lkml.org 
[lkml]   [2020]   [Mar]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/2] KVM: PPC: Book3S HV: check caller of H_SVM_* Hcalls
On Tue, 24 Mar 2020 10:43:23 +1100
Paul Mackerras <paulus@ozlabs.org> wrote:

> On Fri, Mar 20, 2020 at 01:22:48PM +0100, Greg Kurz wrote:
> > On Fri, 20 Mar 2020 11:26:42 +0100
> > Laurent Dufour <ldufour@linux.ibm.com> wrote:
> >
> > > The Hcall named H_SVM_* are reserved to the Ultravisor. However, nothing
> > > prevent a malicious VM or SVM to call them. This could lead to weird result
> > > and should be filtered out.
> > >
> > > Checking the Secure bit of the calling MSR ensure that the call is coming
> > > from either the Ultravisor or a SVM. But any system call made from a SVM
> > > are going through the Ultravisor, and the Ultravisor should filter out
> > > these malicious call. This way, only the Ultravisor is able to make such a
> > > Hcall.
> >
> > "Ultravisor should filter" ? And what if it doesn't (eg. because of a bug) ?
> >
> > Shouldn't we also check the HV bit of the calling MSR as well to
> > disambiguate SVM and UV ?
>
> The trouble with doing that (checking the HV bit) is that KVM does not
> expect to see the HV bit set on an interrupt that occurred while we
> were in the guest, and if it is set, it indicates a serious problem,
> i.e. that an interrupt occurred while we were in the code that
> transitions from host context to guest context, or from guest context
> to host context. In those cases we don't know how much of the
> transition has been completed and therefore whether we have guest
> values or host values in the CPU registers (GPRs, FPRs/VSRs, SPRs).
> If we do see HV set then KVM reports a severe error to userspace which
> should cause userspace to terminate the guest.
>
> Therefore the UV should *always* have the HV bit clear in HSRR1/SRR1
> when transitioning to KVM.
>

Indeed... thanks for the clarification. So I guess we'll just assume
that the UV doesn't reflect these SVM specific hcalls if they happened
to be issued by the guest then.

Cheers,

--
Greg

> Paul.

\
 
 \ /
  Last update: 2020-03-24 15:31    [W:0.118 / U:0.112 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site