[PATCH 3.8 27/91] ipv4: fix race in concurrent ip_route_input_slow()

From: Kamal Mostafa
Date: Thu Jan 02 2014 - 12:27:02 EST -stable review patch. If anyone has any objections, please let me know.


From: Alexei Starovoitov <ast@xxxxxxxxxxxx>

[ Upstream commit dcdfdf56b4a6c9437fc37dbc9cee94a788f9b0c4 ]

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@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
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 0bf88e4..c219f8b 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1724,8 +1724,12 @@ 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;

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