Re: [PATCH] vhost/vsock: accept only packets with the right dst_cid

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Dec 12, 2019 at 07:56:26AM -0500, Michael S. Tsirkin wrote:
> On Thu, Dec 12, 2019 at 01:36:24PM +0100, Stefano Garzarella wrote:
> > On Wed, Dec 11, 2019 at 11:03:07AM -0500, Michael S. Tsirkin wrote:
> > > On Fri, Dec 06, 2019 at 03:39:12PM +0100, Stefano Garzarella wrote:
> > > > When we receive a new packet from the guest, we check if the
> > > > src_cid is correct, but we forgot to check the dst_cid.
> > > > 
> > > > The host should accept only packets where dst_cid is
> > > > equal to the host CID.
> > > > 
> > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > 
> > > Stefano can you clarify the impact pls?
> > 
> > Sure, I'm sorry I didn't do it earlier.
> > 
> > > E.g. is this needed on stable? Etc.
> > 
> > This is a better analysis (I hope) when there is a malformed guest
> > that sends a packet with a wrong dst_cid:
> > - before v5.4 we supported only one transport at runtime, so the sockets
> >   in the host can only receive packets from guests. In this case, if
> >   the dst_cid is wrong, maybe the only issue is that the getsockname()
> >   returns an inconsistent address (the cid returned is the one received
> >   from the guest)
> > 
> > - from v5.4 we support multi-transport, so the L1 VM (e.g. L0 assigned
> >   cid 5 to this VM) can have both Guest2Host and Host2Guest transports.
> >   In this case, we have these possible issues:
> >   - L2 (or L1) guest can use cid 0, 1, and 2 to reach L1 (or L0),
> >     instead we should allow only CID_HOST (2) to reach the level below.
> >     Note: this happens also with not malformed guest that runs Linux v5.4
> >   - if a malformed L2 guest sends a packet with the wrong dst_cid, for example
> >     instead of CID_HOST, it uses the cid assigned by L0 to L1 (5 in this
> >     example), this packets can wrongly queued to a socket on L1 bound to cid 5,
> >     that only expects connections from L0.
> 
> Oh so a security issue?
> 

It seems so, I'll try to see if I can get a real example,
maybe I missed a few checks.

> > 
> > Maybe we really need this only on stable v5.4, but the patch is very simple
> > and should apply cleanly to all stable branches.
> > 
> > What do you think?
> > 
> > Thanks,
> > Stefano
> 
> I'd say it's better to backport to all stable releases where it applies,
> but yes it's only a security issue in 5.4.  Dave could you forward pls?

Yes, I agree with you.

@Dave let me know if I should do it.

Thanks,
Stefano