netfilter: nfnetlink_osf: fix out-of-bounds read on option matching

In nf_osf_match(), the nf_osf_hdr_ctx structure is initialized once
and passed by reference to nf_osf_match_one() for each fingerprint
checked. During TCP option parsing, nf_osf_match_one() advances the
shared ctx->optp pointer.

If a fingerprint perfectly matches, the function returns early without
restoring ctx->optp to its initial state. If the user has configured
NF_OSF_LOGLEVEL_ALL, the loop continues to the next fingerprint.
However, because ctx->optp was not restored, the next call to
nf_osf_match_one() starts parsing from the end of the options buffer.
This causes subsequent matches to read garbage data and fail
immediately, making it impossible to log more than one match or logging
incorrect matches.

Instead of using a shared ctx->optp pointer, pass the context as a
constant pointer and use a local pointer (optp) for TCP option
traversal. This makes nf_osf_match_one() strictly stateless from the
caller's perspective, ensuring every fingerprint check starts at the
correct option offset.

Fixes: 1a6a095 ("netfilter: nfnetlink_osf: add missing fmatch check")
Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: ffmancera

net/netfilter/nfnetlink_osf.c

@@ -64,9 +64,9 @@ struct nf_osf_hdr_ctx {
 static bool nf_osf_match_one(const struct sk_buff *skb,
 			     const struct nf_osf_user_finger *f,
 			     int ttl_check,
-			     struct nf_osf_hdr_ctx *ctx)
+			     const struct nf_osf_hdr_ctx *ctx)
 {
-	const __u8 *optpinit = ctx->optp;
+	const __u8 *optp = ctx->optp;
 	unsigned int check_WSS = 0;
 	int fmatch = FMATCH_WRONG;
 	int foptsize, optnum;
@@ -95,25 +95,25 @@ static bool nf_osf_match_one(const struct sk_buff *skb,
 	check_WSS = f->wss.wc;
 
 	for (optnum = 0; optnum < f->opt_num; ++optnum) {
-		if (f->opt[optnum].kind == *ctx->optp) {
+		if (f->opt[optnum].kind == *optp) {
 			__u32 len = f->opt[optnum].length;
-			const __u8 *optend = ctx->optp + len;
+			const __u8 *optend = optp + len;
 
 			fmatch = FMATCH_OK;
 
-			switch (*ctx->optp) {
+			switch (*optp) {
 			case OSFOPT_MSS:
-				mss = ctx->optp[3];
+				mss = optp[3];
 				mss <<= 8;
-				mss |= ctx->optp[2];
+				mss |= optp[2];
 
 				mss = ntohs((__force __be16)mss);
 				break;
 			case OSFOPT_TS:
 				break;
 			}
 
-			ctx->optp = optend;
+			optp = optend;
 		} else
 			fmatch = FMATCH_OPT_WRONG;
 
@@ -156,9 +156,6 @@ static bool nf_osf_match_one(const struct sk_buff *skb,
 		}
 	}
 
-	if (fmatch != FMATCH_OK)
-		ctx->optp = optpinit;
-
 	return fmatch == FMATCH_OK;
 }