sendmmsg/sendmsg: fix unsafe user pointer access

Dereferencing a user pointer directly from kernel-space without going
through the copy_from_user family of functions is a bad idea. Two of
such usages can be found in the sendmsg code path called from sendmmsg,
added by

commit c71d8ebe7a4496fb7231151cb70a6baa0cb56f9a upstream.
commit 5b47b8038f183b44d2d8ff1c7d11a5c1be706b34 in the 3.0-stable tree.

Usages are performed through memcmp() and memcpy() directly. Fix those
by using the already copied msg_sys structure instead of the __user *msg
structure. Note that msg_sys can be set to NULL by verify_compat_iovec()
or verify_iovec(), which requires additional NULL pointer checks.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>
CC: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: Anton Blanchard <anton@samba.org>
CC: David S. Miller <davem@davemloft.net>
CC: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int __init sock_init(void)
{
	int err;

	/*
	 *      Initialize sock SLAB cache.
	 */

	sk_init();

	/*
	 *      Initialize skbuff SLAB cache
	 */
	skb_init();

	/*
	 *      Initialize the protocols module.
	 */

	init_inodecache();

	err = register_filesystem(&sock_fs_type);
	if (err)
		goto out_fs;
	sock_mnt = kern_mount(&sock_fs_type);
	if (IS_ERR(sock_mnt)) {
		err = PTR_ERR(sock_mnt);
		goto out_mount;
	}

	/* The real protocol initialization is performed in later initcalls.
	 */

#ifdef CONFIG_NETFILTER
	netfilter_init();
#endif

#ifdef CONFIG_NETWORK_PHY_TIMESTAMPING
	skb_timestamping_init();
#endif

out:
	return err;

out_mount:
	unregister_filesystem(&sock_fs_type);
out_fs:
	goto out;
}

static int __init sock_init(void)
{
	int err;

	/*
	 *      Initialize sock SLAB cache.
	 */

	sk_init();

	/*
	 *      Initialize skbuff SLAB cache
	 */
	skb_init();

	/*
	 *      Initialize the protocols module.
	 */

	init_inodecache();

	err = register_filesystem(&sock_fs_type);
	if (err)
		goto out_fs;
	sock_mnt = kern_mount(&sock_fs_type);
	if (IS_ERR(sock_mnt)) {
		err = PTR_ERR(sock_mnt);
		goto out_mount;
	}

	/* The real protocol initialization is performed in later initcalls.
	 */

#ifdef CONFIG_NETFILTER
	netfilter_init();
#endif

#ifdef CONFIG_NETWORK_PHY_TIMESTAMPING
	skb_timestamping_init();
#endif

out:
	return err;

out_mount:
	unregister_filesystem(&sock_fs_type);
out_fs:
	goto out;
}
