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Date:   Fri, 1 Apr 2022 16:20:53 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Janosch Frank <frankja@linux.ibm.com>
Cc:     kvm@vger.kernel.org, borntraeger@de.ibm.com, thuth@redhat.com,
        pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, scgl@linux.ibm.com,
        mimu@linux.ibm.com, nrb@linux.ibm.com
Subject: Re: [PATCH v9 01/18] KVM: s390: pv: leak the topmost page table
 when destroy fails
Message-ID: <20220401162053.5466832d@p-imbrenda>
In-Reply-To: <6b24e1f6-22ee-c0e4-5bde-9eefdccd3619@linux.ibm.com>
References: <20220330122605.247613-1-imbrenda@linux.ibm.com>
        <20220330122605.247613-2-imbrenda@linux.ibm.com>
        <6b24e1f6-22ee-c0e4-5bde-9eefdccd3619@linux.ibm.com>
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On Thu, 31 Mar 2022 15:13:41 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 3/30/22 14:25, Claudio Imbrenda wrote:
> > Each secure guest must have a unique ASCE (address space control
> > element); we must avoid that new guests use the same page for their
> > ASCE, to avoid errors.
> > 
> > Since the ASCE mostly consists of the address of the topmost page table
> > (plus some flags), we must not return that memory to the pool unless
> > the ASCE is no longer in use.
> > 
> > Only a successful Destroy Secure Configuration UVC will make the ASCE
> > reusable again.
> > 
> > If the Destroy Configuration UVC fails, the ASCE cannot be reused for a
> > secure guest (either for the ASCE or for other memory areas). To avoid
> > a collision, it must not be used again. This is a permanent error and
> > the page becomes in practice unusable, so we set it aside and leak it.
> > On failure we already leak other memory that belongs to the ultravisor
> > (i.e. the variable and base storage for a guest) and not leaking the
> > topmost page table was an oversight.
> > 
> > This error (and thus the leakage) should not happen unless the hardware
> > is broken or KVM has some unknown serious bug.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Fixes: 29b40f105ec8d55 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling")
> > ---
> >   arch/s390/include/asm/gmap.h |  2 +
> >   arch/s390/kvm/pv.c           |  9 +++--
> >   arch/s390/mm/gmap.c          | 71 ++++++++++++++++++++++++++++++++++++
> >   3 files changed, 79 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
> > index 40264f60b0da..746e18bf8984 100644
> > --- a/arch/s390/include/asm/gmap.h
> > +++ b/arch/s390/include/asm/gmap.h
> > @@ -148,4 +148,6 @@ void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4],
> >   			     unsigned long gaddr, unsigned long vmaddr);
> >   int gmap_mark_unmergeable(void);
> >   void s390_reset_acc(struct mm_struct *mm);
> > +void s390_remove_old_asce(struct gmap *gmap);
> > +int s390_replace_asce(struct gmap *gmap);
> >   #endif /* _ASM_S390_GMAP_H */
> > diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> > index 7f7c0d6af2ce..3c59ef763dde 100644
> > --- a/arch/s390/kvm/pv.c
> > +++ b/arch/s390/kvm/pv.c
> > @@ -166,10 +166,13 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
> >   	atomic_set(&kvm->mm->context.is_protected, 0);
> >   	KVM_UV_EVENT(kvm, 3, "PROTVIRT DESTROY VM: rc %x rrc %x", *rc, *rrc);
> >   	WARN_ONCE(cc, "protvirt destroy vm failed rc %x rrc %x", *rc, *rrc);
> > -	/* Inteded memory leak on "impossible" error */
> > -	if (!cc)
> > +	/* Intended memory leak on "impossible" error */
> > +	if (!cc) {
> >   		kvm_s390_pv_dealloc_vm(kvm);
> > -	return cc ? -EIO : 0;
> > +		return 0;
> > +	}
> > +	s390_replace_asce(kvm->arch.gmap);
> > +	return -EIO;
> >   }
> >   
> >   int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index dfee0ebb2fac..3b42bf7adb77 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -2714,3 +2714,74 @@ void s390_reset_acc(struct mm_struct *mm)
> >   	mmput(mm);
> >   }
> >   EXPORT_SYMBOL_GPL(s390_reset_acc);
> > +
> > +/**
> > + * s390_remove_old_asce - Remove the topmost level of page tables from the
> > + * list of page tables of the gmap.
> > + * @gmap the gmap whose table is to be removed
> > + *
> > + * This means that it will not be freed when the VM is torn down, and needs
> > + * to be handled separately by the caller, unless an intentional leak is
> > + * intended. Notice that this function will only remove the page from the
> > + * list, the page will still be used as a top level page table (and ASCE).
> > + */
> > +void s390_remove_old_asce(struct gmap *gmap)
> > +{
> > +	struct page *old;
> > +
> > +	old = virt_to_page(gmap->table);
> > +	spin_lock(&gmap->guest_table_lock);
> > +	list_del(&old->lru);
> > +	/*
> > +	 * in case the ASCE needs to be "removed" multiple times, for example
> > +	 * if the VM is rebooted into secure mode several times
> > +	 * concurrently.
> > +	 */  
> 
> How can that happen, what are we protecting against here?

for example if replace_asce fails, and we call it again later. in that
case we have removed the old asce from the list ( = it won't be freed
when the VM terminates), but the ASCE is still in use and still pointed
to. a subsequent call to replace_asce will follow the pointer and try
to remove the same page from the list _again_.

Therefore it's necessary that the page of the ASCE has valid pointers,
so list_del can work (and do nothing) without dereferencing stale or
invalid pointers.

Maybe I should improve the comment

> 
> > +	INIT_LIST_HEAD(&old->lru);
> > +	spin_unlock(&gmap->guest_table_lock);
> > +}
> > +EXPORT_SYMBOL_GPL(s390_remove_old_asce);
> > +
> > +/**
> > + * s390_replace_asce - Try to replace the current ASCE of a gmap with
> > + * another equivalent one.
> > + * @gmap the gmap
> > + *
> > + * If the allocation of the new top level page table fails, the ASCE is not
> > + * replaced.
> > + * In any case, the old ASCE is always removed from the list. Therefore the
> > + * caller has to make sure to save a pointer to it beforehands, unless an
> > + * intentional leak is intended.
> > + */
> > +int s390_replace_asce(struct gmap *gmap)
> > +{
> > +	unsigned long asce;
> > +	struct page *page;
> > +	void *table;
> > +
> > +	s390_remove_old_asce(gmap);
> > +
> > +	page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
> > +	if (!page)
> > +		return -ENOMEM;
> > +	table = page_to_virt(page);
> > +	memcpy(table, gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT));
> > +
> > +	/*
> > +	 * The caller has to deal with the old ASCE, but here we make sure
> > +	 * the new one is properly added to the list of page tables, so that
> > +	 * it will be freed when the VM is torn down.
> > +	 */
> > +	spin_lock(&gmap->guest_table_lock);
> > +	list_add(&page->lru, &gmap->crst_list);
> > +	spin_unlock(&gmap->guest_table_lock);
> > +
> > +	/* Set new table origin while preserving existing ASCE control bits */
> > +	asce = (gmap->asce & ~_ASCE_ORIGIN) | __pa(table);
> > +	WRITE_ONCE(gmap->asce, asce);
> > +	WRITE_ONCE(gmap->mm->context.gmap_asce, asce);
> > +	WRITE_ONCE(gmap->table, table);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(s390_replace_asce);  
>