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@xxxxxxxxxxxxxxx>
> > ---
> > 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
