From: Jiayuan Chen Date: Thu, 28 May 2026 11:09:19 +0000 (+0800) Subject: netfilter: nft_ct: bail out on template ct in get eval X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3027ecbdb5fdf9200251c21d4818e4c447ef78e1;p=thirdparty%2Flinux.git netfilter: nft_ct: bail out on template ct in get eval I noticed this issue while looking at a historic syzbot report [1]. A rule like the one below is enough to trigger the bug: table ip t { chain pre { type filter hook prerouting priority raw; ct zone set 1 ct original saddr 1.2.3.4 accept } } The first expression attaches a per-cpu template ct via nft_ct_set_zone_eval() (nf_ct_tmpl_alloc -> kzalloc, tuple is all zero, nf_ct_l3num(ct) == 0). The next expression then calls nft_ct_get_eval() on the same skb, treats the template as a real ct and hits the 16-byte memcpy path. With dreg at NFT_REG32_15 this overflows past struct nft_regs on the kernel stack; with smaller dreg values it silently clobbers adjacent registers. Reject template ct at the eval entry and in nft_ct_get_fast_eval(), mirroring the check nft_ct_set_eval() already has. Additionally, bound the address copy in NFT_CT_SRC / NFT_CT_DST by priv->len instead of by nf_ct_l3num(ct): nf_ct_get_tuple() zeroes the tuple before pkt_to_tuple() fills in only the protocol-relevant leading bytes, so the trailing bytes of tuple->{src,dst}.u3.all are well-defined zero. priv->len is validated at rule load, so the copy size is now bounded by the destination register rather than by an untrusted field on the conntrack. [1]: https://syzkaller.appspot.com/bug?id=389cf09cb72926114fce90dc85a2c3231dcb647c Fixes: 45d9bcda21f4 ("netfilter: nf_tables: validate len in nft_validate_data_load()") Suggested-by: Florian Westphal Signed-off-by: Jiayuan Chen Signed-off-by: Florian Westphal Signed-off-by: Pablo Neira Ayuso --- diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c index fa2cc556331c..357513c6dcea 100644 --- 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) { @@ -180,12 +180,10 @@ 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); + 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); + memcpy(dest, tuple->dst.u3.all, priv->len); return; case NFT_CT_PROTO_SRC: nft_reg_store16(dest, (__force u16)tuple->src.u.all); 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; }