linux-kernel - [PATCH] net/ipv6: Fix route deleting failure when metric equals 0

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Date: Tue, 14 May 2024 20:11:02 +0800 (CST)
From: <xu.xin16@....com.cn>
To: <davem@...emloft.net>, <dsahern@...nel.org>, <edumazet@...gle.com>,
        <kuba@...nel.org>, <pabeni@...hat.com>
Cc: <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dsahern@...il.com>, <xu.xin16@....com.cn>, <fan.yu9@....com.cn>,
        <yang.yang29@....com.cn>, <si.hao@....com.cn>,
        <zhang.yunkai@....com.cn>, <he.peilin@....com.cn>
Subject: [PATCH] net/ipv6: Fix route deleting failure when metric equals 0

From: xu xin <xu.xin16@....com.cn>

Problem
=========
After commit 67f695134703 ("ipv6: Move setting default metric for routes"),
we noticed that the logic of assigning the default value of fc_metirc
changed in the ioctl process. That is, when users use ioctl(fd, SIOCADDRT,
rt) with a non-zero metric to add a route,  then they may fail to delete a
route with passing in a metric value of 0 to the kernel by ioctl(fd,
SIOCDELRT, rt). But iproute can succeed in deleting it.

As a reference, when using iproute tools by netlink to delete routes with
a metric parameter equals 0, like the command as follows:

	ip -6 route del fe80::/64 via fe81::5054:ff:fe11:3451 dev eth0 metric 0

the user can still succeed in deleting the route entry with the smallest
metric.

Root Reason
===========
After commit 67f695134703 ("ipv6: Move setting default metric for routes"),
When ioctl() pass in SIOCDELRT with a zero metric, rtmsg_to_fib6_config()
will set a defalut value (1024) to cfg->fc_metric in kernel, and in
ip6_route_del() and the line 4074 at net/ipv3/route.c, it will check by

	if (cfg->fc_metric && cfg->fc_metric != rt->fib6_metric)
		continue;

and the condition is true and skip the later procedure (deleting route)
because cfg->fc_metric != rt->fib6_metric. But before that commit,
cfg->fc_metric is still zero there, so the condition is false and it
will do the following procedure (deleting).

Solution
========
In order to keep a consistent behaviour across netlink() and ioctl(), we
should allow to delete a route with a metric value of 0. So we only do
the default setting of fc_metric in route adding.

CC: stable@...r.kernel.org # 5.4+
Fixes: 67f695134703 ("ipv6: Move setting default metric for routes")
Co-developed-by: Fan Yu <fan.yu9@....com.cn>
Signed-off-by: Fan Yu <fan.yu9@....com.cn>
Signed-off-by: xu xin <xu.xin16@....com.cn>
---
 net/ipv6/route.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c43b0616742e..bbc2a0dd9314 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4445,7 +4445,7 @@ static void rtmsg_to_fib6_config(struct net *net,
 		.fc_table = l3mdev_fib_table_by_index(net, rtmsg->rtmsg_ifindex) ?
 			 : RT6_TABLE_MAIN,
 		.fc_ifindex = rtmsg->rtmsg_ifindex,
-		.fc_metric = rtmsg->rtmsg_metric ? : IP6_RT_PRIO_USER,
+		.fc_metric = rtmsg->rtmsg_metric,
 		.fc_expires = rtmsg->rtmsg_info,
 		.fc_dst_len = rtmsg->rtmsg_dst_len,
 		.fc_src_len = rtmsg->rtmsg_src_len,
@@ -4475,6 +4475,9 @@ int ipv6_route_ioctl(struct net *net, unsigned int cmd, struct in6_rtmsg *rtmsg)
 	rtnl_lock();
 	switch (cmd) {
 	case SIOCADDRT:
+		/* Only do the default setting of fc_metric in route adding */
+		if (cfg.fc_metric == 0)
+			cfg.fc_metric = IP6_RT_PRIO_USER;
 		err = ip6_route_add(&cfg, GFP_KERNEL, NULL);
 		break;
 	case SIOCDELRT:
-- 
2.15.2