Re: [PATCH net-next] genetlink: do not parse attributes for families with zero maxattr

On Thu, Oct 10, 2019 at 10:39:58AM +0200, Jiri Pirko wrote:

Wed, Oct 09, 2019 at 06:44:32PM CEST, mkubecek@suse.cz wrote:

quoted

Commit c10e6cf85e7d ("net: genetlink: push attrbuf allocation and parsing
to a separate function") moved attribute buffer allocation and attribute
parsing from genl_family_rcv_msg_doit() into a separate function
genl_family_rcv_msg_attrs_parse() which, unlike the previous code, calls
__nlmsg_parse() even if family->maxattr is 0 (i.e. the family does its own
parsing). The parser error is ignored and does not propagate out of
genl_family_rcv_msg_attrs_parse() but an error message ("Unknown attribute
type") is set in extack and if further processing generates no error or
warning, it stays there and is interpreted as a warning by userspace.

Dumpit requests are not affected as genl_family_rcv_msg_dumpit() bypasses
the call of genl_family_rcv_msg_doit() if family->maxattr is zero. Do the
same also in genl_family_rcv_msg_doit().

This is the original code before the changes:

        if (ops->doit == NULL)
                return -EOPNOTSUPP;

        if (family->maxattr && family->parallel_ops) {
                attrbuf = kmalloc_array(family->maxattr + 1,
                                        sizeof(struct nlattr *),
                                        GFP_KERNEL);
                if (attrbuf == NULL)
                        return -ENOMEM;
        } else
                attrbuf = family->attrbuf;

        if (attrbuf) {
                enum netlink_validation validate = NL_VALIDATE_STRICT;

                if (ops->validate & GENL_DONT_VALIDATE_STRICT)
                        validate = NL_VALIDATE_LIBERAL;

                err = __nlmsg_parse(nlh, hdrlen, attrbuf, family->maxattr,
                                    family->policy, validate, extack);
                if (err < 0)
                        goto out;
        }

Looks like the __nlmsg_parse() is called no matter if maxattr if 0 or
not. It is only considered for allocation of attrbuf. This is in-sync
with the current code.

If family->maxattr is 0, genl_register_family() sets family->attrbuf to
NULL so that attrbuf is also NULL and the whole "if (attrbuf) { ... }"
block is not executed.

Michal