[PATCH -next] serial: 8250: fix return error code in serial8250_request_std_resource()

[Yi Yang]

If port->mapbase = NULL in serial8250_request_std_resource() , it need
return a error code instead of 0. If uart_set_info() fail to request new
regions by serial8250_request_std_resource() but the return value of
serial8250_request_std_resource() is 0, that The system will mistakenly
considers that port resources are successfully applied for. A null
pointer reference is triggered when the port resource is later invoked.

The problem can also be triggered with the following simple program:
----------
  #include <stdio.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <sys/ioctl.h>
  #include <unistd.h>
  #include <errno.h>

  struct serial_struct {
      int type;
      int line;
      unsigned int    port;
      int irq;
      int flags;
      int xmit_fifo_size;
      int custom_divisor;
      int baud_base;
      unsigned short  close_delay;
      char    io_type;
      char    reserved_char[1];
      int hub6;
      unsigned short  closing_wait; /* time to wait before closing */
      unsigned short  closing_wait2; /* no longer used... */
      unsigned char   *iomem_base;
      unsigned short  iomem_reg_shift;
      unsigned int    port_high;
      unsigned long   iomap_base; /* cookie passed into ioremap */
  };

  struct serial_struct str;

  int main(void)
  {
      open("/dev/ttyS0", O_RDWR);
      ioctl(fd, TIOCGSERIAL, &str);
      str.iomem_base = 0;
      ioctl(fd, TIOCSSERIAL, str);
      return 0;
  }
----------

Signed-off-by: Yi Yang <yiyang13@xxxxxxxxxx>
---
 drivers/tty/serial/8250/8250_port.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3e3d784aa628..e1cefa97bdeb 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2961,8 +2961,10 @@ static int serial8250_request_std_resource(struct uart_8250_port *up)
 	case UPIO_MEM32BE:
 	case UPIO_MEM16:
 	case UPIO_MEM:
-		if (!port->mapbase)
+		if (!port->mapbase) {
+			ret = -EFAULT;
 			break;
+		}
 
 		if (!request_mem_region(port->mapbase, size, "serial")) {
 			ret = -EBUSY;
-- 
2.17.1