Re: [RFC PATCH 7/8] mm: secretmem: use AS_INACCESSIBLE to prohibit GUP

[Mike Rapoport <rppt@kernel.org>]

On Tue, Jul 09, 2024 at 11:09:29PM +0200, David Hildenbrand wrote:
> On 09.07.24 15:20, Patrick Roy wrote:
> > Inside of vma_is_secretmem and secretmem_mapping, instead of checking
> > whether a vm_area_struct/address_space has the secretmem ops structure
> > attached to it, check whether the address_space has the AS_INACCESSIBLE
> > bit set. Then set the AS_INACCESSIBLE flag for secretmem's
> > address_space.
> > 
> > This means that get_user_pages and friends are disables for all
> > adress_spaces that set AS_INACCESIBLE. The AS_INACCESSIBLE flag was
> > introduced in commit c72ceafbd12c ("mm: Introduce AS_INACCESSIBLE for
> > encrypted/confidential memory") specifically for guest_memfd to indicate
> > that no reads and writes should ever be done to guest_memfd
> > address_spaces. Disallowing gup seems like a reasonable semantic
> > extension, and means that potential future mmaps of guest_memfd cannot
> > be GUP'd.
> > 
> > Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> > ---
> >   include/linux/secretmem.h | 13 +++++++++++--
> >   mm/secretmem.c            |  6 +-----
> >   2 files changed, 12 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
> > index e918f96881f5..886c8f7eb63e 100644
> > --- a/include/linux/secretmem.h
> > +++ b/include/linux/secretmem.h
> > @@ -8,10 +8,19 @@ extern const struct address_space_operations secretmem_aops;
> >   static inline bool secretmem_mapping(struct address_space *mapping)
> >   {
> > -	return mapping->a_ops == &secretmem_aops;
> > +	return mapping->flags & AS_INACCESSIBLE;
> > +}
> > +
> > +static inline bool vma_is_secretmem(struct vm_area_struct *vma)
> > +{
> > +	struct file *file = vma->vm_file;
> > +
> > +	if (!file)
> > +		return false;
> > +
> > +	return secretmem_mapping(file->f_inode->i_mapping);
> >   }
> 
> That sounds wrong. You should leave *secretmem alone and instead have
> something like inaccessible_mapping that is used where appropriate.
> 
> vma_is_secretmem() should not suddenly succeed on something that is not
> mm/secretmem.c

I'm with David here.
 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.