linux-kernel - Re: [PATCH -next] serial: 8250: fix return error code in serial8250_request_std

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Date:   Mon, 27 Jun 2022 14:09:33 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Yi Yang <yiyang13@...wei.com>, Jiri Slaby <jirislaby@...nel.org>,
        andy.shevchenko@...il.com,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] serial: 8250: fix return error code in
 serial8250_request_std_resource()

On Mon, Jun 20, 2022 at 10:53:24AM +0300, Ilpo Järvinen wrote:
> On Mon, 20 Jun 2022, Yi Yang wrote:
> 
> > 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;
> >   }
> 
> With admin priviledges I guess?
> 
> > ----------
> > 
> > Signed-off-by: Yi Yang <yiyang13@...wei.com>
> > ---
> >  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;
> > 
> 
> I recall reading somewhere that somebody more knowledgeful than me noted 
> that this interface has many ways to shoot oneself in the foot if one 
> really wants to which is why some things are limited to admin only.
> I cannot seem to find that a reference to that now though.

Yes, what could go wrong with allowing userspace to specify memory
locations that a uart might be located at :)

This stuff should all be "locked down" for any system with untrusted
users.

thanks,

greg k-h