Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

[Juergen Gross <jgross@xxxxxxxx>]

On 18.11.21 09:47, Jan Beulich wrote:
> On 18.11.2021 06:32, Juergen Gross wrote:
> > On 18.11.21 03:37, Stefano Stabellini wrote:
> > > --- a/drivers/xen/xenbus/xenbus_probe.c
> > > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > > @@ -951,6 +951,28 @@ static int __init xenbus_init(void)
> > >                 err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> > >                 if (err)
> > >                         goto out_error;
> > > +               /*
> > > +                * Return error on an invalid value.
> > > +                *
> > > +                * Uninitialized hvm_params are zero and return no error.
> > > +                * Although it is theoretically possible to have
> > > +                * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
> > > +                * not zero when valid. If zero, it means that Xenstore hasn't
> > > +                * been properly initialized. Instead of attempting to map a
> > > +                * wrong guest physical address return error.
> > > +                */
> > > +               if (v == 0) {
> > Make this "if (v == ULONG_MAX || v== 0)" instead?
> > This would result in the same err on a new and an old hypervisor
> > (assuming we switch the hypervisor to init params with ~0UL).
> >
> > > +                       err = -ENOENT;
> > > +                       goto out_error;
> > > +               }
> > > +               /*
> > > +                * ULONG_MAX is invalid on 64-bit because is INVALID_PFN.
> > > +                * On 32-bit return error to avoid truncation.
> > > +                */
> > > +               if (v >= ULONG_MAX) {
> > > +                       err = -EINVAL;
> > > +                       goto out_error;
> > > +               }
> > Does it make sense to continue the system running in case of
> > truncation? This would be a 32-bit guest with more than 16TB of RAM
> > and the Xen tools decided to place the Xenstore ring page above the
> > 16TB boundary. This is a completely insane scenario IMO.
> >
> > A proper panic() in this case would make diagnosis of that much
> > easier (me having doubts that this will ever be hit, though).
> While I agree panic() may be an option here (albeit I'm not sure why
> that would be better than trying to cope with 0 and hence without
I could imagine someone wanting to run a guest without Xenstore access,
which BTW will happen in case of a guest created by the hypervisor at
boot time.

> xenbus), I'd like to point out that the amount of RAM assigned to a
> guest is unrelated to the choice of GFNs for the various "magic"
> items.
Yes, but this would still be a major tools problem which probably
would render the whole guest rather unusable.


Juergen

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