netdev - [PATCH net] udp: document udp_rcv_segment special case for looped packets

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 29 Jan 2020 15:20:17 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, pabeni@...hat.com,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] udp: document udp_rcv_segment special case for looped packets

From: Willem de Bruijn <willemb@...gle.com>

Commit 6cd021a58c18a ("udp: segment looped gso packets correctly")
fixes an issue with rare udp gso multicast packets looped onto the
receive path.

The stable backport makes the narrowest change to target only these
packets, when needed. As opposed to, say, expanding __udp_gso_segment,
which is harder to reason to be free from unintended side-effects.

But the resulting code is hardly self-describing.
Document its purpose and rationale.

Signed-off-by: Willem de Bruijn <willemb@...gle.com>

---

Let me know if you'd prefer net-next.
---
 include/net/udp.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/net/udp.h b/include/net/udp.h
index 4a180f2a13e32..e55d5f7658075 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -476,6 +476,13 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
 	if (!inet_get_convert_csum(sk))
 		features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
 
+	/* UDP segmentation expects packets of type CHECKSUM_PARTIAL or
+	 * CHECKSUM_NONE in __udp_gso_segment. UDP GRO indeed builds partial
+	 * packets in udp_gro_complete_segment. As does UDP GSO, verified by
+	 * udp_send_skb. But when those packets are looped in dev_loopback_xmit
+	 * their ip_summed is set to CHECKSUM_UNNECESSARY. Reset in this
+	 * specific case, where PARTIAL is both correct and required.
+	 */
 	if (skb->pkt_type == PACKET_LOOPBACK)
 		skb->ip_summed = CHECKSUM_PARTIAL;
 
-- 
2.25.0.341.g760bfbb309-goog