net: Whitelist the skbuff_head_cache "cb" field

Most callers of put_cmsg() use a "sizeof(foo)" for the length argument.
Within put_cmsg(), a copy_to_user() call is made with a dynamic size, as a
result of the cmsg header calculations. This means that hardened usercopy
will examine the copy, even though it was technically a fixed size and
should be implicitly whitelisted. All the put_cmsg() calls being built
from values in skbuff_head_cache are coming out of the protocol-defined
"cb" field, so whitelist this field entirely instead of creating per-use
bounce buffers, for which there are concerns about performance.

Original report was:

Bad or missing usercopy whitelist? Kernel memory exposure attempt detected from SLAB object 'skbuff_head_cache' (offset 64, size 16)!
WARNING: CPU: 0 PID: 3663 at mm/usercopy.c:81 usercopy_warn+0xdb/0x100 mm/usercopy.c:76
...
 __check_heap_object+0x89/0xc0 mm/slab.c:4426
 check_heap_object mm/usercopy.c:236 [inline]
 __check_object_size+0x272/0x530 mm/usercopy.c:259
 check_object_size include/linux/thread_info.h:112 [inline]
 check_copy_size include/linux/thread_info.h:143 [inline]
 copy_to_user include/linux/uaccess.h:154 [inline]
 put_cmsg+0x233/0x3f0 net/core/scm.c:242
 sock_recv_errqueue+0x200/0x3e0 net/core/sock.c:2913
 packet_recvmsg+0xb2e/0x17a0 net/packet/af_packet.c:3296
 sock_recvmsg_nosec net/socket.c:803 [inline]
 sock_recvmsg+0xc9/0x110 net/socket.c:810
 ___sys_recvmsg+0x2a4/0x640 net/socket.c:2179
 __sys_recvmmsg+0x2a9/0xaf0 net/socket.c:2287
 SYSC_recvmmsg net/socket.c:2368 [inline]
 SyS_recvmmsg+0xc4/0x160 net/socket.c:2352
 entry_SYSCALL_64_fastpath+0x29/0xa0

Reported-by:  <syzbot+e2d6cfb305e9f3911dea@syzkaller.appspotmail.com>
Fixes: 6d07d1cd ("usercopy: Restrict non-usercopy caches to size 0")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>