scm: add user copy checks to put_cmsg()

This is a followup of commit 2558b803 ("net: use a bounce
buffer for copying skb->mark")

x86 and powerpc define user_access_begin, meaning
that they are not able to perform user copy checks
when using user_write_access_begin() / unsafe_copy_to_user()
and friends [1]

Instead of waiting bugs to trigger on other arches,
add a check_object_size() in put_cmsg() to make sure
that new code tested on x86 with CONFIG_HARDENED_USERCOPY=y
will perform more security checks.

[1] We can not generically call check_object_size() from
unsafe_copy_to_user() because UACCESS is enabled at this point.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kees Cook <keescook@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/scm.c

@@ -229,6 +229,8 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 	if (msg->msg_control_is_user) {
 		struct cmsghdr __user *cm = msg->msg_control_user;
 
+		check_object_size(data, cmlen - sizeof(*cm), true);
+
 		if (!user_write_access_begin(cm, cmlen))
 			goto efault;