Re: [PATCH nf] netfilter: nft_ct: fix OOB in NFT_CT_SRC/DST eval

[Florian Westphal <fw@strlen.de>]

Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

[ CC Pablo ]

> > https://sashiko.dev/#/patchset/20260528042620.263828-1-jiayuan.chen%40linux.dev
> > 
> > "This is a pre-existing issue, but does copying only addr_len bytes when
> > priv->len is larger leave the remainder of the register uninitialized?
> > In nft_do_chain(), the register array is allocated on the kernel stack
> > without zero-initialization. If priv->len is 16 and addr_len is 4, only
> > the first 4 bytes are written."
> 
> I just spotted that too.  I think copying priv->len unconditionally
> is enough -- tuple->{src,dst} is zeroed in nf_ct_get_tuple() before the
> protocol pkt_to_tuple callback fills in only the relevant leading bytes,
> so the trailing bytes of tuple->{src,dst}.u3.all are well-defined zeros
> and no wrapper is needed.

Pablo, whats your take?

chain c {
 type filter hook output priority -300; policy accept;
 ct zone set 1
 ct original saddr 0.0.0.0 counter accept
}

Then: ping -c 1 127.0.0.1

should the rule match the template or not?
If not, we need:

diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -78,7 +78,7 @@ static void nft_ct_get_eval(const struct nft_expr *expr,
 		break;
 	}
 
-	if (ct == NULL)
+	if (!ct || nf_ct_is_template(ct))
 		goto err;
 
 	switch (priv->key) {
diff --git a/net/netfilter/nft_ct_fast.c b/net/netfilter/nft_ct_fast.c
index e684c8a91848..ecf7b3a404be 100644
--- a/net/netfilter/nft_ct_fast.c
+++ b/net/netfilter/nft_ct_fast.c
@@ -30,7 +30,7 @@ void nft_ct_get_fast_eval(const struct nft_expr *expr,
 		break;
 	}
 
-	if (!ct) {
+	if (!ct || nf_ct_is_template(ct)) {
 		regs->verdict.code = NFT_BREAK;
 		return;
 	}


If it should match, we need something like this:

diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -180,12 +180,14 @@ static void nft_ct_get_eval(const struct nft_expr *expr,
 	tuple = &ct->tuplehash[priv->dir].tuple;
 	switch (priv->key) {
 	case NFT_CT_SRC:
-		memcpy(dest, tuple->src.u3.all,
-		       nf_ct_l3num(ct) == NFPROTO_IPV4 ? 4 : 16);
+		if (nf_ct_l3num(ct) != nft_pf(pkt))
+			goto err;
+		memcpy(dest, tuple->src.u3.all, priv->len);
 		return;
 	case NFT_CT_DST:
-		memcpy(dest, tuple->dst.u3.all,
-		       nf_ct_l3num(ct) == NFPROTO_IPV4 ? 4 : 16);
+		if (nf_ct_l3num(ct) != nft_pf(pkt))
+			goto err;
+		memcpy(dest, tuple->dst.u3.all, priv->len);
 		return;
 	case NFT_CT_PROTO_SRC:
 		nft_reg_store16(dest, (__force u16)tuple->src.u.all);

I am leaning towards both changes.  What do you think?