netdev - [PATCH net] ipv4: fix race in concurrent ip_route_input

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Date:	Tue, 19 Nov 2013 19:12:34 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: [PATCH net] ipv4: fix race in concurrent ip_route_input_slow()

CPUs can ask for local route via ip_route_input_noref() concurrently.
if nh_rth_input is not cached yet, CPUs will proceed to allocate
equivalent DSTs on 'lo' and then will try to cache them in nh_rth_input
via rt_cache_route()
Most of the time they succeed, but on occasion the following two lines:
	orig = *p;
	prev = cmpxchg(p, orig, rt);
in rt_cache_route() do race and one of the cpus fails to complete cmpxchg.
But ip_route_input_slow() doesn't check the return code of rt_cache_route(),
so dst is leaking. dst_destroy() is never called and 'lo' device
refcnt doesn't go to zero, which can be seen in the logs as:
	unregister_netdevice: waiting for lo to become free. Usage count = 1
Adding mdelay() between above two lines makes it easily reproducible.
Fix it similar to nh_pcpu_rth_output case.

Fixes: d2d68ba9fe8b ("ipv4: Cache input routes in fib_info nexthops.")
Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
---

David,

looks like caacf05e5ad1 ("ipv4: Properly purge netdev references on uncached routes.")
fixed the race for nexthop/rth_output, but missed it for rth_input.
I'm not sure what was the assumption why it's not needed there.
We're definitely seeing it every 12-24hr during nightly tests.
There are several bugs on ubuntu and debian forums with similar description.
Some were closed, since folks struggled to reproduce it.
It took us more than a month to debug it.
Please queue for stable.

Alternative fix:
	rt_free(rth);
	goto local_input;
imo is uglier.
Just like rt_free(rth) followed by re-read of nh_rth_input and re-check_valid

 net/ipv4/route.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f428935..f8da282 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1776,8 +1776,12 @@ local_input:
 		rth->dst.error= -err;
 		rth->rt_flags 	&= ~RTCF_LOCAL;
 	}
-	if (do_cache)
-		rt_cache_route(&FIB_RES_NH(res), rth);
+	if (do_cache) {
+		if (unlikely(!rt_cache_route(&FIB_RES_NH(res), rth))) {
+			rth->dst.flags |= DST_NOCACHE;
+			rt_add_uncached_list(rth);
+		}
+	}
 	skb_dst_set(skb, &rth->dst);
 	err = 0;
 	goto out;
-- 
1.7.9.5

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