[PATCH v3] xfrm: policy: Fix doulbe free in xfrm_policy_timer

[YueHaibing]

After xfrm_add_policy add a policy, its ref is 2, then

                             xfrm_policy_timer
                               read_lock
                               xp->walk.dead is 0
                               ....
                               mod_timer()
xfrm_policy_kill
  policy->walk.dead = 1
  ....
  del_timer(&policy->timer)
    xfrm_pol_put //ref is 1
  xfrm_pol_put  //ref is 0
    xfrm_policy_destroy
      call_rcu
                                 xfrm_pol_hold //ref is 1
                               read_unlock
                               xfrm_pol_put //ref is 0
                                 xfrm_policy_destroy
                                  call_rcu

xfrm_policy_destroy is called twice, which may leads to
double free.

Call Trace:
RIP: 0010:refcount_warn_saturate+0x161/0x210
...
 xfrm_policy_timer+0x522/0x600
 call_timer_fn+0x1b3/0x5e0
 ? __xfrm_decode_session+0x2990/0x2990
 ? msleep+0xb0/0xb0
 ? _raw_spin_unlock_irq+0x24/0x40
 ? __xfrm_decode_session+0x2990/0x2990
 ? __xfrm_decode_session+0x2990/0x2990
 run_timer_softirq+0x5c5/0x10e0

Fix this by use write_lock_bh in xfrm_policy_kill.

Fixes: ea2dea9dacc2 ("xfrm: remove policy lock when accessing policy->walk.dead")
Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
---
v3: Only lock 'policy->walk.dead'
v2: Fix typo 'write_lock_bh'--> 'write_unlock_bh' while unlocking
---
 net/xfrm/xfrm_policy.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index dbda08ec566e..8a4af86a285e 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -434,7 +434,9 @@ EXPORT_SYMBOL(xfrm_policy_destroy);
 
 static void xfrm_policy_kill(struct xfrm_policy *policy)
 {
+	write_lock_bh(&policy->lock);
 	policy->walk.dead = 1;
+	write_unlock_bh(&policy->lock);
 
 	atomic_inc(&policy->genid);
 
-- 
2.17.1