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From: Paul Mackerras <paulus@ozlabs.org>
To: Ram Pai <linuxram@us.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.ibm.com>,
	cclaudio@linux.ibm.com, kvm-ppc@vger.kernel.org,
	Bharata B Rao <bharata@linux.ibm.com>,
	linux-mm@kvack.org, jglisse@redhat.com,
	Ram Pai <linuxram@linux.ibm.com>,
	aneesh.kumar@linux.vnet.ibm.com, paulus@au1.ibm.com,
	sukadev@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org,
	hch@lst.de
Subject: Re: [PATCH v10 7/8] KVM: PPC: Implement H_SVM_INIT_ABORT hcall
Date: Wed, 13 Nov 2019 11:14:27 +1100
Message-ID: <20191113001427.GA17829@oak.ozlabs.ibm.com> (raw)
In-Reply-To: <20191112144555.GE5159@oc0525413822.ibm.com>

On Tue, Nov 12, 2019 at 06:45:55AM -0800, Ram Pai wrote:
> On Tue, Nov 12, 2019 at 10:32:04PM +1100, Paul Mackerras wrote:
> > On Mon, Nov 11, 2019 at 11:52:15PM -0800, Ram Pai wrote:
> > > There is subtle problem removing that code from the assembly.
> > > 
> > > If the H_SVM_INIT_ABORT hcall returns to the ultravisor without clearing
> > > kvm->arch.secure_guest, the hypervisor will continue to think that the
> > > VM is a secure VM.   However the primary reason the H_SVM_INIT_ABORT
> > > hcall was invoked, was to inform the Hypervisor that it should no longer
> > > consider the VM as a Secure VM. So there is a inconsistency there.
> > 
> > Most of the checks that look at whether a VM is a secure VM use code
> > like "if (kvm->arch.secure_guest & KVMPPC_SECURE_INIT_DONE)".  Now
> > since KVMPPC_SECURE_INIT_ABORT is 4, an if statement such as that will
> > take the false branch once we have set kvm->arch.secure_guest to
> > KVMPPC_SECURE_INIT_ABORT in kvmppc_h_svm_init_abort.  So in fact in
> > most places we will treat the VM as a normal VM from then on.  If
> > there are any places where we still need to treat the VM as a secure
> > VM while we are processing the abort we can easily do that too.
> 
> Is the suggestion --  KVMPPC_SECURE_INIT_ABORT should never return back
> to the Ultravisor?   Because removing that assembly code will NOT lead the

No.  The suggestion is that vcpu->arch.secure_guest stays set to
KVMPPC_SECURE_INIT_ABORT until userspace calls KVM_PPC_SVM_OFF.

> Hypervisor back into the Ultravisor.  This is fine with the
> ultravisor. But then the hypervisor will not know where to return to.
> If it wants to return directly to the VM, it wont know to
> which address. It will be in a limbo.
> 
> > 
> > > This is fine, as long as the VM does not invoke any hcall or does not
> > > receive any hypervisor-exceptions.  The moment either of those happen,
> > > the control goes into the hypervisor, the hypervisor services
> > > the exception/hcall and while returning, it will see that the
> > > kvm->arch.secure_guest flag is set and **incorrectly** return
> > > to the ultravisor through a UV_RETURN ucall.  Ultravisor will
> > > not know what to do with it, because it does not consider that
> > > VM as a Secure VM.  Bad things happen.
> > 
> > If bad things happen in the ultravisor because the hypervisor did
> > something it shouldn't, then it's game over, you just lost, thanks for
> > playing.  The ultravisor MUST be able to cope with bogus UV_RETURN
> > calls for a VM that it doesn't consider to be a secure VM.  You need
> > to work out how to handle such calls safely and implement that in the
> > ultravisor.
> 
> Actually we do handle this gracefully in the ultravisor :). 
> We just retun back to the hypervisor saying "sorry dont know what
> to do with it, please handle it yourself".
> 
> However hypervisor would not know what to do with that return, and bad
> things happen in the hypervisor.

Right.  We need something after the "sc 2" to handle the case where
the ultravisor returns with an error from the UV_RETURN.

Paul.

  reply index

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-04  4:17 [PATCH v10 0/8] KVM: PPC: Driver to manage pages of secure guest Bharata B Rao
2019-11-04  4:17 ` [PATCH v10 1/8] mm: ksm: Export ksm_madvise() Bharata B Rao
2019-11-06  4:33   ` Paul Mackerras
2019-11-06  6:45     ` Bharata B Rao
2019-11-07  5:45       ` Paul Mackerras
2019-11-15 14:10         ` Bharata B Rao
2019-11-04  4:17 ` [PATCH v10 2/8] KVM: PPC: Support for running secure guests Bharata B Rao
2019-11-06  4:34   ` Paul Mackerras
2019-11-04  4:17 ` [PATCH v10 3/8] KVM: PPC: Shared pages support for " Bharata B Rao
2019-11-06  4:52   ` Paul Mackerras
2019-11-06  8:22     ` Bharata B Rao
2019-11-06  8:29       ` Bharata B Rao
2019-11-04  4:17 ` [PATCH v10 4/8] KVM: PPC: Radix changes for secure guest Bharata B Rao
2019-11-06  5:58   ` Paul Mackerras
2019-11-06  8:36     ` Bharata B Rao
2019-11-04  4:17 ` [PATCH v10 5/8] KVM: PPC: Handle memory plug/unplug to secure VM Bharata B Rao
2019-11-11  4:25   ` Paul Mackerras
2019-11-04  4:17 ` [PATCH v10 6/8] KVM: PPC: Support reset of secure guest Bharata B Rao
2019-11-11  5:28   ` Paul Mackerras
2019-11-11  6:55     ` Bharata B Rao
2019-11-12  5:34   ` Paul Mackerras
2019-11-13 15:29     ` Bharata B Rao
2019-11-14  5:07       ` Paul Mackerras
2019-11-04  4:17 ` [PATCH v10 7/8] KVM: PPC: Implement H_SVM_INIT_ABORT hcall Bharata B Rao
2019-11-11  4:19   ` Paul Mackerras
2019-11-12  1:01     ` Ram Pai
2019-11-12  5:38       ` Paul Mackerras
2019-11-12  7:52         ` Ram Pai
2019-11-12 11:32           ` Paul Mackerras
2019-11-12 14:45             ` Ram Pai
2019-11-13  0:14               ` Paul Mackerras [this message]
2019-11-13  6:32                 ` Ram Pai
2019-11-13 21:18                   ` Paul Mackerras
2019-11-13 21:50                     ` Ram Pai
2019-11-14  5:08                       ` Paul Mackerras
2019-11-14  7:02                         ` Ram Pai
2019-11-04  4:18 ` [PATCH v10 8/8] KVM: PPC: Ultravisor: Add PPC_UV config option Bharata B Rao
2019-11-06  4:30 ` [PATCH v10 0/8] KVM: PPC: Driver to manage pages of secure guest Paul Mackerras
2019-11-06  6:20   ` Bharata B Rao

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