[PATCH v3,net-next 1/2] ip_gre: fix parsing gre header in ipgre_err

[Haishuang Yan]

gre_parse_header stops parsing when csum_err is encountered, which means
tpi->key is undefined and ip_tunnel_lookup will return NULL improperly.

This patch introduce a NULL pointer as csum_err parameter. Even when
csum_err is encountered, it won't return error and continue parsing gre
header as expected.

Fixes: 9f57c67c379d ("gre: Remove support for sharing GRE protocol hook.")
Reported-by: Jiri Benc <jbenc@xxxxxxxxxx>
Signed-off-by: Haishuang Yan <yanhaishuang@xxxxxxxxxxxxxxxxxxxx>

---
Changes since v3:
  * skb_checksum_simple_validate need to be performed in csum_err case.
---
 net/ipv4/gre_demux.c | 7 ++++---
 net/ipv4/ip_gre.c    | 9 +++------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
index b798862..7efe740 100644
--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c
@@ -86,13 +86,14 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 
 	options = (__be32 *)(greh + 1);
 	if (greh->flags & GRE_CSUM) {
-		if (skb_checksum_simple_validate(skb)) {
+		if (!skb_checksum_simple_validate(skb)) {
+			skb_checksum_try_convert(skb, IPPROTO_GRE, 0,
+						 null_compute_pseudo);
+		} else if (csum_err) {
 			*csum_err = true;
 			return -EINVAL;
 		}
 
-		skb_checksum_try_convert(skb, IPPROTO_GRE, 0,
-					 null_compute_pseudo);
 		options++;
 	}
 
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 8cce0e9..c3385a8 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -232,13 +232,10 @@ static void gre_err(struct sk_buff *skb, u32 info)
 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
 	struct tnl_ptk_info tpi;
-	bool csum_err = false;
 
-	if (gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP),
-			     iph->ihl * 4) < 0) {
-		if (!csum_err)		/* ignore csum errors. */
-			return;
-	}
+	if (gre_parse_header(skb, &tpi, NULL, htons(ETH_P_IP),
+			     iph->ihl * 4) < 0)
+		return;
 
 	if (type == ICMP_DEST_UNREACH && code == ICMP_FRAG_NEEDED) {
 		ipv4_update_pmtu(skb, dev_net(skb->dev), info,
-- 
1.8.3.1