ip: Define usercopy region in IP proto slab cache

The ICMP filters for IPv4 and IPv6 raw sockets need to be copied to/from
userspace. In support of usercopy hardening, this patch defines a region
in the struct proto slab cache in which userspace copy operations are
allowed.

example usage trace:

    net/ipv4/raw.c:
        raw_seticmpfilter(...):
            ...
            copy_from_user(&raw_sk(sk)->filter, ..., optlen)

        raw_geticmpfilter(...):
            ...
            copy_to_user(..., &raw_sk(sk)->filter, len)

    net/ipv6/raw.c:
        rawv6_seticmpfilter(...):
            ...
            copy_from_user(&raw6_sk(sk)->filter, ..., optlen)

        rawv6_geticmpfilter(...):
            ...
            copy_to_user(..., &raw6_sk(sk)->filter, len)

This region is known as the slab cache's usercopy region. Slab caches
can now check that each dynamically sized copy operation involving
cache-managed memory falls entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <[email protected]>
[kees: split from network patch, provide usage trace]
Cc: "David S. Miller" <[email protected]>
Cc: Alexey Kuznetsov <[email protected]>
Cc: Hideaki YOSHIFUJI <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>

net/ipv6/raw.c

@@ -1272,6 +1272,8 @@ struct proto rawv6_prot = {
 	.hash		   = raw_hash_sk,
 	.unhash		   = raw_unhash_sk,
 	.obj_size	   = sizeof(struct raw6_sock),
+	.useroffset	   = offsetof(struct raw6_sock, filter),
+	.usersize	   = sizeof_field(struct raw6_sock, filter),
 	.h.raw_hash	   = &raw_v6_hashinfo,
 #ifdef CONFIG_COMPAT
 	.compat_setsockopt = compat_rawv6_setsockopt,