Re: [PATCH net] ipv4: reject RTNH_F_LINKDOWN for incompatible routes

Andy Gospodarek
2016-07-09T17:23:32Z

On Sat, Jul 09, 2016 at 12:00:15PM +0300, Julian Anastasov wrote:
> Vegard Nossum is reporting for a crash in fib_dump_info (fib_nhs==1)
> when nh_dev = NULL. Problem happens when RTNH_F_LINKDOWN is
> provided from user space for routes that do not use the flag,
> catched with netlink fuzzer.

Can you also include the panic log in the changelog or at a minimum post
it here?

> RTNH_F_LINKDOWN should be used only for link routes, not for
> local routes or for routes with error code. Do not complicate
> fast path with more checks, reject the flag early when configured
> for incompatible routes.

Did the netlink fuzzer (trinity?) happen to check any of the other flags
(liks RTNH_F_DEAD) that are normally set by the kernel but could be
problematic when send down from userspace?

> Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
> Fixes: 0eeb075fad73 ("net: ipv4 sysctl option to ignore routes when nexthop link is down")
> Tested-by: Vegard Nossum <vegard.nossum@oracle.com>
> Signed-off-by: Julian Anastasov <ja@ssi.bg>
> Cc: Andy Gospodarek <gospo@cumulusnetworks.com>
> Cc: Dinesh Dutt <ddutt@cumulusnetworks.com>
> Cc: Scott Feldman <sfeldma@gmail.com>
> ---
> net/ipv4/fib_semantics.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> Note: works for all kernels: net, net-next, 4.4.14, 4.5.7, 4.6.3
>
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index d09173b..b642479 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -1113,7 +1113,8 @@ struct fib_info *fib_create_info(struct fib_config *cfg)
> }
>
> if (fib_props[cfg->fc_type].error) {
> - if (cfg->fc_gw || cfg->fc_oif || cfg->fc_mp)
> + if (cfg->fc_gw || cfg->fc_oif || cfg->fc_mp ||
> + (fi->fib_nh->nh_flags & RTNH_F_LINKDOWN))
> goto err_inval;

It looks a bit odd to use cfg in the existing checkd and fi->fib_nh in
the rest, but not a huge issue since cfg->fc_flags and
fi->fib_nh->nh_flags should be equivalent should be the same for single
and multipath routes.

> goto link_it;
> } else {
> @@ -1136,7 +1137,7 @@ struct fib_info *fib_create_info(struct fib_config *cfg)
> struct fib_nh *nh = fi->fib_nh;
>
> /* Local address is added. */
> - if (nhs != 1 || nh->nh_gw)
> + if (nhs != 1 || nh->nh_gw || (nh->nh_flags & RTNH_F_LINKDOWN))
> goto err_inval;
> nh->nh_scope = RT_SCOPE_NOWHERE;
> nh->nh_dev = dev_get_by_index(net, fi->fib_nh->nh_oif);
> --
> 1.9.3
>