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Subject[PATCH 4.14 123/159] ipv6: grab rt->rt6i_ref before allocating pcpu rt
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wei Wang <weiwan@google.com>


[ Upstream commit a94b9367e044ba672c9f4105eb1516ff6ff4948a ]

After rwlock is replaced with rcu and spinlock, ip6_pol_route() will be
called with only rcu held. That means rt6 route deletion could happen
simultaneously with rt6_make_pcpu_rt(). This could potentially cause
memory leak if rt6_release() is called right before rt6_make_pcpu_rt()
on the same route.

This patch grabs rt->rt6i_ref safely before calling rt6_make_pcpu_rt()
to make sure rt6_release() will not get triggered while
rt6_make_pcpu_rt() is in progress. And rt6_release() is called after
rt6_make_pcpu_rt() is finished.

Note: As we are incrementing rt->rt6i_ref in ip6_pol_route(), there is a
very slim chance that fib6_purge_rt() will be triggered unnecessarily
when deleting a route if ip6_pol_route() running on another thread picks
this route as well and tries to make pcpu cache for it.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv6/route.c | 58 +++++++++++++++++++++++++++----------------------------
1 file changed, 29 insertions(+), 29 deletions(-)

--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1055,7 +1055,6 @@ static struct rt6_info *rt6_get_pcpu_rou

static struct rt6_info *rt6_make_pcpu_route(struct rt6_info *rt)
{
- struct fib6_table *table = rt->rt6i_table;
struct rt6_info *pcpu_rt, *prev, **p;

pcpu_rt = ip6_rt_pcpu_alloc(rt);
@@ -1066,28 +1065,20 @@ static struct rt6_info *rt6_make_pcpu_ro
return net->ipv6.ip6_null_entry;
}

- read_lock_bh(&table->tb6_lock);
- if (rt->rt6i_pcpu) {
- p = this_cpu_ptr(rt->rt6i_pcpu);
- prev = cmpxchg(p, NULL, pcpu_rt);
- if (prev) {
- /* If someone did it before us, return prev instead */
- dst_release_immediate(&pcpu_rt->dst);
- pcpu_rt = prev;
- }
- } else {
- /* rt has been removed from the fib6 tree
- * before we have a chance to acquire the read_lock.
- * In this case, don't brother to create a pcpu rt
- * since rt is going away anyway. The next
- * dst_check() will trigger a re-lookup.
- */
+ dst_hold(&pcpu_rt->dst);
+ p = this_cpu_ptr(rt->rt6i_pcpu);
+ prev = cmpxchg(p, NULL, pcpu_rt);
+ if (prev) {
+ /* If someone did it before us, return prev instead */
+ /* release refcnt taken by ip6_rt_pcpu_alloc() */
+ dst_release_immediate(&pcpu_rt->dst);
+ /* release refcnt taken by above dst_hold() */
dst_release_immediate(&pcpu_rt->dst);
- pcpu_rt = rt;
+ dst_hold(&prev->dst);
+ pcpu_rt = prev;
}
- dst_hold(&pcpu_rt->dst);
+
rt6_dst_from_metrics_check(pcpu_rt);
- read_unlock_bh(&table->tb6_lock);
return pcpu_rt;
}

@@ -1177,19 +1168,28 @@ redo_rt6_select:
if (pcpu_rt) {
read_unlock_bh(&table->tb6_lock);
} else {
- /* We have to do the read_unlock first
- * because rt6_make_pcpu_route() may trigger
- * ip6_dst_gc() which will take the write_lock.
- */
- dst_hold(&rt->dst);
- read_unlock_bh(&table->tb6_lock);
- pcpu_rt = rt6_make_pcpu_route(rt);
- dst_release(&rt->dst);
+ /* atomic_inc_not_zero() is needed when using rcu */
+ if (atomic_inc_not_zero(&rt->rt6i_ref)) {
+ /* We have to do the read_unlock first
+ * because rt6_make_pcpu_route() may trigger
+ * ip6_dst_gc() which will take the write_lock.
+ *
+ * No dst_hold() on rt is needed because grabbing
+ * rt->rt6i_ref makes sure rt can't be released.
+ */
+ read_unlock_bh(&table->tb6_lock);
+ pcpu_rt = rt6_make_pcpu_route(rt);
+ rt6_release(rt);
+ } else {
+ /* rt is already removed from tree */
+ read_unlock_bh(&table->tb6_lock);
+ pcpu_rt = net->ipv6.ip6_null_entry;
+ dst_hold(&pcpu_rt->dst);
+ }
}

trace_fib6_table_lookup(net, pcpu_rt, table->tb6_id, fl6);
return pcpu_rt;
-
}
}
EXPORT_SYMBOL_GPL(ip6_pol_route);

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