Re: [PATCH] nbd: restrict sockets to TCP and UDP

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Sep 09, 2025 at 07:47:09AM -0700, Eric Dumazet wrote:
> On Tue, Sep 9, 2025 at 7:37 AM Jens Axboe <axboe@kernel.dk> wrote:
> >
> > On 9/9/25 8:35 AM, Eric Dumazet wrote:
> > > On Tue, Sep 9, 2025 at 7:04 AM Eric Dumazet <edumazet@google.com> wrote:
> > >>
> > >> On Tue, Sep 9, 2025 at 6:32 AM Richard W.M. Jones <rjones@redhat.com> wrote:
> > >>>
> > >>> On Tue, Sep 09, 2025 at 01:22:43PM +0000, Eric Dumazet wrote:
> > >>>> Recently, syzbot started to abuse NBD with all kinds of sockets.
> > >>>>
> > >>>> Commit cf1b2326b734 ("nbd: verify socket is supported during setup")
> > >>>> made sure the socket supported a shutdown() method.
> > >>>>
> > >>>> Explicitely accept TCP and UNIX stream sockets.
> > >>>
> > >>> I'm not clear what the actual problem is, but I will say that libnbd &
> > >>> nbdkit (which are another NBD client & server, interoperable with the
> > >>> kernel) we support and use NBD over vsock[1].  And we could support
> > >>> NBD over pretty much any stream socket (Infiniband?) [2].
> > >>>
> > >>> [1] https://libguestfs.org/nbd_aio_connect_vsock.3.html
> > >>>     https://libguestfs.org/nbdkit-service.1.html#AF_VSOCK
> > >>> [2] https://libguestfs.org/nbd_connect_socket.3.html
> > >>>
> > >>> TCP and Unix domain sockets are by far the most widely used, but I
> > >>> don't think it's fair to exclude other socket types.
> > >>
> > >> If we have known and supported socket types, please send a patch to add them.
> > >>
> > >> I asked the question last week and got nothing about vsock or other types.
> > >>
> > >> https://lore.kernel.org/netdev/CANn89iLNFHBMTF2Pb6hHERYpuih9eQZb6A12+ndzBcQs_kZoBA@mail.gmail.com/
> > >>
> > >> For sure, we do not want datagram sockets, RAW, netlink, and many others.
> > >
> > > BTW vsock will probably fire lockdep warnings, I see GFP_KERNEL
> > > being used in net/vmw_vsock/virtio_transport.c

CC-ing Stefan & Stefano.  Myself, I'm only using libnbd
(ie. userspace) over vsock, not the kernel client.

> > > So you will have to fix this.
> >
> > Rather than play whack-a-mole with this, would it make sense to mark as
> > socket as "writeback/reclaim" safe and base the nbd decision on that rather
> > than attempt to maintain some allow/deny list of sockets?
> 
> Even if a socket type was writeback/reclaim safe, probably NBD would not support
> arbitrary socket type, like netlink, af_packet, or af_netrom.
> 
> An allow list seems safer to me, with commits with a clear owner.
> 
> If future syzbot reports are triggered, the bisection will point to
> these commits.

From the outside it seems really odd to hard code a list of "good"
socket types into each kernel client that can open a socket.  Normally
if you wanted to restrict socket types wouldn't you do that through
something more flexible like nftables?

Rich.

-- 
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