netdev - Re: [PATCH net] ipv6: make icmp6_send() robust against null skb->dev

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Date:   Tue, 8 Jan 2019 10:46:59 +0100
From:   Piotr Sawicki <p.sawicki2@...tner.samsung.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: make icmp6_send() robust against null
 skb->dev


On 1/8/19 10:21 AM, Eric Dumazet wrote:
> On Tue, Jan 8, 2019 at 12:57 AM Piotr Sawicki
> <p.sawicki2@...tner.samsung.com> wrote:
>> dccp_v6_rcv() calls __sk_receive_skb() which calls sk_filter_trim_cap().
>>
>> sk_filter_trim_cap() should return a value not equal to 0 and cause the skb to be dropped, since icmpv6_send() is called when smack_socket_sock_rcv_skb() returns -EACCES.
>>
>> So, the packet shouldn't be put into the backlog queue.
>>
>> How did it get there?
>>
> I do not believe crash involved a BPF filter at all (My changelog said
> nothing about sk_filter_trim_cap()

Not only BPF but also the LSM subsystem is involved (in this case Smack).

dccp_v6_rcv()
	__sk_receive_skb()

		sk_filter_trim_cap()
			security_sock_rcv_skb()
				smack_sock_rcv_skb()

So, before putting this skb into the backlog queue,

a network packet is checked against Smack rules. If Smack denies access,

the packet is discarded.

__sk_receive_skb()
...
	if (sk_filter_trim_cap <https://elixir.bootlin.com/linux/latest/ident/sk_filter_trim_cap>(sk, skb, trim_cap))
		goto discard_and_relse; ...

> After packet is queued to backlog, the packet circulates, reaching the
> smack_socket_sock_rcv_skb() point.
>
> The stack trace shows only the 2nd phase of the packet, when the user
> process calls release_sock()
>
>
>> Regards,
>>
>> Piotr
>>
>>
>> On 1/4/19 8:00 PM, Eric Dumazet wrote:
>>> syzbot was able to crash one host with the following stack trace :
>>>
>>> kasan: GPF could be caused by NULL-ptr deref or user memory access
>>> general protection fault: 0000 [#1] PREEMPT SMP KASAN
>>> CPU: 0 PID: 8625 Comm: syz-executor4 Not tainted 4.20.0+ #8
>>> RIP: 0010:dev_net include/linux/netdevice.h:2169 [inline]
>>> RIP: 0010:icmp6_send+0x116/0x2d30 net/ipv6/icmp.c:426
>>>  icmpv6_send
>>>  smack_socket_sock_rcv_skb
>>>  security_sock_rcv_skb
>>>  sk_filter_trim_cap
>>>  __sk_receive_skb
>>>  dccp_v6_do_rcv
>>>  release_sock
>>>
>>> This is because a RX packet found socket owned by user and
>>> was stored into socket backlog. Before leaving RCU protected section,
>>> skb->dev was cleared in __sk_receive_skb(). When socket backlog
>>> was finally handled at release_sock() time, skb was fed to
>>> smack_socket_sock_rcv_skb() then icmp6_send()
>>>
>>> We could fix the bug in smack_socket_sock_rcv_skb(), or simply
>>> make icmp6_send() more robust against such possibility.
>>>
>>> In the future we might provide to icmp6_send() the net pointer
>>> instead of infering it.
>>>
>>> Fixes: d66a8acbda92 ("Smack: Inform peer that IPv6 traffic has been blocked")
>>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>>> Cc: Piotr Sawicki <p.sawicki2@...tner.samsung.com>
>>> Cc: Casey Schaufler <casey@...aufler-ca.com>
>>> Reported-by: syzbot <syzkaller@...glegroups.com>
>>> ---
>>>  net/ipv6/icmp.c | 8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
>>> index 5d7aa2c2770ca2b4981d2dd211c3cf0a79a6f9e2..bbcdfd2996926a78c3ea0b274adfa9b5f297efbc 100644
>>> --- a/net/ipv6/icmp.c
>>> +++ b/net/ipv6/icmp.c
>>> @@ -423,10 +423,10 @@ static int icmp6_iif(const struct sk_buff *skb)
>>>  static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
>>>                      const struct in6_addr *force_saddr)
>>>  {
>>> -     struct net *net = dev_net(skb->dev);
>>>       struct inet6_dev *idev = NULL;
>>>       struct ipv6hdr *hdr = ipv6_hdr(skb);
>>>       struct sock *sk;
>>> +     struct net *net;
>>>       struct ipv6_pinfo *np;
>>>       const struct in6_addr *saddr = NULL;
>>>       struct dst_entry *dst;
>>> @@ -437,12 +437,16 @@ static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
>>>       int iif = 0;
>>>       int addr_type = 0;
>>>       int len;
>>> -     u32 mark = IP6_REPLY_MARK(net, skb->mark);
>>> +     u32 mark;
>>>
>>>       if ((u8 *)hdr < skb->head ||
>>>           (skb_network_header(skb) + sizeof(*hdr)) > skb_tail_pointer(skb))
>>>               return;
>>>
>>> +     if (!skb->dev)
>>> +             return;
>>> +     net = dev_net(skb->dev);
>>> +     mark = IP6_REPLY_MARK(net, skb->mark);
>>>       /*
>>>        *      Make sure we respect the rules
>>>        *      i.e. RFC 1885 2.4(e)
>