Re: [PATCH net] ipv6: do not leave garbage in rt->fib6_metrics

[Eric Dumazet <edumazet@google.com>]

On Fri, Oct 5, 2018 at 9:29 AM David Ahern <dsahern@gmail.com> wrote:
>
> On 10/5/18 10:17 AM, Eric Dumazet wrote:
> > In case ip_fib_metrics_init() returns an error, we better
> > rewrite rt->fib6_metrics with &dst_default_metrics so that
> > we do not crash later in ip_fib_metrics_put()
> >
> > Fixes: 767a2217533f ("net: common metrics init helper for FIB entries")
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Reported-by: syzbot <syzkaller@googlegroups.com>
>
> Where is syzbot sending the reports? I don't see it on netdev. I would
> like to understand how it triggered the failure of a 68-byte malloc.
>

Not a public syzbot instance.

And this is not a 68-byte malloc error, simply a ip_metrics_convert()
being cautious with user provided attributes.

You can trivially trigger this with ip command

>
> > ---
> >  net/ipv6/route.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index 6c1d817151cae45421dc976c5ea082b4115650be..74d97addf1af20dda0c2b6a2018e88696f9f7d5a 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -2976,6 +2976,8 @@ static struct fib6_info *ip6_route_info_create(struct fib6_config *cfg,
> >       rt->fib6_metrics = ip_fib_metrics_init(net, cfg->fc_mx, cfg->fc_mx_len);
> >       if (IS_ERR(rt->fib6_metrics)) {
> >               err = PTR_ERR(rt->fib6_metrics);
> > +             /* Do not leave garbage there. */
> > +             rt->fib6_metrics = (struct dst_metrics *)&dst_default_metrics;
> >               goto out;
> >       }
> >
> >
>
> Reviewed-by: David Ahern <dsahern@gmail.com>
>
> Thanks, Eric.