netdev - [net] ipv6/addrconf: fix a null-ptr-deref bug for ip6

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Date:   Fri, 22 Jul 2022 15:41:53 +0800
From:   Ziyang Xuan <william.xuanziyang@...wei.com>
To:     <davem@...emloft.net>, <yoshfuji@...ux-ipv6.org>,
        <dsahern@...nel.org>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <netdev@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>
Subject: [net] ipv6/addrconf: fix a null-ptr-deref bug for ip6_ptr

Change net device's MTU to smaller than IPV6_MIN_MTU or unregister
device while matching route. That may trigger null-ptr-deref bug
for ip6_ptr probability as following.

Reproducer as following:
Firstly, prepare conditions:
$ip netns add ns1
$ip netns add ns2
$ip link add veth1 type veth peer name veth2
$ip link set veth1 netns ns1
$ip link set veth2 netns ns2
$ip netns exec ns1 ip -6 addr add 2001:0db8:0:f101::1/64 dev veth1
$ip netns exec ns2 ip -6 addr add 2001:0db8:0:f101::2/64 dev veth2
$ip netns exec ns1 ifconfig veth1 up
$ip netns exec ns2 ifconfig veth2 up
$ip netns exec ns1 ip -6 route add 2000::/64 dev veth1 metric 1
$ip netns exec ns2 ip -6 route add 2001::/64 dev veth2 metric 1

Secondly, execute the following two commands in two ssh windows
respectively:
$ip netns exec ns1 sh
$while true; do ip -6 addr add 2001:0db8:0:f101::1/64 dev veth1; ip -6 route add 2000::/64 dev veth1 metric 1; ping6 2000::2; done

$ip netns exec ns1 sh
$while true; do ip link set veth1 mtu 1000; ip link set veth1 mtu 1500; sleep 5; done

And in order to increase the probability of reproduce,
we can add mdelay() in find_match() as following:

static bool find_match(struct fib6_nh *nh, u32 fib6_flags,
        if (nh->fib_nh_flags & RTNH_F_DEAD)
                goto out;

+       mdelay(1000);
        if (ip6_ignore_linkdown(nh->fib_nh_dev) &&
            nh->fib_nh_flags & RTNH_F_LINKDOWN &&
            !(strict & RT6_LOOKUP_F_IGNORE_LINKSTATE))

=========================================================
BUG: KASAN: null-ptr-deref in find_match.part.0+0x70/0x134
Read of size 4 at addr 0000000000000308 by task ping6/263

CPU: 2 PID: 263 Comm: ping6 Not tainted 5.19.0-rc7+ #14
Call trace:
 dump_backtrace+0x1a8/0x230
 show_stack+0x20/0x70
 dump_stack_lvl+0x68/0x84
 print_report+0xc4/0x120
 kasan_report+0x84/0x120
 __asan_load4+0x94/0xd0
 find_match.part.0+0x70/0x134
 __find_rr_leaf+0x408/0x470
 fib6_table_lookup+0x264/0x540
 ip6_pol_route+0xf4/0x260
 ip6_pol_route_output+0x58/0x70
 fib6_rule_lookup+0x1a8/0x330
 ip6_route_output_flags_noref+0xd8/0x1a0
 ip6_route_output_flags+0x58/0x160
 ip6_dst_lookup_tail+0x5b4/0x85c
 ip6_dst_lookup_flow+0x98/0x120
 rawv6_sendmsg+0x49c/0xc70
 inet_sendmsg+0x68/0x94
 sock_sendmsg+0x8c/0xb0

It is because ip6_ptr has been assigned to NULL in addrconf_ifdown(),
and ip6_ignore_linkdown() in find_match() accesses ip6_ptr directly.
Although find_match() routine is under rcu_read_lock(), but there is
not synchronize_net() before assign NULL to make rcu grace period end.

So we can add synchronize_net() before assign ip6_ptr to NULL in
addrconf_ifdown() to fix the null-ptr-deref bug.

Fixes: 8814c4b53381 ("[IPV6] ADDRCONF: Convert addrconf_lock to RCU.")
Signed-off-by: Ziyang Xuan <william.xuanziyang@...wei.com>
---
 net/ipv6/addrconf.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 49cc6587dd77..63d33b29ad21 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3757,6 +3757,7 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 		idev->dead = 1;
 
 		/* protected by rtnl_lock */
+		synchronize_net();
 		RCU_INIT_POINTER(dev->ip6_ptr, NULL);
 
 		/* Step 1.5: remove snmp6 entry */
-- 
2.25.1