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

[Greg Kroah-Hartman]

On Wed, Jul 28, 2021 at 01:14:33AM -0500, Gustavo A. R. Silva wrote:
> 
> 
> On 7/28/21 00:55, Greg Kroah-Hartman wrote:
> > 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 :)
> 
> I think addrs should be second. In general, I think all newly added
> non-anonymous structures should be second.

Why not use a local version of the macro like was done in the DRM header
file, to make it always work the same and more obvious what is
happening?  If I were a userspace developer and saw the above, I would
think that the kernel developers have lost it :)

thanks,

greg k-h