[S] Change in openvpn[master]: options: fix use-after-free of DNS options on client connect

Attention is currently required from: cron2, ordex, plaisthos.

Hello cron2, plaisthos, 

I'd like you to reexamine a change. Please visit

    http://gerrit.openvpn.net/c/openvpn/+/1715?usp=email

to look at the new patch set (#2).

The following approvals got outdated and were removed:
Code-Review-1 by cron2


Change subject: options: fix use-after-free of DNS options on client connect
......................................................................

options: fix use-after-free of DNS options on client connect

struct dns_options embeds its own gc_arena. When inherit_context_child()
/inherit_context_top() copy struct options by value, the child shares the
parent's DNS arena. options_detach() detached o->gc but not
o->dns_options.gc, so pre_connect_restore()'s gc_free() (and context
teardown) freed allocations the parent still referenced.

With one or more non-pushed --dhcp-option directives that yield a DNS
entry, a connecting client triggers this and the server crashes
(use-after-free in setenv_dns_options(), reported as a double free).

Detach o->dns_options.gc as well, mirroring the existing o->gc handling.

Change-Id: I49b37b5a90554fa2d4a83c8fc5608dad2a36b835
GitHub: closes openvpn/OpenVPN#1060
Signed-off-by: Antonio Quartulli <[email protected]>
---
M src/openvpn/options.c
1 file changed, 11 insertions(+), 0 deletions(-)


  git pull ssh://gerrit.openvpn.net:29418/openvpn refs/changes/15/1715/2

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 0c2866c..9d4a879 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -1531,7 +1531,18 @@
 void
 options_detach(struct options *o)
 {
+    /* The options struct carries two gc_arena's (one generic and one specific
+     * to the DNS settings), which the by-value options
+     * copy in inherit_context_child()/inherit_context_top() shares with the
+     * source.
+     *
+     * Detach both (i.e. re-inizialize them), otherwise child's call of
+     * gc_free() (or context teardown) would free allocations the source
+     * context still references, leading to a use-after-free (and subsequent
+     * double-free).
+     */
     gc_detach(&o->gc);
+    gc_detach(&o->dns_options.gc);
     o->routes = NULL;
     o->client_nat = NULL;
     clone_push_list(o);

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Gerrit-MessageType: newpatchset
Gerrit-Project: openvpn
Gerrit-Branch: master
Gerrit-Change-Id: I49b37b5a90554fa2d4a83c8fc5608dad2a36b835
Gerrit-Change-Number: 1715
Gerrit-PatchSet: 2
Gerrit-Owner: ordex <[email protected]>
Gerrit-Reviewer: cron2 <[email protected]>
Gerrit-Reviewer: plaisthos <[email protected]>
Gerrit-CC: openvpn-devel <[email protected]>
Gerrit-Attention: plaisthos <[email protected]>
Gerrit-Attention: cron2 <[email protected]>
Gerrit-Attention: ordex <[email protected]>

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