Re: [PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

[Elliot Berman <quic_eberman@quicinc.com>]

On Tue, Aug 06, 2024 at 04:48:22PM +0100, Patrick Roy wrote:
> On Mon, 2024-08-05 at 19:34 +0100, Elliot Berman wrote:
> > Confidential/protected guest virtual machines want to share some memory
> > back with the host Linux. For example, virtqueues allow host and
> > protected guest to exchange data. In MMU-only isolation of protected
> > guest virtual machines, the transition between "shared" and "private"
> > can be done in-place without a trusted hypervisor copying pages.
> > 
> > Add support for this feature and allow Linux to mmap host-accessible
> > pages. When the owner provides an ->accessible() callback in the
> > struct guest_memfd_operations, guest_memfd allows folios to be mapped
> > when the ->accessible() callback returns 0.
> 
> Wouldn't the set of inaccessible folios always match exactly the set of
> folios that have PG_private=1 set? At least guest_memfd instances that
> have GUEST_MEMFD_FLAG_NO_DIRECT_MAP set, having folios without direct
> map entries marked "accessible" sound like it may cause a lot of mayhem
> (as those folios would essentially be secretmem folios, but this time
> without the GUP checks). But even more generally, wouldn't tracking
> accessibility via PG_private be enough?
> 

Mostly that is what is happening. The ->accessible() test allows
guest_memfd to initialize PG_private to the right value and to attempt
to convert private back to shared in the case of host trying to access
the page.

> > To safely make inaccessible:
> > 
> > ```
> > folio = guest_memfd_grab_folio(inode, index, flags);
> > r = guest_memfd_make_inaccessible(inode, folio);
> > if (r)
> >         goto err;
> > 
> > hypervisor_does_guest_mapping(folio);
> > 
> > folio_unlock(folio);
> > ```
> > 
> > hypervisor_does_s2_mapping(folio) should make it so
> > ops->accessible(...) on those folios fails.
> > 
> > The folio lock ensures atomicity.
> > 
> > Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> > ---
> >  include/linux/guest_memfd.h |  7 ++++
> >  mm/guest_memfd.c            | 81 ++++++++++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 87 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
> > index f9e4a27aed67..edcb4ba60cb0 100644
> > --- a/include/linux/guest_memfd.h
> > +++ b/include/linux/guest_memfd.h
> > @@ -16,12 +16,18 @@
> >   * @invalidate_end: called after invalidate_begin returns success. Optional.
> >   * @prepare: called before a folio is mapped into the guest address space.
> >   *           Optional.
> > + * @accessible: called after prepare returns success and before it's mapped
> > + *              into the guest address space. Returns 0 if the folio can be
> > + *              accessed.
> > + *              Optional. If not present, assumes folios are never accessible.
> >   * @release: Called when releasing the guest_memfd file. Required.
> >   */
> >  struct guest_memfd_operations {
> >         int (*invalidate_begin)(struct inode *inode, pgoff_t offset, unsigned long nr);
> >         void (*invalidate_end)(struct inode *inode, pgoff_t offset, unsigned long nr);
> >         int (*prepare)(struct inode *inode, pgoff_t offset, struct folio *folio);
> > +       int (*accessible)(struct inode *inode, struct folio *folio,
> > +                         pgoff_t offset, unsigned long nr);
> >         int (*release)(struct inode *inode);
> >  };
> > 
> > @@ -48,5 +54,6 @@ struct file *guest_memfd_alloc(const char *name,
> >                                const struct guest_memfd_operations *ops,
> >                                loff_t size, unsigned long flags);
> >  bool is_guest_memfd(struct file *file, const struct guest_memfd_operations *ops);
> > +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio);
> > 
> >  #endif
> > diff --git a/mm/guest_memfd.c b/mm/guest_memfd.c
> > index e9d8cab72b28..6b5609932ca5 100644
> > --- a/mm/guest_memfd.c
> > +++ b/mm/guest_memfd.c
> > @@ -9,6 +9,8 @@
> >  #include <linux/pagemap.h>
> >  #include <linux/set_memory.h>
> > 
> > +#include "internal.h"
> > +
> >  static inline int guest_memfd_folio_private(struct folio *folio)
> >  {
> >         unsigned long nr_pages = folio_nr_pages(folio);
> > @@ -89,7 +91,7 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> >                         goto out_err;
> >         }
> > 
> > -       if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> > +       if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
> >                 r = guest_memfd_folio_private(folio);
> >                 if (r)
> >                         goto out_err;
> > @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> >  }
> >  EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
> > 
> > +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
> > +{
> > +       unsigned long gmem_flags = (unsigned long)file->private_data;
> > +       unsigned long i;
> > +       int r;
> > +
> > +       unmap_mapping_folio(folio);
> > +
> > +       /**
> > +        * We can't use the refcount. It might be elevated due to
> > +        * guest/vcpu trying to access same folio as another vcpu
> > +        * or because userspace is trying to access folio for same reason
> > +        *
> > +        * folio_lock serializes the transitions between (in)accessible
> > +        */
> > +       if (folio_maybe_dma_pinned(folio))
> > +               return -EBUSY;
> > +
> > +       if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> > +               r = guest_memfd_folio_private(folio);
> > +               if (r)
> > +                       return r;
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> > +static vm_fault_t gmem_fault(struct vm_fault *vmf)
> > +{
> > +       struct file *file = vmf->vma->vm_file;
> > +       struct inode *inode = file_inode(file);
> > +       const struct guest_memfd_operations *ops = inode->i_private;
> > +       struct folio *folio;
> > +       pgoff_t off;
> > +       int r;
> > +
> > +       folio = guest_memfd_grab_folio(file, vmf->pgoff, GUEST_MEMFD_GRAB_UPTODATE);
> > +       if (!folio)
> > +               return VM_FAULT_SIGBUS;
> > +
> > +       off = vmf->pgoff & (folio_nr_pages(folio) - 1);
> > +       r = ops->accessible(inode, folio, off, 1);
> > +       if (r) {
> 
> This made be stumble at first. I know you say ops->accessible returning
> 0 means "this is accessible", but if I only look at this if-statement it
> reads as "if the folio is accessible, send a SIGBUS", which is not
> what's actually happening.
> 

I'll change it to return a bool :)

> > +               folio_unlock(folio);
> > +               folio_put(folio);
> > +               return VM_FAULT_SIGBUS;
> > +       }
> > +
> > +       guest_memfd_folio_clear_private(folio);
> > +
> > +       vmf->page = folio_page(folio, off);
> > +
> > +       return VM_FAULT_LOCKED;
> > +}
> > +
> > +static const struct vm_operations_struct gmem_vm_ops = {
> > +       .fault = gmem_fault,
> > +};
> > +
> > +static int gmem_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > +       const struct guest_memfd_operations *ops = file_inode(file)->i_private;
> > +
> > +       if (!ops->accessible)
> > +               return -EPERM;
> > +
> > +       /* No support for private mappings to avoid COW.  */
> > +       if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> > +           (VM_SHARED | VM_MAYSHARE))
> > +               return -EINVAL;
> > +
> > +       file_accessed(file);
> > +       vma->vm_ops = &gmem_vm_ops;
> > +       return 0;
> > +}
> > +
> >  static long gmem_punch_hole(struct file *file, loff_t offset, loff_t len)
> >  {
> >         struct inode *inode = file_inode(file);
> > @@ -220,6 +298,7 @@ static int gmem_release(struct inode *inode, struct file *file)
> >  static struct file_operations gmem_fops = {
> >         .open = generic_file_open,
> >         .llseek = generic_file_llseek,
> > +       .mmap = gmem_mmap,
> >         .release = gmem_release,
> >         .fallocate = gmem_fallocate,
> >         .owner = THIS_MODULE,
> > 
> > --
> > 2.34.1
> >