ofproto: Add refcount to ofproto to fix ofproto use-after-free.

From hepeng:
https://patchwork.ozlabs.org/project/openvswitch/patch/20200717015041.82746-1-hepeng.0320@bytedance.com/#2487473

also from guohongzhi <guohongzhi1@huawei.com>:
http://patchwork.ozlabs.org/project/openvswitch/patch/20200306130555.19884-1-guohongzhi1@huawei.com/

also from a discussion about the mixing use of RCU and refcount in the mail
list with Ilya Maximets, William Tu, Ben Pfaf, and Gaëtan Rivet.

A summary, as quoted from Ilya:

"
RCU for ofproto was introduced for one
and only one reason - to avoid freeing ofproto while rules are still
alive.  This was done in commit f416c8d61601 ("ofproto: RCU postpone
rule destruction.").  The goal was to allow using rules without
refcounting them within a single grace period.  And that forced us
to postpone destruction of the ofproto for a single grace period.
Later commit 39c9459355b6 ("Use classifier versioning.") made it
possible for rules to be alive for more than one grace period, so
the commit made ofproto wait for 2 grace periods by double postponing.
As we can see now, that wasn't enough and we have to wait for more
than 2 grace periods in certain cases.
"

In a short, the ofproto should have a longer life time than rule, if
the rule lasts for more than 2 grace periods, the ofproto should live
longer to ensure rule->ofproto is valid. It's hard to predict how long
a ofproto should live, thus we need to use refcount on ofproto to make
things easy. The controversial part is that we have already used RCU postpone
to delay ofproto destrution, if we have to add refcount, is it simpler to
use just refcount without RCU postpone?

IMO, I think going back to the pure refcount solution is more
complicated than mixing using both.

Gaëtan Rive asks some questions on guohongzhi's v2 patch:

during ofproto_rule_create, should we use ofproto_ref
or ofproto_try_ref? how can we make sure the ofproto is alive?

By using RCU, ofproto has three states:

state 1: alive, with refcount >= 1
state 2: dying, with refcount == 0, however pointer is valid
state 3: died, memory freed, pointer might be dangling.

Without using RCU, there is no state 2, thus, we have to be very careful
every time we see a ofproto pointer. In contrast, with RCU, we can be sure
that it's alive at least in this grace peroid, so we can just check if
it is dying by ofproto_try_ref.

This shows that by mixing use of RCU and refcount we can save a lot of work
worrying if ofproto is dangling.

In short, the RCU part makes sure the ofproto is alive when we use it,
and the refcount part makes sure it lives longer enough.

In this patch, I have merged guohongzhi's patch and mine, and fixes
accoring to the previous comments.

Acked-by: Adrian Moreno <amorenoz@redhat.com>
Acked-by: Gaetan Rivet <grive@u256.net>
Acked-by: Mike Pattrick <mkp@redhat.com>
Acked-by: Alin-Gabriel Serdean <aserdean@ovn.org>
Tested-by: Alin-Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Peng He <hepeng.0320@bytedance.com>
Co-authored-by: Hongzhi Guo <guohongzhi1@huawei.com>
Signed-off-by: Hongzhi Guo <guohongzhi1@huawei.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/ofproto/ofproto-provider.h b/ofproto/ofproto-provider.h
index 14b909973..47e96e62e 100644
--- a/ofproto/ofproto-provider.h
+++ b/ofproto/ofproto-provider.h
@@ -143,6 +143,8 @@ struct ofproto {
     /* Variable length mf_field mapping. Stores all configured variable length
      * meta-flow fields (struct mf_field) in a switch. */
     struct vl_mff_map vl_mff_map;
+    /* refcount to this ofproto, held by rule/group/xlate_caches */
+    struct ovs_refcount refcount;
 };
 
 void ofproto_init_tables(struct ofproto *, int n_tables);