Re: [PATCH v4] isa-applesmc: provide OSK forwarding on Apple hosts

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

On Mon, Oct 25, 2021 at 04:53:57PM +0200, Alexander Graf wrote:
> 
> On 25.10.21 16:47, Daniel P. Berrangé wrote:
> > On Mon, Oct 25, 2021 at 04:42:22PM +0200, Alexander Graf wrote:
> > > On 25.10.21 16:22, Daniel P. Berrangé wrote:
> > > > On Mon, Oct 25, 2021 at 12:13:32PM +0200, Alexander Graf wrote:
> > > > > On 22.10.21 18:14, Vladislav Yaroshchuk wrote:
> > > > > > On Apple hosts we can read AppleSMC OSK key directly from host's
> > > > > > SMC and forward this value to QEMU Guest.
> > > > > > 
> > > > > > Usage:
> > > > > > `-device isa-applesmc,hostosk=on`
> > > > > > 
> > > > > > Apple licence allows use and run up to two additional copies
> > > > > > or instances of macOS operating within virtual operating system
> > > > > > environments on each Apple-branded computer that is already running
> > > > > > the Apple Software, for purposes of:
> > > > > > - software development
> > > > > > - testing during software development
> > > > > > - using macOS Server
> > > > > > - personal, non-commercial use
> > > > > > 
> > > > > > Guest macOS requires AppleSMC with correct OSK. The most legal
> > > > > > way to pass it to the Guest is to forward the key from host SMC
> > > > > > without any value exposion.
> > > > > > 
> > > > > > Based on https://web.archive.org/web/20200103161737/osxbook.com/book/bonus/chapter7/tpmdrmmyth/
> > > > > > 
> > > > > > Signed-off-by: Vladislav Yaroshchuk <yaroshchuk2000@gmail.com>
> > > > > > @@ -331,6 +464,25 @@ static void applesmc_isa_realize(DeviceState *dev, Error **errp)
> > > > > >         isa_register_ioport(&s->parent_obj, &s->io_err,
> > > > > >                             s->iobase + APPLESMC_ERR_PORT);
> > > > > > +    if (s->hostosk_flag) {
> > > > > > +        /*
> > > > > > +         * Property 'hostosk' has higher priority than 'osk'
> > > > > > +         * and shadows it.
> > > > > > +         * Free user-provided 'osk' property value
> > > > > > +         */
> > > > > > +        if (s->osk) {
> > > > > > +            warn_report("isa-applesmc.osk is shadowed "
> > > > > > +                        "by isa-applesmc.hostosk");
> > > > > > +            g_free(s->osk);
> > > > > > +        }
> > > > > > +
> > > > > > +        if (!applesmc_read_host_osk(&s->osk, &err)) {
> > > > > > +            /* On host OSK retrieval error report a warning */
> > > > > > +            error_report_err(err);
> > > > > > +            s->osk = default_osk;
> > > > > > +        }
> > > > > > +    }
> > > > > This part is yucky. A few things:
> > > > > 
> > > > > 1) QEMU in general does not fail user requested operations silently. If the
> > > > > user explicitly asked to read the host OSK and we couldn't, it must
> > > > > propagate that error.
> > > > > 2) In tandem to the above, I think the only consistent CX is to make both
> > > > > options mutually exclusive. The easiest way to achieve that IMHO would be to
> > > > > overload the "osk" property. If it is "host", then use the host one.
> > > > > 3) Should we make "osk"="host" the default on macOS as well then? Of course,
> > > > > that one should *not* fail hard when it can't read the key, because it's an
> > > > > implicit request rather than an explicit one.
> > > > The problem with using a magic string value for the existing "osk"
> > > > parameter is that this is not introspectable by management apps.
> > > 
> > > What introspectability would you like to have?
> > Essentially to answer the question
> > 
> >    "Does this QEMU support OSK passthrough from the host"
> > 
> > Mgmt apps like libvirt introspect using various query-XXX QMP commands.
> > For devices, the typical approach is to ask for the list of properties
> > the device supports. If we're just accepting a new magic value on an
> > existing property there is no way to query for existance of that feature.
> > If we add a "host-osk=bool" parameter introspectability is trivially
> > satisfied.
> 
> 
> Ok, the only flow that remains sensible in that case to me sounds like the
> following:

Just need an extra check upfront:

 if (s->osk && s->use_hoist_osk)
     error_setg(errp, ...)
 else
 
> if (s->osk) {
>     /* Use osk */

This should fail hard if the provided value is the wrong length - currently
it falls back with a warning IIUC.

> } else if (s->use_host_osk) {
>     /* Use host OSK. Fail hard if we can't find it */
> } else if (can_use_host_osk) {
>     /* See if we can extract the key from the host. If not, fall back to old
> behavior */
> } else {
>     /* Old fallback behavior */

Was this old fallback behaviour actually useful ? IIUC it means it is using


  static char default_osk[64] = "This is a dummy key. Enter the real key "
                                "using the -osk parameter";

which obviously isn't a valid key that will work with any gust OS that
cares. I guess it at least let QEMU startup, but any the guest OS that
checks the key will be unhappy.

If if don't think default_osk is actually useful, then we could simplify
further to

 if (s->osk && s->use_host_osk) {
     error_setg(errp, ...)
 } else if (s->osk) {
    /* Use osk. Fail hard if invalid (ie wrong length) */
 } else if (s->use_host_osk) {
    /* Use host OSK. Fail hard if we can't find it */
 } else {
    /* try to use host OSK, fail hard if we can't find it or non-OS-X build */
 }

Regards,
Daniel
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