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

Stefan Wiehler [off-list ref] wrote:

quoted

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():

Sure, but RTNL_FLAG_DOIT_UNLOCKED is not set for this function:

        err = rtnl_register_module(THIS_MODULE, RTNL_FAMILY_IP6MR, RTM_GETROUTE,
                                   ip6mr_rtm_getroute, ip6mr_rtm_dumproute, 0);

(0 == flag field).  So RNTL is held.  Would of course be nice to convert it to RCU
eventually but thats an enhancement, not a bug fix, so this must be in
separate changesets, targetting net and net-next, respectively.

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?

{THIS_MODULE, PF_PHONET, RTM_GETADDR, NULL, getaddr_dumpit, 0},
{THIS_MODULE, PF_MPLS, RTM_GETROUTE, mpls_getroute, mpls_dump_routes, 0},

Both get called with RTNL mutex held, but not within an RCU read side
section.

but:
{THIS_MODULE, PF_MPLS, RTM_GETNETCONF, [..] mpls_netconf_dump_devconf,
       	RTNL_FLAG_DUMP_UNLOCKED}, // == no RTNL held

Means: dump callback is invoked without RTNL mutex.  Those functions
are also called without and RCU read-side section.

Both the get (doit) and the dumper function callbacks need to
explicitly opt-in for RTNL-less invocation at register time.