Re: [RFCv2 11/37] KVM: s390/mm: Make pages accessible before destroying the guest

[Thomas Huth <thuth@redhat.com>]

On 03/02/2020 14.19, Christian Borntraeger wrote:
> Before we destroy the secure configuration, we better make all
> pages accessible again. This also happens during reboot, where we reboot
> into a non-secure guest that then can go again into a secure mode. As
> this "new" secure guest will have a new ID we cannot reuse the old page
> state.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  arch/s390/include/asm/pgtable.h |  1 +
>  arch/s390/kvm/pv.c              |  2 ++
>  arch/s390/mm/gmap.c             | 35 +++++++++++++++++++++++++++++++++
>  3 files changed, 38 insertions(+)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 65b6bb47af0a..6e167dcc35f1 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1669,6 +1669,7 @@ extern int vmem_remove_mapping(unsigned long start, unsigned long size);
>  extern int s390_enable_sie(void);
>  extern int s390_enable_skey(void);
>  extern void s390_reset_cmma(struct mm_struct *mm);
> +extern void s390_reset_acc(struct mm_struct *mm);
>  
>  /* s390 has a private copy of get unmapped area to deal with cache synonyms */
>  #define HAVE_ARCH_UNMAPPED_AREA
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index a867b9e9c069..24d802072ac7 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -66,6 +66,8 @@ int kvm_s390_pv_destroy_vm(struct kvm *kvm)
>  	int rc;
>  	u32 ret;
>  
> +	/* make all pages accessible before destroying the guest */
> +	s390_reset_acc(kvm->mm);
>  	rc = uv_cmd_nodata(kvm_s390_pv_handle(kvm),
>  			   UVC_CMD_DESTROY_SEC_CONF, &ret);
>  	WRITE_ONCE(kvm->arch.gmap->se_handle, 0);
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index bf365a09f900..0b00e8d5fa39 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2648,3 +2648,38 @@ void s390_reset_cmma(struct mm_struct *mm)
>  	up_write(&mm->mmap_sem);
>  }
>  EXPORT_SYMBOL_GPL(s390_reset_cmma);
> +
> +/*
> + * make inaccessible pages accessible again
> + */
> +static int __s390_reset_acc(pte_t *ptep, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	pte_t pte = READ_ONCE(*ptep);
> +
> +	if (pte_present(pte))
> +		uv_convert_from_secure(pte_val(pte) & PAGE_MASK);

Is it ok to ignore the return value from uv_convert_from_secure() ?
Problems might go unnoticed ... maybe use at least a WARN_ONCE ?

> +	return 0;
> +}
> +
> +static const struct mm_walk_ops reset_acc_walk_ops = {
> +	.pte_entry		= __s390_reset_acc,
> +};
> +
> +#include <linux/sched/mm.h>
> +void s390_reset_acc(struct mm_struct *mm)
> +{
> +	/*
> +	 * we might be called during
> +	 * reset:                            we walk the pages and clear
> +	 * close of all kvm file descriptor: we walk the pages and clear
> +	 * exit of process on fd closure:    vma already gone, do nothing
> +	 */
> +	if (!mmget_not_zero(mm))
> +		return;
> +	down_read(&mm->mmap_sem);
> +	walk_page_range(mm, 0, TASK_SIZE, &reset_acc_walk_ops, NULL);
> +	up_read(&mm->mmap_sem);
> +	mmput(mm);
> +}
> +EXPORT_SYMBOL_GPL(s390_reset_acc);
> 

Anyway,
Reviewed-by: Thomas Huth <thuth@redhat.com>