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.
Reported-by: syzbot+e2d6cfb305e9f3911dea@syzkaller.appspotmail.com Fixes: 6d07d1cd300f ("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>