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 D748A5AEC79; Thu, 17 Sep 2026 15:42:15 +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=1789659741; cv=none; b=naqMXurNnOJ9d7MBXTFVRlG0ABTPZQHt/W092Ah8u/op0Kj7910siYH6hmK3JVTque5EDTRBqCEt0hsmagyvR5dRM2L+khv6J+Pzbnqa/Jb21+QS0E/VmEt29PsVxJXLyOWwkf6ENZKWhNnE5jwdii43qx15rwmxK1I8IGF4rE0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789659741; c=relaxed/simple; bh=NCdOw0DlN1jbMuQ6vEWHfxvj/P3n5+XmZW5PgV2pufQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=K43hPf/VxMq7IDMQ7sxu8nZqGDkgSGr6Sfjs97/XCwZNYKNkdDesH1dSC3A7F/ele3gM6RIfLfVJVVuj+ZK+9PKlxBvkrsEqr+S7PdvgHZirgEPbw1yHtgkkcWmjSIp9xTqkXfWqZM5Q8f5r67Q0JZgZLUhmgbR19QgfEE7gNc8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=1aNdyUS2; 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="1aNdyUS2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 812771F00898; Thu, 17 Sep 2026 15:42:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789659735; bh=FZZIP6dw47+yzlHQD2w3Tf9PCe3Vt4ynDncFsjcxa5I=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=1aNdyUS2VdL6F8BylMsMjEUw3BNeN7PFULeL9rxxt0wQ92fe4gCel/h6aFkZfbFel ZNgbKEt8LYFInFxUs1Ncx4GaLrYl7Ldnka4unAnR9akLS9kFi9bEK3d0LaGzHgfnlg e1rnbtStKxXiOIvaK5B111mmDziUndhpd2XWOnXA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com, Alice Mikityanska , Willem de Bruijn , Jakub Kicinski , Sasha Levin Subject: [PATCH 7.2 356/733] net: ipv4: Fix UDP length overflow with PMTU discover and big MTU Date: Thu, 17 Sep 2026 16:11:04 +0100 Message-ID: <20260917151400.462533779@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260917151350.597953846@linuxfoundation.org> References: <20260917151350.597953846@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 7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Alice Mikityanska [ Upstream commit b83641e0ab8b20eefcc4cdc5a059f897375291a2 ] This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU. Steps to reproduce: 1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will be set to that MTU in ip_setup_cork. 2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip_append_data, rather than to IP_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of IPv4 header). Device MTU allows it (it's only one byte bigger than IP_MAX_MTU + IPv4 header, and the device MTU is bigger than that). 4. The UDP length in the built packet is 65536, which overflows the 16-bit length field and triggers the WARN in udp_set_len_short. Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward. The Fixes tag points at the first commit where I could reproduce the overflow with IPv4 and IP_PMTUDISC_PROBE. Fixes: daba287b299e ("ipv4: fix DO and PROBE pmtu mode regarding local fragmentation with UFO/CORK") Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/6a6a966c.86abc875.e5c3d.0054.GAE@google.com/ Signed-off-by: Alice Mikityanska Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20260901195714.673548-2-alice.kernel@fastmail.im Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/ipv4/ip_output.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index 74e095b6b7ca0..a24cc8ee11d3e 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -1303,6 +1303,7 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork, cork->fragsize = ip_sk_use_pmtu(sk) ? dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu); + cork->fragsize = min(cork->fragsize, IP_MAX_MTU); if (!inetdev_valid_mtu(cork->fragsize)) return -ENETUNREACH; -- 2.53.0