xfrm: Don't disable preemption while looking up cache state.

For the state cache lookup xfrm_input_state_lookup() first disables
preemption, to remain on the CPU and then retrieves a per-CPU pointer.
Within the preempt-disable section it also acquires
netns_xfrm::xfrm_state_lock, a spinlock_t. This lock must not be
acquired with explicit disabled preemption (such as by get_cpu())
because this lock becomes a sleeping lock on PREEMPT_RT.

To remain on the same CPU is just an optimisation for the CPU local
lookup. The actual modification of the per-CPU variable happens with
netns_xfrm::xfrm_state_lock acquired.

Remove get_cpu() and use the state_cache_input on the current CPU.

Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Closes: https://lore.kernel.org/all/CAADnVQKkCLaj=roayH=Mjiiqz_svdf1tsC3OE4EC0E=mAD+L1A@mail.gmail.com/
Fixes: 81a331a ("xfrm: Add an inbound percpu state cache.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: Sebastian Sewior

net/xfrm/xfrm_state.c

@@ -1150,9 +1150,8 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
 	struct xfrm_hash_state_ptrs state_ptrs;
 	struct hlist_head *state_cache_input;
 	struct xfrm_state *x = NULL;
-	int cpu = get_cpu();
 
-	state_cache_input =  per_cpu_ptr(net->xfrm.state_cache_input, cpu);
+	state_cache_input = raw_cpu_ptr(net->xfrm.state_cache_input);
 
 	rcu_read_lock();
 	hlist_for_each_entry_rcu(x, state_cache_input, state_cache_input) {
@@ -1186,7 +1185,6 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
 
 out:
 	rcu_read_unlock();
-	put_cpu();
 	return x;
 }
 EXPORT_SYMBOL(xfrm_input_state_lookup);