Messages in this thread | | | Date | Thu, 19 Jan 2017 13:03:21 +0100 | From | Greg KH <> | Subject | Re: [kernel-hardening] Re: [PATCH 2/3] Make static usermode helper binaries constant |
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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
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