netdev - [PATCH iproute2] libnetlink: Fix memory leak in __rtnl_talk

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Date:   Wed, 23 Nov 2022 16:15:41 +0000
From:   Gilad Naaman <gnaaman@...venets.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Lahav Daniel Schlesinger <lschlesinger@...venets.com>
Subject: [PATCH iproute2] libnetlink: Fix memory leak in __rtnl_talk_iov()

If `__rtnl_talk_iov()` fails then callers are not expected to free `answer`.

Currently if `NLMSG_ERROR` was received with an error then the netlink
buffer was stored in `answer`, while still returning an error

This leak can be observed by running this snippet over time.
This triggers an `NLMSG_ERROR` because for each neighbour update, `ip`
will try to query for the name of interface 9999 in the wrong netns.
(which in itself is a separate bug)

	set -e

	ip netns del test-a || true
	ip netns add test-a
	ip netns del test-b || true
	ip netns add test-b

	ip -n test-a netns set test-b auto
	ip -n test-a link add veth_a index 9999 type veth peer name veth_b netns test-b
	ip -n test-b link set veth_b up

	ip -n test-a monitor link address prefix neigh nsid label all-nsid > /dev/null &
	monitor_pid=$!
	clean() {
		kill $monitor_pid
		ip netns del test-a
		ip netns del test-b
	}
	trap clean EXIT

	while true; do
		ip -n test-b neigh add dev veth_b 1.2.3.4 lladdr AA:AA:AA:AA:AA:AA
		ip -n test-b neigh del dev veth_b 1.2.3.4
	done

Fixes: 55870df ("Improve batch and dump times by caching link lookups")
Signed-off-by: Lahav Schlesinger <lschlesinger@...venets.com>
Signed-off-by: Gilad Naaman <gnaaman@...venets.com>
---
lib/libnetlink.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/lib/libnetlink.c b/lib/libnetlink.c
index 9af06232..001efc1d 100644
--- a/lib/libnetlink.c
+++ b/lib/libnetlink.c
@@ -1092,14 +1092,19 @@ next:
						rtnl_talk_error(h, err, errfn);
				}

-				if (answer)
-					*answer = (struct nlmsghdr *)buf;
-				else
+				if (i < iovlen) {
					free(buf);
-
-				if (i < iovlen)
					goto next;
-				return error ? -i : 0;
+				}
+
+				if (error) {
+					free(buf);
+					return -i;
+				}
+
+				if (answer)
+					*answer = (struct nlmsghdr *)buf;
+				return 0;
			}

			if (answer) {
--
2.37.1