linux-kernel - Re: [PATCH v5 08/14] KVM: s390: pv: handle secure storage exceptions for normal guests

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Date:   Tue, 12 Oct 2021 10:35:50 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, cohuck@...hat.com, borntraeger@...ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v5 08/14] KVM: s390: pv: handle secure storage
 exceptions for normal guests

On Tue, 12 Oct 2021 10:16:26 +0200
Janosch Frank <frankja@...ux.ibm.com> wrote:

> On 9/20/21 15:24, Claudio Imbrenda wrote:
> > With upcoming patches, normal guests might touch secure pages.
> > 
> > This patch extends the existing exception handler to convert the pages
> > to non secure also when the exception is triggered by a normal guest.
> > 
> > This can happen for example when a secure guest reboots; the first
> > stage of a secure guest is non secure, and in general a secure guest
> > can reboot into non-secure mode.
> > 
> > If the secure memory of the previous boot has not been cleared up
> > completely yet, a non-secure guest might touch secure memory, which
> > will need to be handled properly.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > ---
> >   arch/s390/mm/fault.c | 10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> > index eb68b4f36927..74784581f42d 100644
> > --- a/arch/s390/mm/fault.c
> > +++ b/arch/s390/mm/fault.c
> > @@ -767,6 +767,7 @@ void do_secure_storage_access(struct pt_regs *regs)
> >   	struct vm_area_struct *vma;
> >   	struct mm_struct *mm;
> >   	struct page *page;
> > +	struct gmap *gmap;
> >   	int rc;
> >   
> >   	/*
> > @@ -796,6 +797,14 @@ void do_secure_storage_access(struct pt_regs *regs)
> >   	}
> >   
> >   	switch (get_fault_type(regs)) {
> > +	case GMAP_FAULT:
> > +		gmap = (struct gmap *)S390_lowcore.gmap;
> > +		addr = __gmap_translate(gmap, addr);
> > +		if (IS_ERR_VALUE(addr)) {
> > +			do_fault_error(regs, VM_ACCESS_FLAGS, VM_FAULT_BADMAP);
> > +			break;
> > +		}
> > +		fallthrough;  
> 
> This would trigger an export and not a destroy, right?

correct. but this would only happen for leftover secure pages touched
by non-secure guests, before the background thread could clean them up.

> 
> >   	case USER_FAULT:
> >   		mm = current->mm;
> >   		mmap_read_lock(mm);
> > @@ -824,7 +833,6 @@ void do_secure_storage_access(struct pt_regs *regs)
> >   		if (rc)
> >   			BUG();
> >   		break;
> > -	case GMAP_FAULT:
> >   	default:
> >   		do_fault_error(regs, VM_READ | VM_WRITE, VM_FAULT_BADMAP);
> >   		WARN_ON_ONCE(1);
> >   
>