bpf: rcu lock must not be held when calling copy_to_user()

BUG: sleeping function called from invalid context at mm/memory.c:3732
in_atomic(): 0, irqs_disabled(): 0, pid: 671, name: test_maps
1 lock held by test_maps/671:
 #0:  (rcu_read_lock){......}, at: [<0000000000264190>] map_lookup_elem+0xe8/0x260
Call Trace:
([<0000000000115b7e>] show_trace+0x12e/0x150)
 [<0000000000115c40>] show_stack+0xa0/0x100
 [<00000000009b163c>] dump_stack+0x74/0xc8
 [<000000000017424a>] ___might_sleep+0x23a/0x248
 [<00000000002b58e8>] might_fault+0x70/0xe8
 [<0000000000264230>] map_lookup_elem+0x188/0x260
 [<0000000000264716>] SyS_bpf+0x20e/0x840

Fix it by allocating temporary buffer to store map element value.

Fixes: db20fd2b0108 ("bpf: add lookup/update/delete/iterate methods to BPF maps")
Reported-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/syscall.c

@@ -150,7 +150,7 @@ static int map_lookup_elem(union bpf_attr *attr)
 	int ufd = attr->map_fd;
 	struct fd f = fdget(ufd);
 	struct bpf_map *map;
-	void *key, *value;
+	void *key, *value, *ptr;
 	int err;
 
 	if (CHECK_ATTR(BPF_MAP_LOOKUP_ELEM))
@@ -169,20 +169,29 @@ static int map_lookup_elem(union bpf_attr *attr)
 	if (copy_from_user(key, ukey, map->key_size) != 0)
 		goto free_key;
 
-	err = -ENOENT;
-	rcu_read_lock();
-	value = map->ops->map_lookup_elem(map, key);
+	err = -ENOMEM;
+	value = kmalloc(map->value_size, GFP_USER);
 	if (!value)
-		goto err_unlock;
+		goto free_key;
+
+	rcu_read_lock();
+	ptr = map->ops->map_lookup_elem(map, key);
+	if (ptr)
+		memcpy(value, ptr, map->value_size);
+	rcu_read_unlock();
+
+	err = -ENOENT;
+	if (!ptr)
+		goto free_value;
 
 	err = -EFAULT;
 	if (copy_to_user(uvalue, value, map->value_size) != 0)
-		goto err_unlock;
+		goto free_value;
 
 	err = 0;
 
-err_unlock:
-	rcu_read_unlock();
+free_value:
+	kfree(value);
 free_key:
 	kfree(key);
 err_put: