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

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Message-Id: <20180205.100347.176614123780866781.davem@davemloft.net>
Date:   Mon, 05 Feb 2018 10:03:47 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     keescook@...omium.org
Cc:     syzbot+e2d6cfb305e9f3911dea@...kaller.appspotmail.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        ebiggers3@...il.com, james.morse@....com,
        keun-o.park@...kmatter.ae, labbott@...hat.com, linux-mm@...ck.org,
        mingo@...nel.org
Subject: Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant
 size copies

From: Kees Cook <keescook@...omium.org>
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.