Re: [PATCH v6 02/17] KVM: s390: pv: handle secure storage violations for protected guests

[Janosch Frank]

On 12/3/21 17:57, Claudio Imbrenda wrote:
> With upcoming patches, protected guests will be able to trigger secure
> storage violations in normal operation.
>
> This patch adds handling of secure storage violations for protected
> guests.
>
> Pages that trigger the exception will be made non-secure before
> attempting to use them again for a different secure guest.

I think we should extend this a bit.

With upcoming patches, protected guests will be able to trigger secure 
storage violations in normal operation. This happens if e.g. a protected 
guest is re-booted with lazy destroy enabled and the new guest is also 
protected.

When the new protected guest touches pages that haven't yet been 
destroyed and thus are accounted to the previous protected guest we will 
see the violation exception.

We handle this exception by first trying to destroy the page because we 
expect it to belong to a defunct protected guest where a destroy should 
be possible. If that fails, we will try to do a normal export of the page.


> Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> Acked-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
> ---
>   arch/s390/include/asm/uv.h |  1 +
>   arch/s390/kernel/uv.c      | 55 ++++++++++++++++++++++++++++++++++++++
>   arch/s390/mm/fault.c       | 10 +++++++
>   3 files changed, 66 insertions(+)
>
> diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
> index 72d3e49c2860..cdbd340188ab 100644
> --- a/arch/s390/include/asm/uv.h
> +++ b/arch/s390/include/asm/uv.h
> @@ -356,6 +356,7 @@ static inline int is_prot_virt_host(void)
>   }
>   
>   int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb);
> +int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr);
>   int uv_destroy_owned_page(unsigned long paddr);
>   int uv_convert_from_secure(unsigned long paddr);
>   int uv_convert_owned_from_secure(unsigned long paddr);
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index 386d4e42b8d3..f706456f6261 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -334,6 +334,61 @@ int gmap_convert_to_secure(struct gmap *gmap, unsigned long gaddr)
>   }
>   EXPORT_SYMBOL_GPL(gmap_convert_to_secure);
>   
> +/**
> + * gmap_destroy_page - Destroy a guest page.
> + * @gmap the gmap of the guest
> + * @gaddr the guest address to destroy
> + *
> + * An attempt will be made to destroy the given guest page. If the attempt
> + * fails, an attempt is made to export the page. If both attempts fail, an
> + * appropriate error is returned.
> + */
> +int gmap_destroy_page(struct gmap *gmap, unsigned long gaddr)
> +{
> +	struct vm_area_struct *vma;
> +	unsigned long uaddr;
> +	struct page *page;
> +	int rc;
> +
> +	rc = -EFAULT;
> +	mmap_read_lock(gmap->mm);
> +
> +	uaddr = __gmap_translate(gmap, gaddr);
> +	if (IS_ERR_VALUE(uaddr))
> +		goto out;
> +	vma = vma_lookup(gmap->mm, uaddr);
> +	if (!vma)
> +		goto out;
> +	/*
> +	 * Huge pages should not be able to become secure
> +	 */

Could be one line

> +	if (is_vm_hugetlb_page(vma))
> +		goto out;
> +
> +	rc = 0;
> +	/* we take an extra reference here */

Because?

> +	page = follow_page(vma, uaddr, FOLL_WRITE | FOLL_GET);
> +	if (IS_ERR_OR_NULL(page))
> +		goto out;
> +	rc = uv_destroy_owned_page(page_to_phys(page));
> +	/*
> +	 * Fault handlers can race; it is possible that two CPUs will fault
> +	 * on the same secure page. One CPU can destroy the page, reboot,
> +	 * re-enter secure mode and import it, while the second CPU was
> +	 * stuck at the beginning of the handler. At some point the second
> +	 * CPU will be able to progress, and it will not be able to destroy
> +	 * the page. In that case we do not want to terminate the process,
> +	 * we instead try to export the page.
> +	 */

So when we export we always export a page that's owned by the new guest, 
do I get that right?

> +	if (rc)
> +		rc = uv_convert_owned_from_secure(page_to_phys(page));
> +	put_page(page);
> +out:
> +	mmap_read_unlock(gmap->mm);
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(gmap_destroy_page);
> +
>   /*
>    * To be called with the page locked or with an extra reference! This will
>    * prevent gmap_make_secure from touching the page concurrently. Having 2
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index d30f5986fa85..a1928c89bbfa 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -853,6 +853,16 @@ NOKPROBE_SYMBOL(do_non_secure_storage_access);
>   
>   void do_secure_storage_violation(struct pt_regs *regs)
>   {
> +	unsigned long gaddr = regs->int_parm_long & __FAIL_ADDR_MASK;
> +	struct gmap *gmap = (struct gmap *)S390_lowcore.gmap;
> +
> +	/*
> +	 * If the VM has been rebooted, its address space might still contain
> +	 * secure pages from the previous boot.
> +	 * Clear the page so it can be reused.
> +	 */
> +	if (!gmap_destroy_page(gmap, gaddr))
> +		return;
>   	/*
>   	 * Either KVM messed up the secure guest mapping or the same
>   	 * page is mapped into multiple secure guests.