Re: [PATCH] net: check for NULL net_device in FIB tables

[Vegard Nossum]

On 07/04/2016 09:45 PM, Julian Anastasov wrote:

        Hello,

On Mon, 4 Jul 2016, Vegard Nossum wrote:

struct fib_nh->nh_dev can be NULL, so we should check it before calling
__in_dev_get_rcu on it.

Multiple places seem to want this (and check the return value), so we can
add a convenience wrapper for this.

This fixes a crash in AF_NETLINK sendmsg().

Please double check that I caught all the callers that need the NULL
guard.

        What kind of configuration causes such crash?

It's not very easy to tell, sorry, I was using a netlink fuzzer to find
the crash. But see below, you identified the right bit of code.

-               in_dev = __in_dev_get_rtnl(nh->nh_dev);
+               in_dev = in_dev_get_rtnl(nh->nh_dev);

        fib_rebalance can not crash because for multipath
routes (fib_nhs > 1) all nexthops should have valid nh_dev.

Okay, cool.

                if (in_dev &&
                    IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
@@ -559,7 +559,7 @@ static void fib_rebalance(struct fib_info *fi)
        change_nexthops(fi) {
                int upper_bound;

-               in_dev = __in_dev_get_rtnl(nexthop_nh->nh_dev);
+               in_dev = in_dev_get_rtnl(nexthop_nh->nh_dev);

                if (nexthop_nh->nh_flags & RTNH_F_DEAD) {
                        upper_bound = -1;
@@ -1261,7 +1261,7 @@ int fib_dump_info(struct sk_buff *skb, u32 portid, u32 
seq, int event,
                    nla_put_u32(skb, RTA_OIF, fi->fib_nh->nh_oif))
                        goto nla_put_failure;
                if (fi->fib_nh->nh_flags & RTNH_F_LINKDOWN) {
-                       in_dev = __in_dev_get_rtnl(fi->fib_nh->nh_dev);
+                       in_dev = in_dev_get_rtnl(fi->fib_nh->nh_dev);

        Looks like this is the only place that can crash,
adding extra cycles to the other places is not good. And this
can happen only because fib_create_info() allows RTNH_F_LINKDOWN
to come for routes with error. May be fc_flags should be masked
there? Or there is another place that sets the flag when nh_dev
is NULL?

Indeed, this is the (only) place that actually crashed for me.

                        if (in_dev &&
                            IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev))
                                rtm->rtm_flags |= RTNH_F_DEAD;
@@ -1292,7 +1292,7 @@ int fib_dump_info(struct sk_buff *skb, u32 portid, u32 
seq, int event,

                        rtnh->rtnh_flags = nh->nh_flags & 0xFF;
                        if (nh->nh_flags & RTNH_F_LINKDOWN) {
-                               in_dev = __in_dev_get_rtnl(nh->nh_dev);
+                               in_dev = in_dev_get_rtnl(nh->nh_dev);

        Not needed because fib_nhs > 1

Alright.

Thanks for the review! I can submit a new patch to only check the one
place above that actually crashed. Otherwise, if you think it's better
to go with your fc_flags suggestion, feel free to send a patch for that.
As you can tell, I am not very familiar with this code :-) If you do
send a patch, I can test it easily.


Vegard