Re: [PATCH iptables] nft: fix out-of-bounds read listing an old-revision match

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, Jul 17, 2026 at 06:59:27PM +0000, Omkhar Arasaratnam wrote:
> nft_parse_match() sizes the xt_entry_match buffer from the wire blob
> length reported by the kernel:
> 
>         m = xtables_calloc(1, sizeof(struct xt_entry_match) + mt_len);
>         memcpy(&m->data, mt_info, mt_len);
> 
> but selects the print/save/compare extension purely by name via
> xtables_find_match(), which returns the highest supported revision. When
> the kernel stored an older, smaller revision of the match, mt_len is
> smaller than the resolved extension's userspacesize, and the print
> callback -- and compare_matches(), which memcmp()s userspacesize bytes
> -- read past the mt_len-sized allocation.
> 
> For example a conntrack match stored as xt_conntrack_mtinfo1 (152 bytes)
> is printed by the rev3 callback conntrack_dump(), which reads
> xt_conntrack_mtinfo3 fields (info->origsrc_port_high at offset 154), two
> bytes past the buffer. Listing such a ruleset with iptables-nft -L reads
> out of bounds.

Is this a hypothetical crash? How can this happen in practise? These
revisions are very old, you would need to pick a very old iptables
version which possibly does not supports nftables to trigger this?

> Size the allocation to the resolved extension when its blob is larger,
> just as nft_create_match() and __nft_create_target() already do, so the
> print and compare paths stay in bounds. Matches whose stored blob is at
> least as large as the extension are unaffected.
>
> Fixes: cdc78b1d6bd7 ("nft: convert rule into a command state structure")
> Signed-off-by: Omkhar Arasaratnam <omkhar@linkedin.com>
> ---
>  iptables/nft-ruleparse.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/iptables/nft-ruleparse.c b/iptables/nft-ruleparse.c
> index 26a605cf..2c5aa9ce 100644
> --- a/iptables/nft-ruleparse.c
> +++ b/iptables/nft-ruleparse.c
> @@ -638,7 +638,10 @@ static void nft_parse_match(struct nft_xt_ctx *ctx, struct nftnl_expr *e)
>                 return;
>         }
> 
> -       m = xtables_calloc(1, sizeof(struct xt_entry_match) + mt_len);
> +       /* Kernel blob may be smaller than the resolved extension (older
> +        * revision); size to the extension so print/compare stay in bounds. */
> +       m = xtables_calloc(1, sizeof(struct xt_entry_match) +
> +                             (mt_len < match->size ? match->size : mt_len));
>         memcpy(&m->data, mt_info, mt_len);
>         m->u.match_size = mt_len + XT_ALIGN(sizeof(struct xt_entry_match));
>         m->u.user.revision = nftnl_expr_get_u32(e, NFTNL_EXPR_TG_REV);
> --
> 2.34.1