bpf_lru_list: Read double-checked variable once without lock

For double-checked locking in bpf_common_lru_push_free(), node->type is
read outside the critical section and then re-checked under the lock.
However, concurrent writes to node->type result in data races.

For example, the following concurrent access was observed by KCSAN:

  write to 0xffff88801521bc22 of 1 bytes by task 10038 on cpu 1:
   __bpf_lru_node_move_in        kernel/bpf/bpf_lru_list.c:91
   __local_list_flush            kernel/bpf/bpf_lru_list.c:298
   ...
  read to 0xffff88801521bc22 of 1 bytes by task 10043 on cpu 0:
   bpf_common_lru_push_free      kernel/bpf/bpf_lru_list.c:507
   bpf_lru_push_free             kernel/bpf/bpf_lru_list.c:555
   ...

Fix the data races where node->type is read outside the critical section
(for double-checked locking) by marking the access with READ_ONCE() as
well as ensuring the variable is only accessed once.

Fixes: 3a08c2fd ("bpf: LRU List")
Reported-by: syzbot+3536db46dfa58c573458@syzkaller.appspotmail.com
Reported-by: syzbot+516acdb03d3e27d91bcd@syzkaller.appspotmail.com
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210209112701.3341724-1-elver@google.com

kernel/bpf/bpf_lru_list.c

@@ -502,13 +502,14 @@ struct bpf_lru_node *bpf_lru_pop_free(struct bpf_lru *lru, u32 hash)
 static void bpf_common_lru_push_free(struct bpf_lru *lru,
 				     struct bpf_lru_node *node)
 {
+	u8 node_type = READ_ONCE(node->type);
 	unsigned long flags;
 
-	if (WARN_ON_ONCE(node->type == BPF_LRU_LIST_T_FREE) ||
-	    WARN_ON_ONCE(node->type == BPF_LRU_LOCAL_LIST_T_FREE))
+	if (WARN_ON_ONCE(node_type == BPF_LRU_LIST_T_FREE) ||
+	    WARN_ON_ONCE(node_type == BPF_LRU_LOCAL_LIST_T_FREE))
 		return;
 
-	if (node->type == BPF_LRU_LOCAL_LIST_T_PENDING) {
+	if (node_type == BPF_LRU_LOCAL_LIST_T_PENDING) {
 		struct bpf_lru_locallist *loc_l;
 
 		loc_l = per_cpu_ptr(lru->common_lru.local_list, node->cpu);