xfrm: policy: Fix doulbe free in xfrm_policy_timer

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 <[e-mail]>
Acked-by: Timo Teräs <[e-mail]>
Acked-by: Herbert Xu <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/xfrm/xfrm_policy.c

···
 
 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);