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

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Date:   Fri, 22 Jul 2022 11:00:01 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Ziyang Xuan <william.xuanziyang@...wei.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.

>         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



>                 RCU_INIT_POINTER(dev->ip6_ptr, NULL);
>
>                 /* Step 1.5: remove snmp6 entry */
> --
> 2.25.1
>