Re: [PATCH 19/64] ip: Use struct_group() for memcpy() regions

[Greg Kroah-Hartman]

On Tue, Jul 27, 2021 at 01:58:10PM -0700, Kees Cook wrote:
> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> field bounds checking for memcpy(), memmove(), and memset(), avoid
> intentionally writing across neighboring fields.
> 
> Use struct_group() in struct flowi4, struct ipv4hdr, and struct ipv6hdr
> around members saddr and daddr, so they can be referenced together. This
> will allow memcpy() and sizeof() to more easily reason about sizes,
> improve readability, and avoid future warnings about writing beyond the
> end of saddr.
> 
> "pahole" shows no size nor member offset changes to struct flowi4.
> "objdump -d" shows no meaningful object code changes (i.e. only source
> line number induced differences.)
> 
> Note that since this is a UAPI header, struct_group() has been open
> coded.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/net/flow.h            |  6 ++++--
>  include/uapi/linux/if_ether.h | 12 ++++++++++--
>  include/uapi/linux/ip.h       | 12 ++++++++++--
>  include/uapi/linux/ipv6.h     | 12 ++++++++++--
>  net/core/flow_dissector.c     | 10 ++++++----
>  net/ipv4/ip_output.c          |  6 ++----
>  6 files changed, 42 insertions(+), 16 deletions(-)
> 
> diff --git a/include/net/flow.h b/include/net/flow.h
> index 6f5e70240071..f1a3b6c8eae2 100644
> --- a/include/net/flow.h
> +++ b/include/net/flow.h
> @@ -81,8 +81,10 @@ struct flowi4 {
>  #define flowi4_multipath_hash	__fl_common.flowic_multipath_hash
>  
>  	/* (saddr,daddr) must be grouped, same order as in IP header */
> -	__be32			saddr;
> -	__be32			daddr;
> +	struct_group(addrs,
> +		__be32			saddr;
> +		__be32			daddr;
> +	);
>  
>  	union flowi_uli		uli;
>  #define fl4_sport		uli.ports.sport
> diff --git a/include/uapi/linux/if_ether.h b/include/uapi/linux/if_ether.h
> index a0b637911d3c..8f5667b2ea92 100644
> --- a/include/uapi/linux/if_ether.h
> +++ b/include/uapi/linux/if_ether.h
> @@ -163,8 +163,16 @@
>  
>  #if __UAPI_DEF_ETHHDR
>  struct ethhdr {
> -	unsigned char	h_dest[ETH_ALEN];	/* destination eth addr	*/
> -	unsigned char	h_source[ETH_ALEN];	/* source ether addr	*/
> +	union {
> +		struct {
> +			unsigned char h_dest[ETH_ALEN];	  /* destination eth addr */
> +			unsigned char h_source[ETH_ALEN]; /* source ether addr	  */
> +		};
> +		struct {
> +			unsigned char h_dest[ETH_ALEN];	  /* destination eth addr */
> +			unsigned char h_source[ETH_ALEN]; /* source ether addr	  */
> +		} addrs;

A union of the same fields in the same structure in the same way?

Ah, because struct_group() can not be used here?  Still feels odd to see
in a userspace-visible header.

> +	};
>  	__be16		h_proto;		/* packet type ID field	*/
>  } __attribute__((packed));
>  #endif
> diff --git a/include/uapi/linux/ip.h b/include/uapi/linux/ip.h
> index e42d13b55cf3..33647a37e56b 100644
> --- a/include/uapi/linux/ip.h
> +++ b/include/uapi/linux/ip.h
> @@ -100,8 +100,16 @@ struct iphdr {
>  	__u8	ttl;
>  	__u8	protocol;
>  	__sum16	check;
> -	__be32	saddr;
> -	__be32	daddr;
> +	union {
> +		struct {
> +			__be32	saddr;
> +			__be32	daddr;
> +		} addrs;
> +		struct {
> +			__be32	saddr;
> +			__be32	daddr;
> +		};

Same here (except you named the first struct addrs, not the second,
unlike above).


> +	};
>  	/*The options start here. */
>  };
>  
> diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
> index b243a53fa985..1c26d32e733b 100644
> --- a/include/uapi/linux/ipv6.h
> +++ b/include/uapi/linux/ipv6.h
> @@ -130,8 +130,16 @@ struct ipv6hdr {
>  	__u8			nexthdr;
>  	__u8			hop_limit;
>  
> -	struct	in6_addr	saddr;
> -	struct	in6_addr	daddr;
> +	union {
> +		struct {
> +			struct	in6_addr	saddr;
> +			struct	in6_addr	daddr;
> +		} addrs;
> +		struct {
> +			struct	in6_addr	saddr;
> +			struct	in6_addr	daddr;
> +		};

addrs first?  Consistancy is key :)

thanks,

greg k-h