mqueue: fix a use-after-free in sys_mq_notify()

The retry logic for netlink_attachskb() inside sys_mq_notify()
is nasty and vulnerable:

1) The sock refcnt is already released when retry is needed
2) The fd is controllable by user-space because we already
   release the file refcnt

so we when retry but the fd has been just closed by user-space
during this small window, we end up calling netlink_detachskb()
on the error path which releases the sock again, later when
the user-space closes this socket a use-after-free could be
triggered.

Setting 'sock' to NULL here should be sufficient to fix it.

Reported-by: GeneBlue <geneblue.mail@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static int __init init_mqueue_fs(void)
{
	int error;

	mqueue_inode_cachep = kmem_cache_create("mqueue_inode_cache",
				sizeof(struct mqueue_inode_info), 0,
				SLAB_HWCACHE_ALIGN|SLAB_ACCOUNT, init_once);
	if (mqueue_inode_cachep == NULL)
		return -ENOMEM;

	/* ignore failures - they are not fatal */
	mq_sysctl_table = mq_register_sysctl_table();

	error = register_filesystem(&mqueue_fs_type);
	if (error)
		goto out_sysctl;

	spin_lock_init(&mq_lock);

	error = mq_init_ns(&init_ipc_ns);
	if (error)
		goto out_filesystem;

	return 0;

out_filesystem:
	unregister_filesystem(&mqueue_fs_type);
out_sysctl:
	if (mq_sysctl_table)
		unregister_sysctl_table(mq_sysctl_table);
	kmem_cache_destroy(mqueue_inode_cachep);
	return error;
}

static int __init init_mqueue_fs(void)
{
	int error;

	mqueue_inode_cachep = kmem_cache_create("mqueue_inode_cache",
				sizeof(struct mqueue_inode_info), 0,
				SLAB_HWCACHE_ALIGN|SLAB_ACCOUNT, init_once);
	if (mqueue_inode_cachep == NULL)
		return -ENOMEM;

	/* ignore failures - they are not fatal */
	mq_sysctl_table = mq_register_sysctl_table();

	error = register_filesystem(&mqueue_fs_type);
	if (error)
		goto out_sysctl;

	spin_lock_init(&mq_lock);

	error = mq_init_ns(&init_ipc_ns);
	if (error)
		goto out_filesystem;

	return 0;

out_filesystem:
	unregister_filesystem(&mqueue_fs_type);
out_sysctl:
	if (mq_sysctl_table)
		unregister_sysctl_table(mq_sysctl_table);
	kmem_cache_destroy(mqueue_inode_cachep);
	return error;
}
