On Tue, Sep 9, 2025 at 8:19 AM Richard W.M. Jones [off-list ref] wrote:

On Tue, Sep 09, 2025 at 07:47:09AM -0700, Eric Dumazet wrote:

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On Tue, Sep 9, 2025 at 7:37 AM Jens Axboe [off-list ref] wrote:

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On 9/9/25 8:35 AM, Eric Dumazet wrote:

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On Tue, Sep 9, 2025 at 7:04 AM Eric Dumazet [off-list ref] wrote:

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On Tue, Sep 9, 2025 at 6:32 AM Richard W.M. Jones [off-list ref] wrote:

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On Tue, Sep 09, 2025 at 01:22:43PM +0000, Eric Dumazet wrote:

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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/ (local)

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.

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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?

nftables is user policy.

We need a kernel that will not crash, even if nftables is not
compiled/loaded/used .

Rich.

--
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