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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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