Re: [PATCH net v6 09/10] ip6mr: Lock RCU before ip6mr_get_table() call in ip6mr_rtm_getroute()

quoted

When IPV6_MROUTE_MULTIPLE_TABLES is enabled, multicast routing tables
must be read under RCU or RTNL lock.

Fixes: d1db275dd3f6 ("ipv6: ip6mr: support multiple tables")
Signed-off-by: Stefan Wiehler <redacted>
---
 net/ipv6/ip6mr.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index b169b27de7e1..39aac81a30f1 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c

@@ -2633,27 +2633,31 @@ static int ip6mr_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
              grp = nla_get_in6_addr(tb[RTA_DST]);
      tableid = tb[RTA_TABLE] ? nla_get_u32(tb[RTA_TABLE]) : 0;

+     rcu_read_lock();

AFAICS ip6mr_rtm_getroute() runs with RTNL held, so I don't see
why this patch is needed.

That's true, but it's called neither with RCU nor RTNL lock when
RTNL_FLAG_DOIT_UNLOCKED is set in rtnetlink_rcv_msg():

if (flags & RTNL_FLAG_DOIT_UNLOCKED) {
	doit = link->doit;
	rcu_read_unlock();
	if (doit)
		err = doit(skb, nlh, extack);
	module_put(owner);
	return err;
}
rcu_read_unlock();

I realized now that I completely misunderstood how ip6mr_rtm_dumproute() gets
called - it should be still safe though because mpls_netconf_dump_devconf() and
getaddr_dumpit() hold the RCU lock while mpls_dump_routes() asserts that the
RTNL lock is held. Is that observation correct?