Re: [PATCH v1 2/2] KVM: s390: pv: fix race when making a page secure

[David Hildenbrand <david@xxxxxxxxxx>]

On 13.02.25 21:16, David Hildenbrand wrote:
> On 13.02.25 21:07, Claudio Imbrenda wrote:
> > Holding the pte lock for the page that is being converted to secure is
> > needed to avoid races. A previous commit removed the locking, which
> > caused issues. Fix by locking the pte again.
> >
> > Fixes: 5cbe24350b7d ("KVM: s390: move pv gmap functions into kvm")
>
> If you found this because of my report about the changed locking,
> consider adding a Suggested-by / Reported-y.
>
> > Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> > ---
> >    arch/s390/include/asm/uv.h |  2 +-
> >    arch/s390/kernel/uv.c      | 19 +++++++++++++++++--
> >    arch/s390/kvm/gmap.c       | 12 ++++++++----
> >    3 files changed, 26 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
> > index b11f5b6d0bd1..46fb0ef6f984 100644
> > --- a/arch/s390/include/asm/uv.h
> > +++ b/arch/s390/include/asm/uv.h
> > @@ -631,7 +631,7 @@ int uv_pin_shared(unsigned long paddr);
> >    int uv_destroy_folio(struct folio *folio);
> >    int uv_destroy_pte(pte_t pte);
> >    int uv_convert_from_secure_pte(pte_t pte);
> > -int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb);
> > +int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb);
> >    int uv_convert_from_secure(unsigned long paddr);
> >    int uv_convert_from_secure_folio(struct folio *folio);
> >    
> > diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> > index 9f05df2da2f7..de3c092da7b9 100644
> > --- a/arch/s390/kernel/uv.c
> > +++ b/arch/s390/kernel/uv.c
> > @@ -245,7 +245,7 @@ static int expected_folio_refs(struct folio *folio)
> >     * Context: The caller must hold exactly one extra reference on the folio
> >     *          (it's the same logic as split_folio())
> >     */
> > -int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb)
> > +static int __make_folio_secure(struct folio *folio, unsigned long hva, struct uv_cb_header *uvcb)
> >    {
> >    	int expected, cc = 0;
> >    
> > @@ -277,7 +277,22 @@ int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb)
> >    		return -EAGAIN;
> >    	return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> >    }
> > -EXPORT_SYMBOL_GPL(make_folio_secure);
> > +
> > +int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb)
> > +{
> > +	spinlock_t *ptelock;
> > +	pte_t *ptep;
> > +	int rc;
> > +
> > +	ptep = get_locked_pte(mm, hva, &ptelock);
> > +	if (!ptep)
> > +		return -ENXIO;
> > +	rc = __make_folio_secure(page_folio(pte_page(*ptep)), hva, uvcb);
> > +	pte_unmap_unlock(ptep, ptelock);
> > +
> > +	return rc;
> > +}
> > +EXPORT_SYMBOL_GPL(make_hva_secure);
> >    
> >    /*
> >     * To be called with the folio locked or with an extra reference! This will
> > diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
> > index fc4d490d25a2..e56c0ab5fec7 100644
> > --- a/arch/s390/kvm/gmap.c
> > +++ b/arch/s390/kvm/gmap.c
> > @@ -55,7 +55,7 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
> >    	return atomic_read(&mm->context.protected_count) > 1;
> >    }
> >    
> > -static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
> > +static int __gmap_make_secure(struct gmap *gmap, struct page *page, unsigned long hva, void *uvcb)
> >    {
> >    	struct folio *folio = page_folio(page);
> >    	int rc;
> > @@ -83,7 +83,7 @@ static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
> >    		return -EAGAIN;
> >    	if (should_export_before_import(uvcb, gmap->mm))
> >    		uv_convert_from_secure(folio_to_phys(folio));
> > -	rc = make_folio_secure(folio, uvcb);
> > +	rc = make_hva_secure(gmap->mm, hva, uvcb);
> >    	folio_unlock(folio);
> >    
> >    	/*
> > @@ -120,6 +120,7 @@ static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
> >    int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
> >    {
> >    	struct kvm *kvm = gmap->private;
> > +	unsigned long vmaddr;
> >    	struct page *page;
> >    	int rc = 0;
> >    
> > @@ -127,8 +128,11 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
> >    
> >    	page = gfn_to_page(kvm, gpa_to_gfn(gaddr));
> >    	mmap_read_lock(gmap->mm);
> > -	if (page)
> > -		rc = __gmap_make_secure(gmap, page, uvcb);
> > +	vmaddr = gfn_to_hva(gmap->private, gpa_to_gfn(gaddr));
> > +	if (kvm_is_error_hva(vmaddr))
> > +		rc = -ENXIO;
> > +	if (!rc && page)
> > +		rc = __gmap_make_secure(gmap, page, vmaddr, uvcb);
> >    	kvm_release_page_clean(page);
> >    	mmap_read_unlock(gmap->mm);
> >    
>
> You effectively make the code more complicated and inefficient than
> before. Now you effectively walk the page table twice in the common
> small-folio case ...
>
> Can we just go back to the old handling that we had before here?

I'll note that there is still the possibility for a different race: 
nothing guarantees that the page you looked up using gfn_to_hva() will 
still be mapped when you perform the get_locked_pte(). Not sure what 
would happen if we would have a different page mapped.

You could re-verify it is still there, but then, doing two page table 
walks is still more than required in the common case where we can just 
perform the conversion.

--
Cheers,

David / dhildenb