netdev - [PATCH net-next 10/10] netfilter: xt_tcpmss: check remaining length before reading optlen

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Message-ID: <20260120191803.22208-11-fw@strlen.de>
Date: Tue, 20 Jan 2026 20:18:03 +0100
From: Florian Westphal <fw@...len.de>
To: <netdev@...r.kernel.org>
Cc: Paolo Abeni <pabeni@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	<netfilter-devel@...r.kernel.org>,
	pablo@...filter.org
Subject: [PATCH net-next 10/10] netfilter: xt_tcpmss: check remaining length before reading optlen

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@...me>
Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/netfilter/xt_tcpmss.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/xt_tcpmss.c b/net/netfilter/xt_tcpmss.c
index 37704ab01799..0d32d4841cb3 100644
--- a/net/netfilter/xt_tcpmss.c
+++ b/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;
-- 
2.52.0