From dbdfaae9609629a9569362e3b8f33d0a20fd783c Mon Sep 17 00:00:00 2001 From: Weiming Shi Date: Thu, 19 Mar 2026 15:32:44 +0800 Subject: [PATCH] nfnetlink_osf: validate individual option lengths in fingerprints nfnl_osf_add_callback() validates opt_num bounds and string NUL-termination but does not check individual option length fields. A zero-length option causes nf_osf_match_one() to enter the option matching loop even when foptsize sums to zero, which matches packets with no TCP options where ctx->optp is NULL: Oops: general protection fault KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: 0010:nf_osf_match_one (net/netfilter/nfnetlink_osf.c:98) Call Trace: nf_osf_match (net/netfilter/nfnetlink_osf.c:227) xt_osf_match_packet (net/netfilter/xt_osf.c:32) ipt_do_table (net/ipv4/netfilter/ip_tables.c:293) nf_hook_slow (net/netfilter/core.c:623) ip_local_deliver (net/ipv4/ip_input.c:262) ip_rcv (net/ipv4/ip_input.c:573) Additionally, an MSS option (kind=2) with length < 4 causes out-of-bounds reads when nf_osf_match_one() unconditionally accesses optp[2] and optp[3] for MSS value extraction. While RFC 9293 section 3.2 specifies that the MSS option is always exactly 4 bytes (Kind=2, Length=4), the check uses "< 4" rather than "!= 4" because lengths greater than 4 do not cause memory safety issues -- the buffer is guaranteed to be at least foptsize bytes by the ctx->optsize == foptsize check. Reject fingerprints where any option has zero length, or where an MSS option has length less than 4, at add time rather than trusting these values in the packet matching hot path. Fixes: 11eeef41d5f6 ("netfilter: passive OS fingerprint xtables match") Reported-by: Xiang Mei Signed-off-by: Weiming Shi Signed-off-by: Florian Westphal --- net/netfilter/nfnetlink_osf.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/net/netfilter/nfnetlink_osf.c b/net/netfilter/nfnetlink_osf.c index 94e3eac5743a..45d9ad231a92 100644 --- a/net/netfilter/nfnetlink_osf.c +++ b/net/netfilter/nfnetlink_osf.c @@ -302,7 +302,9 @@ static int nfnl_osf_add_callback(struct sk_buff *skb, { struct nf_osf_user_finger *f; struct nf_osf_finger *kf = NULL, *sf; + unsigned int tot_opt_len = 0; int err = 0; + int i; if (!capable(CAP_NET_ADMIN)) return -EPERM; @@ -318,6 +320,17 @@ static int nfnl_osf_add_callback(struct sk_buff *skb, if (f->opt_num > ARRAY_SIZE(f->opt)) return -EINVAL; + for (i = 0; i < f->opt_num; i++) { + if (!f->opt[i].length || f->opt[i].length > MAX_IPOPTLEN) + return -EINVAL; + if (f->opt[i].kind == OSFOPT_MSS && f->opt[i].length < 4) + return -EINVAL; + + tot_opt_len += f->opt[i].length; + if (tot_opt_len > MAX_IPOPTLEN) + return -EINVAL; + } + if (!memchr(f->genre, 0, MAXGENRELEN) || !memchr(f->subtype, 0, MAXGENRELEN) || !memchr(f->version, 0, MAXGENRELEN))