Re: [RFC PATCH 3/3] mm: perform the mapping_map_writable() check after call_mmap()

[Lorenzo Stoakes]

On Fri, Apr 21, 2023 at 11:06:28AM +0200, Jan Kara wrote:
> On Mon 03-04-23 23:28:32, Lorenzo Stoakes wrote:
> > In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to
> > clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap()
> > handler to do so. We would otherwise fail the mapping_map_writable() check
> > before we had the opportunity to avoid it.
> >
> > This patch moves this check after the call_mmap() invocation. Only memfd
> > actively denies write access causing a potential failure here (in
> > memfd_add_seals()), so there should be no impact on non-memfd cases.
> >
> > This patch makes the userland-visible change that MAP_SHARED, PROT_READ
> > mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed.
> >
> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > ---
> >  mm/mmap.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index c96dcce90772..a166e9f3c474 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2596,17 +2596,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  	vma->vm_pgoff = pgoff;
> >
> >  	if (file) {
> > -		if (is_shared_maywrite(vm_flags)) {
> > -			error = mapping_map_writable(file->f_mapping);
> > -			if (error)
> > -				goto free_vma;
> > -		}
> > -
> >  		vma->vm_file = get_file(file);
> >  		error = call_mmap(file, vma);
> >  		if (error)
> >  			goto unmap_and_free_vma;
> >
> > +		if (vma_is_shared_maywrite(vma)) {
> > +			error = mapping_map_writable(file->f_mapping);
> > +			if (error)
> > +				goto unmap_and_free_vma;
>
> Shouldn't we rather jump to close_and_free_vma?

You're right, we may need to call vma->vm_ops->close() to match the
->mmap().

>
> 								Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR