Re: [kernel-hardening] Re: [PATCH 2/3] Make static usermode helper binaries constant

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 17, 2017 at 04:29:19PM +0100, Greg KH wrote:
> On Tue, Jan 17, 2017 at 10:19:11AM -0500, J. Bruce Fields wrote:
> > On Tue, Jan 17, 2017 at 08:13:47AM +0100, Greg KH wrote:
> > > On Mon, Jan 16, 2017 at 04:25:55PM -0500, J. Bruce Fields wrote:
> > > > On Mon, Jan 16, 2017 at 05:50:31PM +0100, Greg KH wrote:
> > > > > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > > 
> > > > > There are a number of usermode helper binaries that are "hard coded" in
> > > > > the kernel today, so mark them as "const" to make it harder for someone
> > > > > to change where the variables point to.
> > > > > 
> > > > ...
> > > > > --- a/drivers/pnp/pnpbios/core.c
> > > > > +++ b/drivers/pnp/pnpbios/core.c
> > > > > @@ -98,6 +98,7 @@ static struct completion unload_sem;
> > > > >   */
> > > > >  static int pnp_dock_event(int dock, struct pnp_docking_station_info *info)
> > > > >  {
> > > > > +	static char const sbin_pnpbios[] = "/sbin/pnpbios";
> > > > >  	char *argv[3], **envp, *buf, *scratch;
> > > > >  	int i = 0, value;
> > > > >  
> > > > > @@ -112,7 +113,7 @@ static int pnp_dock_event(int dock, struct pnp_docking_station_info *info)
> > > > >  	 * integrated into the driver core and use the usual infrastructure
> > > > >  	 * like sysfs and uevents
> > > > >  	 */
> > > > > -	argv[0] = "/sbin/pnpbios";
> > > > > +	argv[0] = (char *)sbin_pnpbios;
> > > > 
> > > > So here and elsewhere, can attackers write to argv[0] instead of to the
> > > > memory where the string lives?
> > > 
> > > Yes, they could, it would be a very "tight" race to do that (have to
> > > write after the assignment and before the call_usermodehelper_exec()
> > > runs).  However, the kernel does not run argv[0], it just passes it to
> > > the binary you specify in path, so for this example, the correct program
> > > would still be run by the kernel.
> > 
> > In this case it's argv[0] that will be passed to call_usermodehelper as
> > path, but.... OK, this argv array and the various function call
> > arguments are all just data on the stack, so I guess it's all about
> > equivalent.
> 
> Kind of, nice catch, I'll change the call to usermodehelper to use
> sbin_pnpbios here, as that's the right thing to do.

Oops, no, the patch was doing the right thing here, you missed the next
chunk of the patch:


@@ -139,7 +140,7 @@ static int pnp_dock_event(int dock, struct pnp_docking_station_info *info)
                           info->location_id, info->serial, info->capabilities);
        envp[i] = NULL;

-       value = call_usermodehelper(argv [0], argv, envp, UMH_WAIT_EXEC);
+       value = call_usermodehelper(sbin_pnpbios, argv, envp, UMH_WAIT_EXEC);
        kfree(buf);
        kfree(envp);
        return 0;


So it's ok.

thanks,

greg k-h