[PATCH net-next 1/2] ip_tunnel: use WRITE_ONCE in ip_tunnel_encap_setup

[Eric Dumazet <edumazet@google.com>]

Update ip_tunnel_encap_setup() to use WRITE_ONCE() when writing
to encap fields (type, sport, dport, flags) and hlen fields.
This ensures that concurrent lockless readers (like fill_info)
do not see torn writes.

Also remove the unsafe memset() on t->encap which could cause
concurrent readers to transiently see zeroed fields.
Removing it also fixes a bug where t->encap was left cleared
even if ip_encap_hlen() failed, resulting in partial configuration.

Fixes: 56328486539d ("net: Changes to ip_tunnel to support foo-over-udp encapsulation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/ip_tunnel.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 9d114bd575f928b0ab46ef3007e456692d82b497..f7bbdd1bd323b1973ec1ea1d93f0ed6ab703bcd3 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -491,19 +491,17 @@ int ip_tunnel_encap_setup(struct ip_tunnel *t,
 {
 	int hlen;
 
-	memset(&t->encap, 0, sizeof(t->encap));
-
 	hlen = ip_encap_hlen(ipencap);
 	if (hlen < 0)
 		return hlen;
 
-	t->encap.type = ipencap->type;
-	t->encap.sport = ipencap->sport;
-	t->encap.dport = ipencap->dport;
-	t->encap.flags = ipencap->flags;
+	WRITE_ONCE(t->encap.type, ipencap->type);
+	WRITE_ONCE(t->encap.sport, ipencap->sport);
+	WRITE_ONCE(t->encap.dport, ipencap->dport);
+	WRITE_ONCE(t->encap.flags, ipencap->flags);
 
-	t->encap_hlen = hlen;
-	t->hlen = t->encap_hlen + t->tun_hlen;
+	WRITE_ONCE(t->encap_hlen, hlen);
+	WRITE_ONCE(t->hlen, hlen + t->tun_hlen);
 
 	return 0;
 }
-- 
2.55.0.rc0.799.gd6f94ed593-goog