[PATCH net] net/flow: fix fc->percpu NULL pointer dereference

[Hangbin Liu <liuhangbin@gmail.com>]

Now we will force to do garbage collection if any policy removed in
xfrm_policy_flush(). But during xfrm_net_exit(). We call flow_cache_fini()
first and set set fc->percpu to NULL. Then after we call xfrm_policy_fini()
-> frxm_policy_flush() -> flow_cache_flush(), we will get NULL pointer
dereference when check percpu_empty. The code path looks like:

flow_cache_fini()
  - fc->percpu = NULL
xfrm_policy_fini()
  - xfrm_policy_flush()
    - xfrm_garbage_collect()
      - flow_cache_flush()
        - flow_cache_percpu_empty()
	  - fcp = per_cpu_ptr(fc->percpu, cpu)

To reproduce, just add ipsec in netns and then remove the netns.

Fixes: 35db06912189 ("xfrm: do the garbage collection after flushing policy")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 net/core/flow.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/core/flow.c b/net/core/flow.c
index f7f5d19..321fc53 100644
--- a/net/core/flow.c
+++ b/net/core/flow.c
@@ -332,10 +332,13 @@ static int flow_cache_percpu_empty(struct flow_cache *fc, int cpu)
 	struct flow_cache_percpu *fcp;
 	unsigned int i;
 
-	fcp = per_cpu_ptr(fc->percpu, cpu);
-	for (i = 0; i < flow_cache_hash_size(fc); i++)
-		if (!hlist_empty(&fcp->hash_table[i]))
-			return 0;
+	if (fc->percpu) {
+		fcp = per_cpu_ptr(fc->percpu, cpu);
+		for (i = 0; i < flow_cache_hash_size(fc); i++)
+			if (!hlist_empty(&fcp->hash_table[i]))
+				return 0;
+	}
+
 	return 1;
 }
 
-- 
2.5.5