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

Julian Anastasov
2016-07-09T20:11:57Z

Hello,

On Sat, 9 Jul 2016, Andy Gospodarek wrote:

> 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?

Now after Vegard posted it, I'll include in v2.

> > 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?

My guess is that fib_flush will release it soon or
later. I preferred to reject the RTNH_F_LINKDOWN flag only
for some kind of routes but another alternative is to always
reject both RTNH_F_LINKDOWN and RTNH_F_DEAD: RTNH_F_LINKDOWN
is recalculated and there is no good reason user space to
provide initial value for flag that is maintained by kernel.

> > 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.

Using fc_flags works too for the above case. In fact,
it goes also to fib_flags, so we should have our checks there.
But it is true that RTNH_F_LINKDOWN is not used from fib_flags,
I think, we already had a chance to talk about it on 27 Oct 2015.

May be we can reject the both flags once for
rtnh_flags in fib_get_nhs() and then for fc_flags in
fib_create_info().

Regards