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Date: Fri, 22 Jul 2022 19:06:41 +0800
From: "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>
To: Eric Dumazet <edumazet@...gle.com>
CC: David Miller <davem@...emloft.net>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
David Ahern <dsahern@...nel.org>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
netdev <netdev@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [net] ipv6/addrconf: fix a null-ptr-deref bug for ip6_ptr
> On Fri, Jul 22, 2022 at 9:42 AM Ziyang Xuan
> <william.xuanziyang@...wei.com> wrote:
>>
>> 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);
>
> But adding a mdelay() in an rcu_read_lock() should not be possible.
>
> I guess this means _this_ function is not properly using rcu protection.
This just to increase the probability during reproducing.
The problem needs ip6_ptr assigned to NULL in addrconf_ifdown() firstly,
then accesses ip6_ptr without any NULL check in ip6_ignore_linkdown().
cpu0 cpu1
fib6_table_lookup [ under rcu_read_lock() ]
__find_rr_leaf [ traverse fib6_info list ]
addrconf_notify [ NETDEV_CHANGEMTU ]
addrconf_ifdown
RCU_INIT_POINTER(dev->ip6_ptr, NULL)
find_match
ip6_ignore_linkdown
static inline bool ip6_ignore_linkdown(const struct net_device *dev)
{
const struct inet6_dev *idev = __in6_dev_get(dev);
// without NULL check, access idev directly. If idev is NULL, null-ptr-deref occur.
return !!idev->cnf.ignore_routes_with_linkdown;
}
>
>> 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.
>>
>
> This is not how RCU works.
>
>> So we can add synchronize_net() before assign ip6_ptr to NULL in
>> addrconf_ifdown() to fix the null-ptr-deref bug.
>
> This does not make sense to me.
>
>>
>> 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();
>
> I do not think we want yet another expensive synchronize_net(),
> especially before setting ip6_ptr to NULL
Maybe the following solution can be considered.
diff --git a/include/net/addrconf.h b/include/net/addrconf.h
index f7506f08e505..c04f359655b8 100644
--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -405,6 +405,9 @@ static inline bool ip6_ignore_linkdown(const struct net_device *dev)
{
const struct inet6_dev *idev = __in6_dev_get(dev);
+ if (unlikely(!idev))
+ return true;
+
return !!idev->cnf.ignore_routes_with_linkdown;
}
>
>
>
>> RCU_INIT_POINTER(dev->ip6_ptr, NULL);
>>
>> /* Step 1.5: remove snmp6 entry */
>> --
>> 2.25.1
>>
> .
>
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