netlink: fix potential sleeping issue in mqueue_flush_file

I analyze the potential sleeping issue of the following processes:
Thread A                                Thread B
...                                     netlink_create  //ref = 1
do_mq_notify                            ...
  sock = netlink_getsockbyfilp          ...     //ref = 2
  info->notify_sock = sock;             ...
...                                     netlink_sendmsg
...                                       skb = netlink_alloc_large_skb  //skb->head is vmalloced
...                                       netlink_unicast
...                                         sk = netlink_getsockbyportid //ref = 3
...                                         netlink_sendskb
...                                           __netlink_sendskb
...                                             skb_queue_tail //put skb to sk_receive_queue
...                                         sock_put //ref = 2
...                                     ...
...                                     netlink_release
...                                       deferred_put_nlk_sk //ref = 1
mqueue_flush_file
  spin_lock
  remove_notification
    netlink_sendskb
      sock_put  //ref = 0
        sk_free
          ...
          __sk_destruct
            netlink_sock_destruct
              skb_queue_purge  //get skb from sk_receive_queue
                ...
                __skb_queue_purge_reason
                  kfree_skb_reason
                    __kfree_skb
                    ...
                    skb_release_all
                      skb_release_head_state
                        netlink_skb_destructor
                          vfree(skb->head)  //sleeping while holding spinlock

In netlink_sendmsg, if the memory pointed to by skb->head is allocated by
vmalloc, and is put to sk_receive_queue queue, also the skb is not freed.
When the mqueue executes flush, the sleeping bug will occur. Use
vfree_atomic instead of vfree in netlink_skb_destructor to solve the issue.

Fixes: c05cdb1 ("netlink: allow large data transfers from user-space")
Signed-off-by: Zhengchao Shao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: zhengchaoshao

net/netlink/af_netlink.c

@@ -374,7 +374,7 @@ static void netlink_skb_destructor(struct sk_buff *skb)
 	if (is_vmalloc_addr(skb->head)) {
 		if (!skb->cloned ||
 		    !atomic_dec_return(&(skb_shinfo(skb)->dataref)))
-			vfree(skb->head);
+			vfree_atomic(skb->head);
 
 		skb->head = NULL;
 	}