Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use shareability to guard faulting

[Fuad Tabba]

Hi Ackerley,

On Fri, 30 May 2025 at 19:32, Ackerley Tng <ackerleytng@xxxxxxxxxx> wrote:
>
> Fuad Tabba <tabba@xxxxxxxxxx> writes:
>
> > Hi,
> >
> > .. snip..
> >
> >> I noticed that in [1], the kvm_gmem_mmap() does not check the range.
> >> So, the WARN() here can be hit when userspace mmap() an area larger than the
> >> inode size and accesses the out of band HVA.
> >>
> >> Maybe limit the mmap() range?
> >>
> >> @@ -1609,6 +1620,10 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
> >>         if (!kvm_gmem_supports_shared(file_inode(file)))
> >>                 return -ENODEV;
> >>
> >> +       if (vma->vm_end - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT) > i_size_read(file_inode(file)))
> >> +               return -EINVAL;
> >> +
> >>         if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> >>             (VM_SHARED | VM_MAYSHARE)) {
> >>                 return -EINVAL;
> >>
> >> [1] https://lore.kernel.org/all/20250513163438.3942405-8-tabba@xxxxxxxxxx/
> >
> > I don't think we want to do that for a couple of reasons. We catch
> > such invalid accesses on faulting, and, by analogy, afaikt, neither
> > secretmem nor memfd perform a similar check on mmap (nor do
> > memory-mapped files in general).
> >
> > There are also valid reasons why a user would want to deliberately
> > mmap more memory than the backing store, knowing that it's only going
> > to fault what it's going to use, e.g., alignment.
> >
>
> This is a good point.
>
> I think there's no check against the inode size on faulting now though?
> v10's [1] kvm_gmem_fault_shared() calls kvm_gmem_get_folio()
> straightaway.
>
> We should add a check like [2] to kvm_gmem_fault_shared().

Yes! I mistakenly thought that kvm_gmem_get_folio() had such a check,
I just verified that it doesn't. I have added the check, as well as a
new selftest to make sure we don't miss it in the future.

Thanks!
/fuad

> [1] https://lore.kernel.org/all/20250513163438.3942405-8-tabba@xxxxxxxxxx/
> [2] https://github.com/torvalds/linux/blob/8477ab143069c6b05d6da4a8184ded8b969240f5/mm/filemap.c#L3373
>
> > Cheers,
> > /fuad
> >
> >
> >> > +     return xa_to_value(entry);
> >> > +}
> >> > +
> >> > +static struct folio *kvm_gmem_get_shared_folio(struct inode *inode, pgoff_t index)
> >> > +{
> >> > +     if (kvm_gmem_shareability_get(inode, index) != SHAREABILITY_ALL)
> >> > +             return ERR_PTR(-EACCES);
> >> > +
> >> > +     return kvm_gmem_get_folio(inode, index);
> >> > +}
> >> > +
> >> > +#else
> >> > +
> >> > +static int kvm_gmem_shareability_setup(struct maple_tree *mt, loff_t size, u64 flags)
> >> > +{
> >> > +     return 0;
> >> > +}
> >> > +
> >> > +static inline struct folio *kvm_gmem_get_shared_folio(struct inode *inode, pgoff_t index)
> >> > +{
> >> > +     WARN_ONCE("Unexpected call to get shared folio.")
> >> > +     return NULL;
> >> > +}
> >> > +
> >> > +#endif /* CONFIG_KVM_GMEM_SHARED_MEM */
> >> > +
> >> >  static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
> >> >                                   pgoff_t index, struct folio *folio)
> >> >  {
> >> > @@ -333,7 +404,7 @@ static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf)
> >> >
> >> >       filemap_invalidate_lock_shared(inode->i_mapping);
> >> >
> >> > -     folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> >> > +     folio = kvm_gmem_get_shared_folio(inode, vmf->pgoff);
> >> >       if (IS_ERR(folio)) {
> >> >               int err = PTR_ERR(folio);
> >> >
> >> > @@ -420,8 +491,33 @@ static struct file_operations kvm_gmem_fops = {
> >> >       .fallocate      = kvm_gmem_fallocate,
> >> >  };
> >> >
> >> > +static void kvm_gmem_free_inode(struct inode *inode)
> >> > +{
> >> > +     struct kvm_gmem_inode_private *private = kvm_gmem_private(inode);
> >> > +
> >> > +     kfree(private);
> >> > +
> >> > +     free_inode_nonrcu(inode);
> >> > +}
> >> > +
> >> > +static void kvm_gmem_destroy_inode(struct inode *inode)
> >> > +{
> >> > +     struct kvm_gmem_inode_private *private = kvm_gmem_private(inode);
> >> > +
> >> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> >> > +     /*
> >> > +      * mtree_destroy() can't be used within rcu callback, hence can't be
> >> > +      * done in ->free_inode().
> >> > +      */
> >> > +     if (private)
> >> > +             mtree_destroy(&private->shareability);
> >> > +#endif
> >> > +}
> >> > +
> >> >  static const struct super_operations kvm_gmem_super_operations = {
> >> >       .statfs         = simple_statfs,
> >> > +     .destroy_inode  = kvm_gmem_destroy_inode,
> >> > +     .free_inode     = kvm_gmem_free_inode,
> >> >  };
> >> >
> >> >  static int kvm_gmem_init_fs_context(struct fs_context *fc)
> >> > @@ -549,12 +645,26 @@ static const struct inode_operations kvm_gmem_iops = {
> >> >  static struct inode *kvm_gmem_inode_make_secure_inode(const char *name,
> >> >                                                     loff_t size, u64 flags)
> >> >  {
> >> > +     struct kvm_gmem_inode_private *private;
> >> >       struct inode *inode;
> >> > +     int err;
> >> >
> >> >       inode = alloc_anon_secure_inode(kvm_gmem_mnt->mnt_sb, name);
> >> >       if (IS_ERR(inode))
> >> >               return inode;
> >> >
> >> > +     err = -ENOMEM;
> >> > +     private = kzalloc(sizeof(*private), GFP_KERNEL);
> >> > +     if (!private)
> >> > +             goto out;
> >> > +
> >> > +     mt_init(&private->shareability);
> >> Wrap the mt_init() inside "#ifdef CONFIG_KVM_GMEM_SHARED_MEM" ?
> >>
> >> > +     inode->i_mapping->i_private_data = private;
> >> > +
> >> > +     err = kvm_gmem_shareability_setup(private, size, flags);
> >> > +     if (err)
> >> > +             goto out;
> >> > +
> >> >       inode->i_private = (void *)(unsigned long)flags;
> >> >       inode->i_op = &kvm_gmem_iops;
> >> >       inode->i_mapping->a_ops = &kvm_gmem_aops;
> >> > @@ -566,6 +676,11 @@ static struct inode *kvm_gmem_inode_make_secure_inode(const char *name,
> >> >       WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
> >> >
> >> >       return inode;
> >> > +
> >> > +out:
> >> > +     iput(inode);
> >> > +
> >> > +     return ERR_PTR(err);
> >> >  }
> >> >
> >> >  static struct file *kvm_gmem_inode_create_getfile(void *priv, loff_t size,
> >> > @@ -654,6 +769,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
> >> >       if (kvm_arch_vm_supports_gmem_shared_mem(kvm))
> >> >               valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
> >> >
> >> > +     if (flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED)
> >> > +             valid_flags |= GUEST_MEMFD_FLAG_INIT_PRIVATE;
> >> > +
> >> >       if (flags & ~valid_flags)
> >> >               return -EINVAL;
> >> >
> >> > @@ -842,6 +960,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> >> >       if (!file)
> >> >               return -EFAULT;
> >> >
> >> > +     filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
> >> > +
> >> >       folio = __kvm_gmem_get_pfn(file, slot, index, pfn, &is_prepared, max_order);
> >> >       if (IS_ERR(folio)) {
> >> >               r = PTR_ERR(folio);
> >> > @@ -857,8 +977,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> >> >               *page = folio_file_page(folio, index);
> >> >       else
> >> >               folio_put(folio);
> >> > -
> >> >  out:
> >> > +     filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
> >> >       fput(file);
> >> >       return r;
> >> >  }
> >> > --
> >> > 2.49.0.1045.g170613ef41-goog
> >> >
> >> >