[PATCH] hw/gpio: npcm7xx: fixup out-of-bounds access

[Patrick Venture]

The reg isn't validated to be a possible register before
it's dereferenced for one case.  The mmio space registered
for the gpio device is 4KiB but there aren't that many
registers in the struct.

Google-Bug-Id: 397469048
Change-Id: I2fb8d0d3d41422baab22e8fc7e9fadd0f2ee7068
Signed-off-by: Patrick Venture <vent...@google.com>
---
 hw/gpio/npcm7xx_gpio.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/hw/gpio/npcm7xx_gpio.c b/hw/gpio/npcm7xx_gpio.c
index 23e67424c9..2916056fae 100644
--- a/hw/gpio/npcm7xx_gpio.c
+++ b/hw/gpio/npcm7xx_gpio.c
@@ -220,8 +220,6 @@ static void npcm7xx_gpio_regs_write(void *opaque, hwaddr 
addr, uint64_t v,
         return;
     }
 
-    diff = s->regs[reg] ^ value;
-
     switch (reg) {
     case NPCM7XX_GPIO_TLOCK1:
     case NPCM7XX_GPIO_TLOCK2:
@@ -242,6 +240,7 @@ static void npcm7xx_gpio_regs_write(void *opaque, hwaddr 
addr, uint64_t v,
     case NPCM7XX_GPIO_PU:
     case NPCM7XX_GPIO_PD:
     case NPCM7XX_GPIO_IEM:
+        diff = s->regs[reg] ^ value;
         s->regs[reg] = value;
         npcm7xx_gpio_update_pins(s, diff);
         break;
-- 
2.48.1.658.g4767266eb4-goog