Subject: Re: [iproute PATCH v2 1/7] ipntable: Make sure filter.name is NULL-terminated

From: Phil Sutter <[email-masked]>
Date: Fri, 18 Aug 2017 18:52:23 +0200

On Fri, Aug 18, 2017 at 04:32:47PM +0000, David Laight wrote:
> From: Phil Sutter
> > Sent: 18 August 2017 11:52
> > On Fri, Aug 18, 2017 at 09:19:16AM +0000, David Laight wrote:
> > > From: Phil Sutter
> > > > Sent: 17 August 2017 18:09
> > > > To: Stephen Hemminger
> > > > Cc: [email-masked]
> > > > Subject: [iproute PATCH v2 1/7] ipntable: Make sure filter.name is NULL-terminated
> > > >
> > > > Signed-off-by: Phil Sutter <[email-masked]>
> > > > ---
> > > >  ip/ipntable.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/ip/ipntable.c b/ip/ipntable.c
> > > > index 879626ee4f491..7be1f04d33d90 100644
> > > > --- a/ip/ipntable.c
> > > > +++ b/ip/ipntable.c
> > > > @@ -633,7 +633,8 @@ static int ipntable_show(int argc, char **argv)
> > > >  		} else if (strcmp(*argv, "name") == 0) {
> > > >  			NEXT_ARG();
> > > >
> > > > -			strncpy(filter.name, *argv, sizeof(filter.name));
> > > > +			strncpy(filter.name, *argv, sizeof(filter.name) - 1);
> > > > +			filter.name[sizeof(filter.name) - 1] = '\0';
> > >
> > > Why not check for overflow instead?
> > > 			if (filter.name[sizeof(filter.name) - 1])
> > > 				usage("filer name too long");
> > 
> > sizeof(filter.name) is 1024, which is maybe a bit over the top for
> > something a user would input. So I found a better way avoiding all this
> > at once: I made filter.name a const char *, then just assigned *argv to
> > it. This should be safe since rtnl_dump_filter() and therefore
> > print_ntable() callback is called from inside ipntable_show() so *argv
> > is not accessed outside of it's scope.
> > 
> > What do you think?
> 
> There isn't a scope problem, *argv is program data (written to unusable
> stack space by the kernel during exec.

Ah, thanks for the info!

> If the filter is done in userpace it is ok, but I'd have thought
> it would be passed to kernel later on?

No, it just calls sends RTM_GETNEIGHTBL to kernel and throws away
anything that doesn't match the filter. So filtering happens in
user space exclusively.

Thanks, Phil