netfilter: xt_tcpmss: check remaining length before reading optlen

jira VULN-184557
cve CVE-2026-43190
commit-author Florian Westphal <fw@strlen.de>
commit 735ee85

Quoting reporter:
  In net/netfilter/xt_tcpmss.c (lines 53-68), the TCP option parser reads
 op[i+1] directly without validating the remaining option length.

  If the last byte of the option field is not EOL/NOP (0/1), the code attempts
  to index op[i+1]. In the case where i + 1 == optlen, this causes an
  out-of-bounds read, accessing memory past the optlen boundary
  (either reading beyond the stack buffer _opt or the
  following payload).

	Reported-by: sungzii <sungzii@pm.me>
	Signed-off-by: Florian Westphal <fw@strlen.de>
(cherry picked from commit 735ee85)
	Signed-off-by: CIQ Kernel Automation <ciq_kernel_automation@ciq.com>

Author: CIQ Kernel Automation

net/netfilter/xt_tcpmss.c

@@ -61,7 +61,7 @@ tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
 			return (mssval >= info->mss_min &&
 				mssval <= info->mss_max) ^ info->invert;
 		}
-		if (op[i] < 2)
+		if (op[i] < 2 || i == optlen - 1)
 			i++;
 		else
 			i += op[i+1] ? : 1;