[PATCH net] gso: fix GSO_DODGY bit handling for related protocols

[Yan Zhai <yan@cloudflare.com>]

SKB_GSO_DODGY bit indicates a GSO packet comes from an untrusted source.
The canonical way is to recompute the gso_segs to avoid device driver
issues. Afterwards, the DODGY bit can be removed to avoid re-check at the
egress of later devices, e.g. packets can egress to a vlan device backed
by a real NIC.

Commit 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4
packets.") checks DODGY bit for UDP, but for packets that can be fed
directly to the device after gso_segs reset, it actually falls through
to fragmentation [1].

Commit 90017accff61 ("sctp: Add GSO support") and commit 3820c3f3e417
("[TCP]: Reset gso_segs if packet is dodgy") both didn't remove the DODGY
bit after recomputing gso_segs.

This change fixes the GSO_UDP_L4 handling case, and remove the DODGY bit
at other places.

Fixes: 90017accff61 ("sctp: Add GSO support")
Fixes: 3820c3f3e417 ("[TCP]: Reset gso_segs if packet is dodgy")
Fixes: 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4 packets.")
Signed-off-by: Yan Zhai <yan@cloudflare.com>

---
[1]:
https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@mail.gmail.com/

---
 net/ipv4/tcp_offload.c |  1 +
 net/ipv4/udp_offload.c | 19 +++++++++++++++----
 net/ipv6/udp_offload.c | 19 +++++++++++++++----
 net/sctp/offload.c     |  2 ++
 4 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index 8311c38267b5..f9b93708c22e 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -87,6 +87,7 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
 		/* Packet is from an untrusted source, reset gso_segs. */
 
 		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
+		skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
 
 		segs = NULL;
 		goto out;
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 75aa4de5b731..bd29cf19bb6b 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -388,11 +388,22 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
 	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 		goto out;
 
-	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
-	    !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
-		return __udp_gso_segment(skb, features, false);
-
 	mss = skb_shinfo(skb)->gso_size;
+
+	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
+		if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
+			/* Packet is from an untrusted source, reset actual gso_segs */
+			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
+								 mss);
+			skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
+
+			segs = NULL;
+			goto out;
+		} else {
+			return __udp_gso_segment(skb, features, false);
+		}
+	}
+
 	if (unlikely(skb->len <= mss))
 		goto out;
 
diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index ad3b8726873e..6857d9f7bd06 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -43,11 +43,22 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 		if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 			goto out;
 
-		if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
-		    !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
-			return __udp_gso_segment(skb, features, true);
-
 		mss = skb_shinfo(skb)->gso_size;
+
+		if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
+			if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
+				/* Packet is from an untrusted source, reset actual gso_segs */
+				skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
+									 mss);
+				skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
+
+				segs = NULL;
+				goto out;
+			} else {
+				return __udp_gso_segment(skb, features, true);
+			}
+		}
+
 		if (unlikely(skb->len <= mss))
 			goto out;
 
diff --git a/net/sctp/offload.c b/net/sctp/offload.c
index 502095173d88..3d2b44db0d42 100644
--- a/net/sctp/offload.c
+++ b/net/sctp/offload.c
@@ -65,6 +65,8 @@ static struct sk_buff *sctp_gso_segment(struct sk_buff *skb,
 		skb_walk_frags(skb, frag_iter)
 			pinfo->gso_segs++;
 
+		pinfo->gso_type &= ~SKB_GSO_DODGY;
+
 		segs = NULL;
 		goto out;
 	}
-- 
2.30.2