Date:   Fri, 11 Oct 2019 10:54:13 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Ido Schimmel <idosch@...sch.org>, Martin KaFai Lau <kafai@...com>
Cc:     Jesse Hathaway <jesse@...ki-mvuki.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Race condition in route lookup

On Fri, Oct 11, 2019 at 8:42 AM Ido Schimmel <idosch@...sch.org> wrote:
>
> On Fri, Oct 11, 2019 at 09:36:51AM -0500, Jesse Hathaway wrote:
> > On Thu, Oct 10, 2019 at 3:31 AM Ido Schimmel <idosch@...sch.org> wrote:
> > > I think it's working as expected. Here is my theory:
> > >
> > > If CPU0 is executing both the route get request and forwarding packets
> > > through the directly connected interface, then the following can happen:
> > >
> > > <CPU0, t0> - In process context, per-CPU dst entry cached in the nexthop
> > > is found. Not yet dumped to user space
> > >
> > > <Any CPU, t1> - Routes are added / removed, therefore invalidating the
> > > cache by bumping 'net->ipv4.rt_genid'
> > >
> > > <CPU0, t2> - In softirq, packet is forwarded through the nexthop. The
> > > cached dst entry is found to be invalid. Therefore, it is replaced by a
> > > newer dst entry. dst_dev_put() is called on old entry which assigns the
> > > blackhole netdev to 'dst->dev'. This netdev has an ifindex of 0 because
> > > it is not registered.
> > >
> > > <CPU0, t3> - After softirq finished executing, your route get request
> > > from t0 is resumed and the old dst entry is dumped to user space with
> > > ifindex of 0.
> > >
> > > I tested this on my system using your script to generate the route get
> > > requests. I pinned it to the same CPU forwarding packets through the
> > > nexthop. To constantly invalidate the cache I created another script
> > > that simply adds and removes IP addresses from an interface.
> > >
> > > If I stop the packet forwarding or the script that invalidates the
> > > cache, then I don't see any '*' answers to my route get requests.
> >
> > Thanks for the reply and analysis Ido, I tested with an additional script which
> > adds and deletes a route in a loop, as you also saw this increased the
> > frequency of blackhole route replies from the first script.
> >
> > Questions:
> >
> > 1. We saw this behavior occurring with TCP connections traversing our routers,
> > though I was able to reproduce it with only local route requests on our router.
> > Would you expect this same behavior for TCP traffic only in the kernel which
> > does not go to userspace?
>
> Yes, the problem is in the input path where received packets need to be
> forwarded.
>
> >
> > 2. These blackhole routes occur even though our main routing table is not
> > changing, however a separate route table managed by bird on the Linux router is
> > changing. Is this still expected behavior given that the ip-rules and main
> > route table used by these route requests are not changing?
>
> Yes, there is a per-netns counter that is incremented whenever cached
> dst entries need to be invalidated. Since it is per-netns it is
> incremented regardless of the routing table to which your insert the
> route.
>
> >
> > 3. We were previously rejecting these packets with an iptables rule which sent
> > an ICMP prohibited message to the sender, this caused TCP connections to break
> > with a EHOSTUNREACH, should we be silently dropping these packets instead?
> >
> > 4. If we should just be dropping these packets, why does the kernel not drop
> > them instead of letting them traverse the iptables rules?
>
> I actually believe the current behavior is a bug that needs to be fixed.
> See below.
>
> >
> > > BTW, the blackhole netdev was added in 5.3. I assume (didn't test) that
> > > with older kernel versions you'll see 'lo' instead of '*'.
> >
> > Yes indeed! Thanks for solving that mystery as well, our routers are running
> > 5.1, but we upgraded to 5.4-rc2 to determine whether the issue was still
> > present in the latest kernel.
>
> Do you remember when you started seeing this behavior? I think it
> started in 4.13 with commit ffe95ecf3a2e ("Merge branch
> 'net-remove-dst-garbage-collector-logic'").
>
> Let me add Wei to see if/how this can be fixed.
>
> Wei, in case you don't have the original mail with the description of
> the problem, it can be found here [1].
>
> I believe that the issue Jesse is experiencing is the following:
>
> <CPU A, t0> - Received packet A is forwarded and cached dst entry is
> taken from the nexthop ('nhc->nhc_rth_input'). Calls skb_dst_set()
>
> <t1> - Given Jesse has busy routers ("ingesting full BGP routing tables
> from multiple ISPs"), route is added / deleted and rt_cache_flush() is
> called
>
> <CPU B, t2> - Received packet B tries to use the same cached dst entry
> from t0, but rt_cache_valid() is no longer true and it is replaced in
> rt_cache_route() by the newer one. This calls dst_dev_put() on the
> original dst entry which assigns the blackhole netdev to 'dst->dev'
>
> <CPU A, t3> - dst_input(skb) is called on packet A and it is dropped due
> to 'dst->dev' being the blackhole netdev
>
> The following patch "fixes" the problem for me:
>
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 42221a12bdda..1c67bdb80fd5 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1482,7 +1482,6 @@ static bool rt_cache_route(struct fib_nh_common *nhc, struct rtable *rt)
>         prev = cmpxchg(p, orig, rt);
>         if (prev == orig) {
>                 if (orig) {
> -                       dst_dev_put(&orig->dst);
>                         dst_release(&orig->dst);
>                 }
>         } else {
>
> But if this dst entry is cached in some inactive socket and the netdev
> on which it took a reference needs to be unregistered, then we can
> potentially wait forever. No?
>
Yes. That's exactly the reason we need to free the dev here.
Otherwise as you described, we will see "unregister_netdevice: waiting
for xxx to become free. Usage count = x" flushing the screen... Not
fun...


> I'm thinking that it can be fixed by making 'nhc_rth_input' per-CPU, in
> a similar fashion to what Eric did in commit d26b3a7c4b3b ("ipv4: percpu
> nh_rth_output cache").
>
Hmm... Yes... I would think a per-CPU input cache should work for the
case above.
Another idea is: instead of calling dst_dev_put() in rt_cache_route()
to switch out the dev, we call, rt_add_uncached_list() to add this
obsolete dst cache to the uncached list. And if the device gets
unregistered, rt_flush_dev() takes care of all dst entries in the
uncached list. I think that would work too.

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index dc1f510a7c81..ee618d4234ce 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1482,7 +1482,7 @@ static bool rt_cache_route(struct fib_nh_common
*nhc, struct rtable *rt)
        prev = cmpxchg(p, orig, rt);
        if (prev == orig) {
                if (orig) {
-                       dst_dev_put(&orig->dst);
+                       rt_add_uncached_list(orig);
                        dst_release(&orig->dst);
                }
        } else {

+ Martin for his idea and input.

> Two questions:
>
> 1. Do you agree with the above analysis?
> 2. Do you have a simpler/better solution in mind?
>
> Thanks
>
> [1] https://lore.kernel.org/netdev/CANSNSoVM1Uo106xfJtGpTyXNed8kOL4JiXqf3A1eZHBa7z3=yg@mail.gmail.com/T/#medece9445d617372b4842d44525ef0d3ba1ea083

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