Re: [PATCH v3 2/3] tools: build qemu-vmsr-helper

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Feb 21, 2024 at 02:19:11PM +0100, Anthony Harivel wrote:
> Daniel P. Berrangé, Jan 29, 2024 at 20:45:
> > On Mon, Jan 29, 2024 at 08:33:21PM +0100, Paolo Bonzini wrote:
> > > On Mon, Jan 29, 2024 at 7:53 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > > > diff --git a/meson.build b/meson.build
> > > > > index d0329966f1b4..93fc233b0891 100644
> > > > > --- a/meson.build
> > > > > +++ b/meson.build
> > > > > @@ -4015,6 +4015,11 @@ if have_tools
> > > > >                 dependencies: [authz, crypto, io, qom, qemuutil,
> > > > >                                libcap_ng, mpathpersist],
> > > > >                 install: true)
> > > > > +
> > > > > +    executable('qemu-vmsr-helper', files('tools/i386/qemu-vmsr-helper.c'),
> > > >
> > > > I'd suggest 'tools/x86/' since this works fine on 64-bit too
> > > 
> > > QEMU tends to use i386 in the source to mean both 32- and 64-bit.
> >
> > One day we should rename that to x86 too :-)
> >
> > > > You never answered my question from the previous posting of this
> > > >
> > > > This check is merely validating the the thread ID in the message
> > > > is a child of the process ID connected to the socket. Any process
> > > > on the entire host can satisfy this requirement.
> > > >
> > > > I don't see what is limiting this to only QEMU as claimed by the
> > > > commit message, unless you're expecting the UNIX socket permissions
> > > > to be such that only processes under the qemu:qemu user:group pair
> > > > can access to the socket ? That would be a libvirt based permissions
> > > > assumption though.
> > > 
> > > Yes, this is why the systemd socket uses 600, like
> > > contrib/systemd/qemu-pr-helper.socket. The socket can be passed via
> > > SCM_RIGHTS by libvirt, or its permissions can be changed (e.g. 660 and
> > > root:kvm would make sense on a Debian system), or a separate helper
> > > can be started by libvirt.
> > > 
> > > Either way, the policy is left to the user rather than embedding it in
> > > the provided systemd unit.
> >
> > Ok, this code needs a comment to explain that we're relying on
> > socket permissions to control who/what can access the daemon,
> > combined with this PID+TID check to validate it is not spoofing
> > its identity, as without context the TID check looks pointless.
> 
> Hi Daniel,
> 
> would you prefer a comment in the code or a security section in the doc 
> (i.e docs/specs/rapl-msr.rst) ?

I think it is worth creating a docs/specs/rapl-msr.rst to explain the
overall design & usage & security considerations.

With regards,
Daniel
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