[nf-next PATCH] netfilter: nf_tables: Introduce NFTA_RULE_ALT_EXPRESSIONS

[nf-next PATCH] netfilter: nf_tables: Introduce NFTA_RULE_ALT_EXPRESSIONS

[Phil Sutter]

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 allows for newer user space to provide a rule representation
understood by older user space while still able to verify the rule's
actual expressions applied to packets.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 include/net/netfilter/nf_tables.h        | 12 ++++++
 include/uapi/linux/netfilter/nf_tables.h |  3 ++
 net/netfilter/nf_tables_api.c            | 47 +++++++++++++++++++++---
 3 files changed, 56 insertions(+), 6 deletions(-)

diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
index e69ce23566eab..b08e01d19e835 100644
--- a/include/net/netfilter/nf_tables.h
+++ b/include/net/netfilter/nf_tables.h
@@ -946,10 +946,21 @@ struct nft_expr_ops {
 	void				*data;
 };
 
+/**
+ *	struct nft_alt_expr - nft_tables rule alternate expressions
+ *	@dlen: length of @data
+ *	@data: blob used as payload of NFTA_RULE_EXPRESSIONS attribute
+ */
+struct nft_alt_expr {
+	int	dlen;
+	char	data[];
+};
+
 /**
  *	struct nft_rule - nf_tables rule
  *
  *	@list: used internally
+ *	@alt_expr: Expression blob to dump instead of live data
  *	@handle: rule handle
  *	@genmask: generation mask
  *	@dlen: length of expression data
@@ -958,6 +969,7 @@ struct nft_expr_ops {
  */
 struct nft_rule {
 	struct list_head		list;
+	struct nft_alt_expr		*alt_expr;
 	u64				handle:42,
 					genmask:2,
 					dlen:12,
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,
 	__NFTA_RULE_MAX
 };
 #define NFTA_RULE_MAX		(__NFTA_RULE_MAX - 1)
diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index 6269b0d9977c6..d9b95a19bb028 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -3064,14 +3064,33 @@ static int nf_tables_fill_rule_info(struct sk_buff *skb, struct net *net,
 	if (chain->flags & NFT_CHAIN_HW_OFFLOAD)
 		nft_flow_rule_stats(chain, rule);
 
-	list = nla_nest_start_noflag(skb, NFTA_RULE_EXPRESSIONS);
-	if (list == NULL)
-		goto nla_put_failure;
-	nft_rule_for_each_expr(expr, next, rule) {
-		if (nft_expr_dump(skb, NFTA_LIST_ELEM, expr, reset) < 0)
+	if (rule->alt_expr) {
+		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);
 	}
-	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);
+		if (!alt_expr) {
+			err = -ENOMEM;
+			goto err_release_expr;
+		}
+		alt_expr->dlen = dlen;
+		nla_memcpy(alt_expr->data,
+			   nla[NFTA_RULE_ALT_EXPRESSIONS], dlen);
+	}
+
 	if (nla[NFTA_RULE_USERDATA]) {
 		ulen = nla_len(nla[NFTA_RULE_USERDATA]);
 		if (ulen > 0)
@@ -3572,6 +3606,7 @@ static int nf_tables_newrule(struct sk_buff *skb, const struct nfnl_info *info,
 	rule->handle = handle;
 	rule->dlen   = size;
 	rule->udata  = ulen ? 1 : 0;
+	rule->alt_expr = alt_expr;
 
 	if (ulen) {
 		udata = nft_userdata(rule);
-- 
2.38.0

Re: [nf-next PATCH] netfilter: nf_tables: Introduce NFTA_RULE_ALT_EXPRESSIONS

[Florian Westphal]

Phil Sutter <phil@nwl.cc> 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?

Perhaps its better to use two distinct attributes?

NFTA_RULE_EXPRESSIONS_COMPAT  (input)
NFTA_RULE_EXPRESSIONS_ACTUAL  (output)?

The switcheroo of ALT (old crap on input, actual live ruleset on output)
is very unintuitive.

> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
>  include/net/netfilter/nf_tables.h        | 12 ++++++
>  include/uapi/linux/netfilter/nf_tables.h |  3 ++
>  net/netfilter/nf_tables_api.c            | 47 +++++++++++++++++++++---
>  3 files changed, 56 insertions(+), 6 deletions(-)
> 
> diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
> index e69ce23566eab..b08e01d19e835 100644
> --- a/include/net/netfilter/nf_tables.h
> +++ b/include/net/netfilter/nf_tables.h
> @@ -946,10 +946,21 @@ struct nft_expr_ops {
>  	void				*data;
>  };
>  
> +/**
> + *	struct nft_alt_expr - nft_tables rule alternate expressions
> + *	@dlen: length of @data
> + *	@data: blob used as payload of NFTA_RULE_EXPRESSIONS attribute
> + */
> +struct nft_alt_expr {
> +	int	dlen;
> +	char	data[];
> +};
> +
>  /**
>   *	struct nft_rule - nf_tables rule
>   *
>   *	@list: used internally
> + *	@alt_expr: Expression blob to dump instead of live data
>   *	@handle: rule handle
>   *	@genmask: generation mask
>   *	@dlen: length of expression data
> @@ -958,6 +969,7 @@ struct nft_expr_ops {
>   */
>  struct nft_rule {
>  	struct list_head		list;
> +	struct nft_alt_expr		*alt_expr;
>  	u64				handle:42,
>  					genmask:2,
>  					dlen:12,
> 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.

> +		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?

> -	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.

> +		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?

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...).

Re: [nf-next PATCH] netfilter: nf_tables: Introduce NFTA_RULE_ALT_EXPRESSIONS

[Phil Sutter]

Hi,

On Fri, Dec 16, 2022 at 02:16:41AM +0100, Florian Westphal wrote:
> Phil Sutter <phil@nwl.cc> 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

Re: [nf-next PATCH] netfilter: nf_tables: Introduce NFTA_RULE_ALT_EXPRESSIONS

[Florian Westphal]

Phil Sutter <phil@nwl.cc> wrote:
> > Or at least mention that this is for iptables-nft/NFT_COMPAT sake?
> 
> I could move it into NFTA_RULE_COMPAT.

Fine with me.

> > 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?

Yes, NFTA_RULE_EXPRESSIONS is the 'compat kludge',
NFTA_RULE_EXPRESSIONS_ACTUAL the active one.

> > 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.

Yep.

> Adding the two attributes above and using *_COMPAT for input only and
> *_ACTUAL for output only is probably a trivial change though.

Right.  I don't mind but its perhaps a bit clearer as to what is going
on.

> [...]
> > > 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[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?

Actually you don't need a maxlen at all, sorry.  I saw the nla_len and
thought NLA_BINARY but you could do this:

-       [NFTA_RULE_EXPRESSIONS] = { .type = NLA_NESTED },
+       [NFTA_RULE_EXPRESSIONS] = NLA_POLICY_NESTED_ARRAY(nft_expr_policy),


... and use the same for the COMPAT_EXPRESSIONS (or whatever name
will be chosen).

(This won't do full validation of all attributes because the policy
 for expr 'X' isn't known at compile time).

> > > +		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?

I was just wondering if we should at least verify that netlink
attributes and sizes match, not that the expressions themselves are
parseable/available.

But its impossible to do so if the expression isn't available because
we then have no policy either.

So perhaps above NLA_POLICY_NESTED_ARRAY() is enough after all.