.h
@@ -7,6 +7,7 @@
  * Same as asm-generic/percpu.h, except that we store the per cpu offset
  * in the paca. Based on the x86-64 implementation.
  */
+#define PER_CPU_OFFSET_POISON 0xfeeeeeeeeeeeeeeeULL
 
 #ifdef CONFIG_SMP
 
diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index be8db402e963..c33de70596e7 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -198,7 +198,7 @@ void __init initialise_paca(struct paca_struct *new_paca, int cpu)
 	new_paca->hw_cpu_id = 0xffff;
 	new_paca->kexec_state = KEXEC_STATE_NONE;
 	new_paca->__current = &init_task;
-	new_paca->data_offset = 0xfeeeeeeeeeeeeeeeULL;
+	new_paca->data_offset = PER_CPU_OFFSET_POISON;
 #ifdef CONFIG_PPC_64S_HASH_MMU
 	new_paca->slb_shadow_ptr = NULL;
 #endif
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index a0dee7354fe6..039a4eac03c3 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -362,6 +362,7 @@ void __init early_setup(unsigned long dt_ptr)
 	 * So set up a temporary paca. It will be replaced below once we know
 	 * what CPU we are on.
 	 */
+	__per_cpu_offset[0] = 0;
 	initialise_paca(&boot_paca, 0);
 	fixup_boot_paca(&boot_paca);
 	WARN_ON(local_paca != 0);
@@ -828,7 +829,7 @@ static __init int pcpu_cpu_to_node(int cpu)
 	return early_cpu_to_node(cpu);
 }
 
-unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
+unsigned long __per_cpu_offset[NR_CPUS] __read_mostly = { [0 ... NR_CPUS-1 ] = PER_CPU_OFFSET_POISON };
 EXPORT_SYMBOL(__per_cpu_offset);
 
 void __init setup_per_cpu_areas(void)
-- 
2.37.2