Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Tue, Oct 20, 2020 at 01:25:59AM -0700, John Hubbard wrote:
> On 10/19/20 11:18 PM, Kirill A. Shutemov wrote:
> > New helpers copy_from_guest()/copy_to_guest() to be used if KVM memory
> > protection feature is enabled.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >   include/linux/kvm_host.h |  4 ++
> >   virt/kvm/kvm_main.c      | 90 +++++++++++++++++++++++++++++++---------
> >   2 files changed, 75 insertions(+), 19 deletions(-)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 05e3c2fb3ef7..380a64613880 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -504,6 +504,7 @@ struct kvm {
> >   	struct srcu_struct irq_srcu;
> >   	pid_t userspace_pid;
> >   	unsigned int max_halt_poll_ns;
> > +	bool mem_protected;
> >   };
> >   #define kvm_err(fmt, ...) \
> > @@ -728,6 +729,9 @@ void kvm_set_pfn_dirty(kvm_pfn_t pfn);
> >   void kvm_set_pfn_accessed(kvm_pfn_t pfn);
> >   void kvm_get_pfn(kvm_pfn_t pfn);
> > +int copy_from_guest(void *data, unsigned long hva, int len, bool protected);
> > +int copy_to_guest(unsigned long hva, const void *data, int len, bool protected);
> > +
> >   void kvm_release_pfn(kvm_pfn_t pfn, bool dirty, struct gfn_to_pfn_cache *cache);
> >   int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> >   			int len);
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index cf88233b819a..a9884cb8c867 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2313,19 +2313,70 @@ static int next_segment(unsigned long len, int offset)
> >   		return len;
> >   }
> > +int copy_from_guest(void *data, unsigned long hva, int len, bool protected)
> > +{
> > +	int offset = offset_in_page(hva);
> > +	struct page *page;
> > +	int npages, seg;
> > +
> > +	if (!protected)
> > +		return __copy_from_user(data, (void __user *)hva, len);
> > +
> > +	might_fault();
> > +	kasan_check_write(data, len);
> > +	check_object_size(data, len, false);
> > +
> > +	while ((seg = next_segment(len, offset)) != 0) {
> > +		npages = get_user_pages_unlocked(hva, 1, &page, 0);
> > +		if (npages != 1)
> > +			return -EFAULT;
> > +		memcpy(data, page_address(page) + offset, seg);
> 
> Hi Kirill!
> 
> OK, so the copy_from_guest() is a read-only case for gup, which I think is safe
> from a gup/pup + filesystem point of view, but see below about copy_to_guest()...
> 
> > +		put_page(page);
> > +		len -= seg;
> > +		hva += seg;
> > +		offset = 0;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +int copy_to_guest(unsigned long hva, const void *data, int len, bool protected)
> > +{
> > +	int offset = offset_in_page(hva);
> > +	struct page *page;
> > +	int npages, seg;
> > +
> > +	if (!protected)
> > +		return __copy_to_user((void __user *)hva, data, len);
> > +
> > +	might_fault();
> > +	kasan_check_read(data, len);
> > +	check_object_size(data, len, true);
> > +
> > +	while ((seg = next_segment(len, offset)) != 0) {
> > +		npages = get_user_pages_unlocked(hva, 1, &page, FOLL_WRITE);
> 
> 
> Should copy_to_guest() use pin_user_pages_unlocked() instead of gup_unlocked?
> We wrote a  "Case 5" in Documentation/core-api/pin_user_pages.rst, just for this
> situation, I think:
> 
> 
> CASE 5: Pinning in order to write to the data within the page
> -------------------------------------------------------------
> Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
> write to a page's data, unpin" can cause a problem. Case 5 may be considered a
> superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
> other words, if the code is neither Case 1 nor Case 2, it may still require
> FOLL_PIN, for patterns like this:
> 
> Correct (uses FOLL_PIN calls):
>     pin_user_pages()
>     write to the data within the pages
>     unpin_user_pages()

Right. I didn't internalize changes in GUP interface yet. Will update.

-- 
 Kirill A. Shutemov