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

[Hangbin Liu]

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 <liuhang...@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