Re: io_uring, IORING_OP_RECVMSG and ancillary data

[Jann Horn <jannh@google.com>]

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.