From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 481B72EBB8D; Tue, 21 Jul 2026 20:37:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784666260; cv=none; b=n7aGxVkCcFuF9qW7xuyqPgZQe4blfVmxloKNzXLvB+neykSeUV1LPX1aRInhxRcvG4Bjwws8PT0TsLCl2dp4U4dR5JrOaGXx3QhHMIfABfyIbfAC46RTYj8bocKZyJOo+pHHvBV3dDghz6OX2fvMqJAdQI27xUwu3krakRXU0eE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784666260; c=relaxed/simple; bh=oM/swLmCN44sK9yXnxkTR4KG8Q4bXbo83U0ldaOwUPw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=JK777jedN6B6vbUP6ZH7cHaBUAj31bFNVssBcZjw+PRXcdyyqQea6UYjGhrHQPjEE2s1rueD6J6D0DTRxaIypz21pBE97cfArGaU5TbRICL78fTwtxrk+uPyOrfwgQwxaVPpkO0rwxslg6TYBJkR79glcJwTEIY8IQvgCtDUB78= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=GVkH8WSh; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="GVkH8WSh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ADCD61F00A3A; Tue, 21 Jul 2026 20:37:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784666259; bh=/Yj828DES4Psxz3bL6JBMvx9BrbmI9eRc+d+On8dqaM=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=GVkH8WSheJ3VX4EcWscGA14SvmaJkvOd6TUh5F2MYEItfOZWCKSmFYD5zCbs2TfMh PQM3vi+pYr7jffH6V0hHy3BBV4Q+quUfZ0z3yoTWg/ddMlggYRJhaCcZcORGmldcGn m/MGS6SDdGYo9qGJNilfmBiaoURkc5y0wPQfxhHE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+ebdb22d461c904fc3cb2@syzkaller.appspotmail.com, Eric Dumazet , Kuniyuki Iwashima , Jakub Kicinski , Sasha Levin Subject: [PATCH 6.6 0598/1266] tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) Date: Tue, 21 Jul 2026 17:17:15 +0200 Message-ID: <20260721152455.247115945@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152441.786066624@linuxfoundation.org> References: <20260721152441.786066624@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet [ Upstream commit 2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22 ] When MTU is large, ip6_default_advmss() can return IPV6_MAXPLEN (65535). This is interpreted by TCP as mss_clamp, allowing the MSS to reach 65535. However, 0xFFFF is also used as a magic value GSO_BY_FRAGS in the kernel. If a TCP packet with gso_size=0xFFFF is passed to skb_segment(), it will be mistakenly treated as GSO_BY_FRAGS, leading to a NULL pointer dereference because local TCP packets do not use frag_list. Fix this by returning min(IPV6_MAXPLEN, GSO_BY_FRAGS - 1) (65534) from ip6_default_advmss() when MTU is large. Also update the stale comment in ip6_default_advmss() which suggested that IPV6_MAXPLEN is returned to mean "any MSS". Fixes: 3953c46c3ac7 ("sk_buff: allow segmenting based on frag sizes") Reported-by: syzbot+ebdb22d461c904fc3cb2@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/6a2c3193.8812e0fc.3c3fa4.0001.GAE@google.com/T/#u Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260612162517.83394-1-edumazet@google.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/ipv6/route.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 6d80e17c04c0d8..c3257e8d575485 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -3251,11 +3251,11 @@ static unsigned int ip6_default_advmss(const struct dst_entry *dst) /* * Maximal non-jumbo IPv6 payload is IPV6_MAXPLEN and * corresponding MSS is IPV6_MAXPLEN - tcp_header_size. - * IPV6_MAXPLEN is also valid and means: "any MSS, - * rely only on pmtu discovery" + * Limit the default MSS to GSO_BY_FRAGS - 1 to avoid + * collision with the GSO_BY_FRAGS magic value (0xFFFF). */ if (mtu > IPV6_MAXPLEN - sizeof(struct tcphdr)) - mtu = IPV6_MAXPLEN; + mtu = min_t(unsigned int, IPV6_MAXPLEN, GSO_BY_FRAGS - 1); return mtu; } -- 2.53.0