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SubjectRe: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies
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From: Kees Cook <keescook@chromium.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.

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