Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies

[David Miller]

From: Kees Cook <keescook@xxxxxxxxxxxx>
Date: Fri, 2 Feb 2018 02:27:49 -0800

> @@ -343,6 +343,14 @@ struct ucred {
>  
>  extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
>  extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
> +/*
> + * Provide a bounce buffer for copying cmsg data to userspace when the
> + * target memory isn't already whitelisted for hardened usercopy.
> + */
> +#define put_cmsg_whitelist(_msg, _level, _type, _ptr) ({		\
> +		typeof(*(_ptr)) _val = *(_ptr);				\
> +		put_cmsg(_msg, _level, _type, sizeof(_val), &_val);	\
> +	})

I understand what you are trying to achieve, but it's at a real cost
here.  Some of these objects are structures, for example the struct
sock_extended_err is 16 bytes.

And now we're going to copy it twice, once into the on-stack copy,
and then once again into the CMSG blob.

Please find a way to make hardened user copy happy without adding
new overhead.

Thank you.