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 193B840803D; Tue, 21 Jul 2026 18:01:18 +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=1784656879; cv=none; b=gQcXzQzrcNwwm+Olp4XtsweB0ehmSbXfwMbncjCjeWBBPH4v5zedjOPu6l4v0PiMGdWt8pwARWeK8D6smwh2Ff04NngiXx2DASSgqk8lG2n48Hg7jCndvPht8Z7c+09CF9POkE6ZYch1+uGgXL0gN78RtqpxLnLycOBiIZOAf5Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784656879; c=relaxed/simple; bh=l2rydaHCVAVqz1OvtyKcrMVg4qalK/Do8kL1cpc4I8Y=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=hhCf4zr4gZsoAHWJOWnozitW+kuo/4v7cSQfZvituojAKS3j75XxCUXIQ3CG2CApl/Y+RXULmlu0dPfIjJ9m3b1GDm2KM+X66v9F4N91Z8QYnd9yQGbfugMjgbay+QPuqVrPHhRZDM4udJuh56pbXhR1fcfBl/LHRaBWw4OdVXA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=dM7OZKdO; 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="dM7OZKdO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7FE641F000E9; Tue, 21 Jul 2026 18:01:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784656878; bh=gLBB0pvqkQHrEnGJPN3gbrFpvqoSFJBrn4snChmbV/w=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=dM7OZKdOWB4ZDKli+6sj544bRLd9j8tIKnNVv7ppSG7jB5mQxiMZRAEWY4PIjAxTF lWxXj3vRK07To5NaOUPDMGNpzoE9ZN1Cwi8ozNEaXObS+mvlrs6MF5K6SIx1gj7icq 9wFCARZsk17+yChiVCE7dhDdBYd5VJxj0lyC1h0w= 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 6.18 0551/1611] tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) Date: Tue, 21 Jul 2026 17:11:07 +0200 Message-ID: <20260721152527.772401553@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152514.750365251@linuxfoundation.org> References: <20260721152514.750365251@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 6.18-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 f89220929c4e24..b34f6a3e705192 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -3268,11 +3268,11 @@ 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