[PATCH 5.4 024/147] Revert "ipv6: add mtu lock check in __ip6_rt_update_pmtu"

[Greg Kroah-Hartman]

From: "Maciej Åenczykowski" <maze@xxxxxxxxxx>

[ Upstream commit 09454fd0a4ce23cb3d8af65066c91a1bf27120dd ]

This reverts commit 19bda36c4299ce3d7e5bce10bebe01764a655a6d:

| ipv6: add mtu lock check in __ip6_rt_update_pmtu
|
| Prior to this patch, ipv6 didn't do mtu lock check in ip6_update_pmtu.
| It leaded to that mtu lock doesn't really work when receiving the pkt
| of ICMPV6_PKT_TOOBIG.
|
| This patch is to add mtu lock check in __ip6_rt_update_pmtu just as ipv4
| did in __ip_rt_update_pmtu.

The above reasoning is incorrect.  IPv6 *requires* icmp based pmtu to work.
There's already a comment to this effect elsewhere in the kernel:

  $ git grep -p -B1 -A3 'RTAX_MTU lock'
  net/ipv6/route.c=4813=

  static int rt6_mtu_change_route(struct fib6_info *f6i, void *p_arg)
  ...
    /* In IPv6 pmtu discovery is not optional,
       so that RTAX_MTU lock cannot disable it.
       We still use this lock to block changes
       caused by addrconf/ndisc.
    */

This reverts to the pre-4.9 behaviour.

Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
Cc: Xin Long <lucien.xin@xxxxxxxxx>
Cc: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Maciej Åenczykowski <maze@xxxxxxxxxx>
Fixes: 19bda36c4299 ("ipv6: add mtu lock check in __ip6_rt_update_pmtu")
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv6/route.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2728,8 +2728,10 @@ static void __ip6_rt_update_pmtu(struct
 	const struct in6_addr *daddr, *saddr;
 	struct rt6_info *rt6 = (struct rt6_info *)dst;
 
-	if (dst_metric_locked(dst, RTAX_MTU))
-		return;
+	/* Note: do *NOT* check dst_metric_locked(dst, RTAX_MTU)
+	 * IPv6 pmtu discovery isn't optional, so 'mtu lock' cannot disable it.
+	 * [see also comment in rt6_mtu_change_route()]
+	 */
 
 	if (iph) {
 		daddr = &iph->daddr;