On Mon, Jan 29, 2024 at 7:53 PM Daniel P. Berrangé <berra...@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.
Either is fine by me though.
> > + dependencies: [authz, crypto, io, qom, qemuutil,
> > + libcap_ng, mpathpersist],
> > + install: true)
>
> Shouldn't this executable() call be conditional though, so this
> is only built for x86 host targets.
Yes. Also should be 32- and 64-bit (careful because Meson uses 'x86' for
32-bit).
> > +static void compute_default_paths(void)
> > +{
> > + socket_path = g_build_filename("/run", "qemu-vmsr-helper.sock", NULL);
> > + pidfile = g_build_filename("/run", "qemu-vmsr-helper.pid", NULL);
> > +}
>
> We shouldn't be hardcoding /run, we need to honour --prefix and
> --localstatedir args given to configure. /var/run is a symlink
> to /run so the end result ends up the same AFAIK
Indeed; just copy from scsi/qemu-pr-helper.c.
> 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.
> > + if (capng_update(CAPNG_ADD, CAPNG_EFFECTIVE | CAPNG_PERMITTED,
> > + CAP_SYS_RAWIO) < 0) {
> > + return -1;
> > + }
> > +
> > + /*
> > + * Change user/group id, retaining the capabilities.
> > + * Because file descriptors are passed via SCM_RIGHTS,
> > + * we don't need supplementary groups (and in fact the helper
> > + * can run as "nobody").
> > + */
> > + if (capng_change_id(uid != -1 ? uid : getuid(),
> > + gid != -1 ? gid : getgid(),
> > + CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING)) {
> > + return -1;
> > + }
>
> Does this actually work ? IIUC, the file that requires privileges
> is /dev/cpu/%u/msr, and we're opening that fresh on every request,
> so how can this run as anything other than root ?
Agreed, the capabilities can be dropped but the uid and gid cannot.
Paolo
