Re: io_uring, IORING_OP_RECVMSG and ancillary data

[Jens Axboe <axboe@kernel.dk>]

On 4/27/20 1:20 PM, Jann Horn wrote:
> On Sat, Apr 25, 2020 at 10:23 PM Jens Axboe <axboe@kernel.dk> wrote:
>> On 4/25/20 11:29 AM, Andreas Smas wrote:
>>> Hi,
>>>
>>> Tried to use io_uring with OP_RECVMSG with ancillary buffers (for my
>>> particular use case I'm using SO_TIMESTAMP for incoming UDP packets).
>>>
>>> These submissions fail with EINVAL due to the check in __sys_recvmsg_sock().
>>>
>>> The following hack fixes the problem for me and I get valid timestamps
>>> back. Not suggesting this is the real fix as I'm not sure what the
>>> implications of this is.
>>>
>>> Any insight into this would be much appreciated.
>>
>> It was originally disabled because of a security issue, but I do think
>> it's safe to enable again.
>>
>> Adding the io-uring list and Jann as well, leaving patch intact below.
>>
>>> diff --git a/net/socket.c b/net/socket.c
>>> index 2dd739fba866..689f41f4156e 100644
>>> --- a/net/socket.c
>>> +++ b/net/socket.c
>>> @@ -2637,10 +2637,6 @@ long __sys_recvmsg_sock(struct socket *sock,
>>> struct msghdr *msg,
>>>                         struct user_msghdr __user *umsg,
>>>                         struct sockaddr __user *uaddr, unsigned int flags)
>>>  {
>>> -       /* disallow ancillary data requests from this path */
>>> -       if (msg->msg_control || msg->msg_controllen)
>>> -               return -EINVAL;
>>> -
>>>         return ____sys_recvmsg(sock, msg, umsg, uaddr, flags, 0);
>>>  }
> 
> I think that's hard to get right. In particular, unix domain sockets
> can currently pass file descriptors in control data - so you'd need to
> set the file_table flag for recvmsg and sendmsg. And I'm not sure
> whether, to make this robust, there should be a whitelist of types of
> control messages that are permitted to be used with io_uring, or
> something like that...
> 
> I think of ancillary buffers as being kind of like ioctl handlers in
> this regard.

Good point. I'll send out something that hopefully will be enough to
be useful, whole not allowing anything randomly.

-- 
Jens Axboe