linux-kernel - Re: [PATCH v5 05/14] KVM: s390: pv: leak the topmost page table when destroy fails

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Date:   Tue, 12 Oct 2021 09:58:19 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     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 05/14] KVM: s390: pv: leak the topmost page table when
 destroy fails

On 9/20/21 15:24, Claudio Imbrenda wrote:
> Each secure guest must have a unique address space control element and
> we must avoid that new guests use the same ASCE, to avoid errors.
> Since the ASCE mostly consists of the topmost page table address (and
> 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 should not happen unless the hardware is broken or KVM has
> some unknown serious bug.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>

Fixes tag?

> ---
>   arch/s390/include/asm/gmap.h |  2 ++
>   arch/s390/kvm/pv.c           |  4 ++-
>   arch/s390/mm/gmap.c          | 55 ++++++++++++++++++++++++++++++++++++
>   3 files changed, 60 insertions(+), 1 deletion(-)
> 
> 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 00d272d134c2..76b0d64ce8fa 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -168,9 +168,11 @@ 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 */
> +	/* Intended memory leak on "impossible" error */

Rather unrelated

>   	if (!cc)
>   		kvm_s390_pv_dealloc_vm(kvm);
> +	else
> +		s390_replace_asce(kvm->arch.gmap);
>   	return cc ? -EIO : 0;

Might make more sense now to do an early return so we don't have the 
ternary if here.

>   }
>   
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 9bb2c7512cd5..5a138f6220c4 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2706,3 +2706,58 @@ void s390_reset_acc(struct mm_struct *mm)
>   	mmput(mm);
>   }
>   EXPORT_SYMBOL_GPL(s390_reset_acc);
> +
> +/*
> + * Remove the topmost level of page tables from the list of page tables of
> + * the gmap.
> + * 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.
> + */
> +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);
> +	spin_unlock(&gmap->guest_table_lock);
> +	/* in case the ASCE needs to be "removed" multiple times */
> +	INIT_LIST_HEAD(&old->lru);
> +}
> +EXPORT_SYMBOL_GPL(s390_remove_old_asce);

Is this used anywhere else than below?
This can be static, no?

> +
> +/*
> + * Try to replace the current ASCE with another equivalent one.
> + * If the allocation of the new top level page table fails, the ASCE is not
> + * replaced.
> + * In any case, the old ASCE is 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));
> +
> +	spin_lock(&gmap->guest_table_lock);
> +	list_add(&page->lru, &gmap->crst_list);
> +	spin_unlock(&gmap->guest_table_lock);
> +
> +	asce = (gmap->asce & ~PAGE_MASK) | __pa(table);
> +	WRITE_ONCE(gmap->asce, asce);

Are you sure we don't need the mm in write lock?

> +	WRITE_ONCE(gmap->mm->context.gmap_asce, asce);

This is usually changed with the context lock held.

> +	WRITE_ONCE(gmap->table, table);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(s390_replace_asce);
>