[PATCH] powerpc: fix use-after-free on fixup_port_irq()

[Wen Yang]

There is a possible use-after-free issue in the fixup_port_irq():

460 static void __init fixup_port_irq(int index,
461                                   struct device_node *np,
462                                   struct plat_serial8250_port *port)
463 {
...
469         if (!virq && legacy_serial_infos[index].irq_check_parent) {
470                 np = of_get_parent(np);  --> modified here.
...
474                 of_node_put(np); ---> released here
475         }
...
481 #ifdef CONFIG_SERIAL_8250_FSL
482   if (of_device_is_compatible(np, "fsl,ns16550")) --> dereferenced here
...
484 #endif
485 }

We solve this problem by introducing a new parent_np variable.

Fixes: 9deaa53ac7fa ("serial: add irq handler for Freescale 16550 errata.")
Signed-off-by: Wen Yang <wen.yan...@zte.com.cn>
Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org>
Cc: Paul Mackerras <pau...@samba.org>
Cc: Michael Ellerman <m...@ellerman.id.au>
Cc: Rob Herring <r...@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-ker...@vger.kernel.org
---
 arch/powerpc/kernel/legacy_serial.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/legacy_serial.c 
b/arch/powerpc/kernel/legacy_serial.c
index 7cea597..0105f3e 100644
--- a/arch/powerpc/kernel/legacy_serial.c
+++ b/arch/powerpc/kernel/legacy_serial.c
@@ -461,17 +461,18 @@ static void __init fixup_port_irq(int index,
                                  struct device_node *np,
                                  struct plat_serial8250_port *port)
 {
+       struct device_node *parent_np;
        unsigned int virq;
 
        DBG("fixup_port_irq(%d)\n", index);
 
        virq = irq_of_parse_and_map(np, 0);
        if (!virq && legacy_serial_infos[index].irq_check_parent) {
-               np = of_get_parent(np);
-               if (np == NULL)
+               parent_np = of_get_parent(np);
+               if (parent_np == NULL)
                        return;
-               virq = irq_of_parse_and_map(np, 0);
-               of_node_put(np);
+               virq = irq_of_parse_and_map(parent_np, 0);
+               of_node_put(parent_np);
        }
        if (!virq)
                return;
-- 
2.9.5