Hi,
On Fri, Dec 16, 2022 at 02:16:41AM +0100, Florian Westphal wrote:
> Phil Sutter <phil@xxxxxx> wrote:
> > With identical content as NFTA_RULE_EXPRESSIONS, data in this attribute
> > is dumped in place of the live expressions, which in turn are dumped as
> > NFTA_RULE_ALT_EXPRESSIONS.
>
> This name isn't very descriptive.
>
> Or at least mention that this is for iptables-nft/NFT_COMPAT sake?
I could move it into NFTA_RULE_COMPAT.
> Perhaps its better to use two distinct attributes?
>
> NFTA_RULE_EXPRESSIONS_COMPAT (input)
> NFTA_RULE_EXPRESSIONS_ACTUAL (output)?
With NFTA_RULE_EXPRESSIONS_ACTUAL replacing NFTA_RULE_EXPRESSIONS?
> The switcheroo of ALT (old crap on input, actual live ruleset on output)
> is very unintuitive.
Well, providing something that replaces the content of
NFTA_RULE_EXPRESSIONS is the basic concept of this approach. Restricting
it to output only is not possible since old user space uses it for
input. I guess one could make new user space use the *_COMPAT/*_ACTUAL
attributes above but kernel still has to fall back to plain
*_EXPRESSIONS.
Adding the two attributes above and using *_COMPAT for input only and
*_ACTUAL for output only is probably a trivial change though.
[...]
> > diff --git a/include/uapi/linux/netfilter/nf_tables.h b/include/uapi/linux/netfilter/nf_tables.h
> > index cfa844da1ce61..2dff92f527429 100644
> > --- a/include/uapi/linux/netfilter/nf_tables.h
> > +++ b/include/uapi/linux/netfilter/nf_tables.h
> > @@ -247,6 +247,8 @@ enum nft_chain_attributes {
> > * @NFTA_RULE_USERDATA: user data (NLA_BINARY, NFT_USERDATA_MAXLEN)
> > * @NFTA_RULE_ID: uniquely identifies a rule in a transaction (NLA_U32)
> > * @NFTA_RULE_POSITION_ID: transaction unique identifier of the previous rule (NLA_U32)
> > + * @NFTA_RULE_CHAIN_ID: add the rule to chain by ID, alternative to @NFTA_RULE_CHAIN (NLA_U32)
> > + * @NFTA_RULE_ALT_EXPRESSIONS: expressions to swap with @NFTA_RULE_EXPRESSIONS for dumps (NLA_NESTED: nft_expr_attributes)
> > */
> > enum nft_rule_attributes {
> > NFTA_RULE_UNSPEC,
> > @@ -261,6 +263,7 @@ enum nft_rule_attributes {
> > NFTA_RULE_ID,
> > NFTA_RULE_POSITION_ID,
> > NFTA_RULE_CHAIN_ID,
> > + NFTA_RULE_ALT_EXPRESSIONS,
>
> You need to update the nla_policy too.
ACK, thanks!
> > + if (nla_put(skb, NFTA_RULE_EXPRESSIONS,
> > + rule->alt_expr->dlen, rule->alt_expr->data) < 0)
> > + goto nla_put_failure;
> > + } else {
> > + list = nla_nest_start_noflag(skb, NFTA_RULE_EXPRESSIONS);
> > + if (!list)
> > goto nla_put_failure;
> > +
> > + nft_rule_for_each_expr(expr, next, rule) {
> > + if (nft_expr_dump(skb, NFTA_LIST_ELEM, expr, reset) < 0)
> > + goto nla_put_failure;
> > + }
> > + nla_nest_end(skb, list);
> > + }
> > +
> > + if (rule->alt_expr) {
> > + list = nla_nest_start_noflag(skb, NFTA_RULE_ALT_EXPRESSIONS);
> > + if (!list)
> > + goto nla_put_failure;
> > +
> > + nft_rule_for_each_expr(expr, next, rule) {
> > + if (nft_expr_dump(skb, NFTA_LIST_ELEM, expr, reset) < 0)
> > + goto nla_put_failure;
> > + }
> > + nla_nest_end(skb, list);
>
> How does userspace know if NFTA_RULE_EXPRESSIONS is the backward annotated
> kludge or the live/real ruleset? Check for presence of ALT attribute?
Yes, by presence check. iptables-nft may default to ALT_EXPRESSIONS or
parse both into iptables_command_state objects for comparison.
> > - nla_nest_end(skb, list);
> >
> > if (rule->udata) {
> > struct nft_userdata *udata = nft_userdata(rule);
> > @@ -3366,6 +3385,7 @@ static void nf_tables_rule_destroy(const struct nft_ctx *ctx,
> > nf_tables_expr_destroy(ctx, expr);
> > expr = next;
> > }
> > + kfree(rule->alt_expr);
> > kfree(rule);
> > }
> >
> > @@ -3443,6 +3463,7 @@ static int nf_tables_newrule(struct sk_buff *skb, const struct nfnl_info *info,
> > struct nft_rule *rule, *old_rule = NULL;
> > struct nft_expr_info *expr_info = NULL;
> > u8 family = info->nfmsg->nfgen_family;
> > + struct nft_alt_expr *alt_expr = NULL;
> > struct nft_flow_rule *flow = NULL;
> > struct net *net = info->net;
> > struct nft_userdata *udata;
> > @@ -3556,6 +3577,19 @@ static int nf_tables_newrule(struct sk_buff *skb, const struct nfnl_info *info,
> > if (size >= 1 << 12)
> > goto err_release_expr;
> >
> > + if (nla[NFTA_RULE_ALT_EXPRESSIONS]) {
> > + int dlen = nla_len(nla[NFTA_RULE_ALT_EXPRESSIONS]);
> > + alt_expr = kvmalloc(sizeof(*alt_expr) + dlen, GFP_KERNEL);
>
> Once nla_policy provides a maxlen this is fine.
Hmm. I could define this max length as
| NFT_RULE_MAXEXPRS * max(nft_expr_ops::size)
given the currently existing nft_expr_ops instances in kernel. This
isn't pretty accurate though. Or I do a "fake" parsing of the nested
expression list only checking the number of elements? IIUC, this is the
only restriction applied to NFTA_RULE_EXPRESSIONS - at least I don't see
the size of each NFTA_LIST_ELEM being checked against its
nft_expr_ops::size. Or am I missing something?
> > + nla_memcpy(alt_expr->data,
> > + nla[NFTA_RULE_ALT_EXPRESSIONS], dlen);
>
> Hmm, I wonder if this isn't a problem.
> The kernel will now dump arbitrary data to userspace, whereas without
> this patch the nla data is generated by kernel, i.e. never malformed.
>
> I wonder if the alt blob needs to go through some type of validation
> too?
Given that the kernel doesn't use the ALT data, I liked the fact that it
may contain expressions it doesn't even support anymore. Upholding this
allows to disable nft_compat in kernel once iptables-nft produces native
expressions for everything.
I get your point about dumping tainted data. Does a shallow syntactic
check suffice, is it required to verify each expression's syntax or even
its semantics?
> Also I think that this attribute should always be ignored for
> NFT_COMPAT=n builds, we should document this kludge is only for
> iptables-nft sake (or rather, incorrect a**umptions by 3rd
> party wrappers of iptables userspace...).
Fine with me. For now, iptables-nft is not usable without NFT_COMPAT
anyway. If so, we could still relax this into ALT_EXPRESSIONS but no
compat interface itself.
Thanks, Phil
