CVE-2026-23397: nfnetlink_osf: validate individual option lengths in fingerprints

In the Linux kernel, the following vulnerability has been resolved:

nfnetlinkosf: validate individual option lengths in fingerprints

nfnlosfaddcallback() validates optnum bounds and string NUL-termination but does not check individual option length fields. A zero-length option causes nfosfmatchone() 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:nfosfmatchone (net/netfilter/nfnetlinkosf.c:98) Call Trace: nfosfmatch (net/netfilter/nfnetlinkosf.c:227) xtosfmatchpacket (net/netfilter/xtosf.c:32) iptdotable (net/ipv4/netfilter/iptables.c:293) nfhookslow (net/netfilter/core.c:623) iplocaldeliver (net/ipv4/ipinput.c:262) iprcv (net/ipv4/ipinput.c:573)

Additionally, an MSS option (kind=2) with length < 4 causes out-of-bounds reads when nfosfmatchone() 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.