Subject: Re: [RFC v1 2/2] mm/gup/writeback: add callbacks for inaccessible pages

From: Claudio Imbrenda <[email-masked]>
Date: Sat, 29 Feb 2020 11:49:19 +0100

On Fri, 28 Feb 2020 16:08:23 -0800
John Hubbard <[email-masked]> wrote:

> On 2/28/20 8:08 AM, Christian Borntraeger wrote:
> > Andrew,
> > 
> > while patch 1 is a fixup for the FOLL_PIN work in your patch queue,
> > I would really love to see this patch in 5.7. The exploitation code
> > of kvm/s390 is in Linux next also scheduled for 5.7.
> > 
> > Christian
> > 
> > On 28.02.20 16:43, Claudio Imbrenda wrote:  
> >> With the introduction of protected KVM guests on s390 there is now
> >> a concept of inaccessible pages. These pages need to be made
> >> accessible before the host can access them.
> >>
> >> While cpu accesses will trigger a fault that can be resolved, I/O
> >> accesses will just fail.  We need to add a callback into
> >> architecture code for places that will do I/O, namely when
> >> writeback is started or when a page reference is taken.
> >>
> >> This is not only to enable paging, file backing etc, it is also
> >> necessary to protect the host against a malicious user space. For
> >> example a bad QEMU could simply start direct I/O on such protected
> >> memory.  We do not want userspace to be able to trigger I/O errors
> >> and thus we the logic is "whenever somebody accesses that page
> >> (gup) or  
> 
> 
> I actually kind of like the sound of that: "We the logic of the
> kernel, in order to form a more perfect computer..." :)
> 
> Probably this wording is what you want, though:
> 
> "thus the logic is "whenever somebody (gup) accesses that page or"
> 
> 
> ...
> >> @@ -458,7 +457,6 @@ static struct page *follow_page_pte(struct
> >> vm_area_struct *vma, }
> >>  
> >>  	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> >> -		int ret;
> >>  		get_page(page);
> >>  		pte_unmap_unlock(ptep, ptl);
> >>  		lock_page(page);
> >> @@ -475,6 +473,14 @@ static struct page *follow_page_pte(struct
> >> vm_area_struct *vma, page = ERR_PTR(-ENOMEM);
> >>  		goto out;
> >>  	}
> >> +	if (flags & FOLL_PIN) {  
> 
> 
> What about FOLL_GET? Unless your calling code has some sort of
> BUG_ON(flags & FOLL_GET), I'm not sure it's a good idea to leave that
> case unhandled.

if I understood the semantics of FOLL_PIN correctly, then we don't need
to make the page accessible for FOLL_GET. FOLL_PIN indicates intent to
access the content of the page, whereas FOLL_GET is only for the struct
page. 

if we are not touching the content of the page, there is no need to
make it accessible 

> >> +		ret = arch_make_page_accessible(page);
> >> +		if (ret) {
> >> +			unpin_user_page(page);
> >> +			page = ERR_PTR(ret);
> >> +			goto out;
> >> +		}
> >> +	}
> >>  	if (flags & FOLL_TOUCH) {
> >>  		if ((flags & FOLL_WRITE) &&
> >>  		    !pte_dirty(pte) && !PageDirty(page))
> >> @@ -2143,6 +2149,13 @@ static int gup_pte_range(pmd_t pmd,
> >> unsigned long addr, unsigned long end, 
> >>  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
> >>  
> >> +		if (flags & FOLL_PIN) {
> >> +			ret = arch_make_page_accessible(page);
> >> +			if (ret) {
> >> +				unpin_user_page(page);  
> 
> 
> Same concern as above, about leaving FOLL_GET unhandled.

and same answer as above :)