[PATCH net 04/14] netfilter: nfnetlink_osf: fix divide-by-zero in OSF_WSS_MODULO

Pablo Neira Ayuso
2026-04-16T01:31:14Z

From: Xiang Mei <xmei5@asu.edu>

nf_osf_match_one() computes ctx->window % f->wss.val in the
OSF_WSS_MODULO branch with no guard for f->wss.val == 0. A
CAP_NET_ADMIN user can add such a fingerprint via nfnetlink; a
subsequent matching TCP SYN divides by zero and panics the kernel.

Reject the bogus fingerprint in nfnl_osf_add_callback() above the
per-option for-loop. f->wss is per-fingerprint, not per-option, so
the check must run regardless of f->opt_num (including 0). Also
reject wss.wc >= OSF_WSS_MAX; nf_osf_match_one() already treats that
as "should not happen".

Crash:
Oops: divide error: 0000 [#1] SMP KASAN NOPTI
RIP: 0010:nf_osf_match_one (net/netfilter/nfnetlink_osf.c:98)
Call Trace:
<IRQ>
nf_osf_match (net/netfilter/nfnetlink_osf.c:220)
xt_osf_match_packet (net/netfilter/xt_osf.c:32)
ipt_do_table (net/ipv4/netfilter/ip_tables.c:348)
nf_hook_slow (net/netfilter/core.c:622)
ip_local_deliver (net/ipv4/ip_input.c:265)
ip_rcv (include/linux/skbuff.h:1162)
__netif_receive_skb_one_core (net/core/dev.c:6181)
process_backlog (net/core/dev.c:6642)
__napi_poll (net/core/dev.c:7710)
net_rx_action (net/core/dev.c:7945)
handle_softirqs (kernel/softirq.c:622)

Fixes: 11eeef41d5f6 ("netfilter: passive OS fingerprint xtables match")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Suggested-by: Florian Westphal <fw@strlen.de>
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
net/netfilter/nfnetlink_osf.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c
index 45d9ad231a92..70172ca07858 100644
--- a/net/netfilter/nfnetlink_osf.c
+++ b/net/netfilter/nfnetlink_osf.c
@@ -320,6 +320,10 @@ static int nfnl_osf_add_callback(struct sk_buff *skb,
if (f->opt_num > ARRAY_SIZE(f->opt))
return -EINVAL;

+ if (f->wss.wc >= OSF_WSS_MAX ||
+ (f->wss.wc == OSF_WSS_MODULO && f->wss.val == 0))
+ return -EINVAL;
+
for (i = 0; i < f->opt_num; i++) {
if (!f->opt[i].length || f->opt[i].length > MAX_IPOPTLEN)
return -EINVAL;
--
2.47.3