Message-ID: <0848356a-07df-c243-db82-4b034f425656@partner.samsung.com>
Date:   Tue, 8 Jan 2019 11:36:17 +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 11:06 AM, Eric Dumazet wrote:
> On Tue, Jan 8, 2019 at 1:47 AM Piotr Sawicki
> <p.sawicki2@...tner.samsung.com> wrote:
>>
>> 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; ...
>>
> Crash did not involve sk_filter_trim_cap() here...
>
> Not sure what you are trying to say.


Are you sure that sk_filter_trim_cap() is not on the stack trace?

Maybe I'm missing something. How should I read the below dump?

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 <---- here
 __sk_receive_skb
 dccp_v6_do_rcv
 release_sock


>>> 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)
>

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