Re: [PATCH] net: recvmsg: Unconditionally zero struct sockaddr_storage

[Alexander Potapenko <glider@google.com>]

On Wed, Nov 1, 2017 at 7:23 PM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Nov 1, 2017 at 5:48 AM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>> Eric Dumazet <eric.dumazet@gmail.com> writes:
>>
>>> On Tue, 2017-10-31 at 09:14 -0700, Kees Cook wrote:
>>>> Some protocols do not correctly wipe the contents of the on-stack
>>>> struct sockaddr_storage sent down into recvmsg() (e.g. SCTP), and leak
>>>> kernel stack contents to userspace. This wipes it unconditionally before
>>>> per-protocol handlers run.
>>>>
>>>> Note that leaks like this are mitigated by building with
>>>> CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y
>>>>
>>>> Reported-by: Alexander Potapenko <glider@google.com>
>>>> Cc: "David S. Miller" <davem@davemloft.net>
>>>> Cc: netdev@vger.kernel.org
>>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>>> ---
>>>>  net/socket.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/net/socket.c b/net/socket.c
>>>> index c729625eb5d3..34183f4fbdf8 100644
>>>> --- a/net/socket.c
>>>> +++ b/net/socket.c
>>>> @@ -2188,6 +2188,7 @@ static int ___sys_recvmsg(struct socket *sock, struct user_msghdr __user *msg,
>>>>      struct sockaddr __user *uaddr;
>>>>      int __user *uaddr_len = COMPAT_NAMELEN(msg);
>>>>
>>>> +    memset(&addr, 0, sizeof(addr));
>>>>      msg_sys->msg_name = &addr;
>>>>
>>>
>>> This kind of patch comes every year.
>>>
>>> Standard answer is : We fix the buggy protocol, we do not make
>>> everything slower just because we are lazy.
>>>
>>> struct sockaddr is 128 bytes, but IPV4 only uses a fraction of it.
>>>
>>> Also memset() is using long word stores, so next 4-byte or 2-byte stores
>>> on same location hit a performance problem on x86.
>>>
>>> By adding all these defensive programming, we would give strong
>>> incentives to bypass the kernel for networking. That would be bad.
>>
>> In this case it looks like the root cause is something in sctp
>> not filling in the ipv6 sin6_scope_id.
>>
>> Which is not only a leak but a correctness bug.
>>
>> I ran the reproducer test program and while none of the leak checkers
>> are telling me anything I have gotten as far as seeing that the returned
>> length is correct and sometimes nonsense.
>>
>> Hmm.
>>
>> At a quick look it looks like all that is necessary is to do this:
>>
>> diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
>> index 51c488769590..6301913d0516 100644
>> --- a/net/sctp/ipv6.c
>> +++ b/net/sctp/ipv6.c
>> @@ -807,9 +807,10 @@ static void sctp_inet6_skb_msgname(struct sk_buff *skb, char *msgname,
>>                 addr->v6.sin6_flowinfo = 0;
>>                 addr->v6.sin6_port = sh->source;
>>                 addr->v6.sin6_addr = ipv6_hdr(skb)->saddr;
>> -               if (ipv6_addr_type(&addr->v6.sin6_addr) & IPV6_ADDR_LINKLOCAL) {
>> +               if (ipv6_addr_type(&addr->v6.sin6_addr) & IPV6_ADDR_LINKLOCAL)
>>                         addr->v6.sin6_scope_id = sctp_v6_skb_iif(skb);
>> -               }
>> +               else
>> +                       addr->v6.sin6_scope_id = 0;
>>         }
>>
>>         *addr_len = sctp_v6_addr_to_user(sctp_sk(skb->sk), addr);
>>
>> Eric
>>
>
> Thanks for digging into this Eric! Alexander, can you confirm this
> fixes it for you when CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL is not
> enabled?
Sorry, I accidentally missed this patch.
Yes, I confirm it fixes the problem with
CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL disabled.
> -Kees
>
> --
> Kees Cook
> Pixel Security



-- 
Alexander Potapenko
Software Engineer

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