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

[Kees Cook]

On Wed, Nov 1, 2017 at 5:48 AM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Eric Dumazet <eric.dumazet@xxxxxxxxx> 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@xxxxxxxxxx>
>>> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
>>> Cc: netdev@xxxxxxxxxxxxxxx
>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> ---
>>>  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?

-Kees

-- 
Kees Cook
Pixel Security