Re: [PATCH v16 06/11] mm: introduce memfd_secret system call to create "secret" memory areas

Michal Hocko
2021-01-26T07:16:50Z

On Mon 25-01-21 23:36:18, Mike Rapoport wrote:
> On Mon, Jan 25, 2021 at 06:01:22PM +0100, Michal Hocko wrote:
> > On Thu 21-01-21 14:27:18, Mike Rapoport wrote:
> > > From: Mike Rapoport <rppt@linux.ibm.com>
> > >
> > > Introduce "memfd_secret" system call with the ability to create memory
> > > areas visible only in the context of the owning process and not mapped not
> > > only to other processes but in the kernel page tables as well.
> > >
> > > The user will create a file descriptor using the memfd_secret() system
> > > call. The memory areas created by mmap() calls from this file descriptor
> > > will be unmapped from the kernel direct map and they will be only mapped in
> > > the page table of the owning mm.
> > >
> > > The secret memory remains accessible in the process context using uaccess
> > > primitives, but it is not accessible using direct/linear map addresses.
> > >
> > > Functions in the follow_page()/get_user_page() family will refuse to return
> > > a page that belongs to the secret memory area.
> > >
> > > A page that was a part of the secret memory area is cleared when it is
> > > freed.
> > >
> > > The following example demonstrates creation of a secret mapping (error
> > > handling is omitted):
> > >
> > > fd = memfd_secret(0);
> > > ftruncate(fd, MAP_SIZE);
> > > ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> >
> > I do not see any access control or permission model for this feature.
> > Is this feature generally safe to anybody?
>
> The mappings obey memlock limit. Besides, this feature should be enabled
> explicitly at boot with the kernel parameter that says what is the maximal
> memory size secretmem can consume.

Why is such a model sufficient and future proof? I mean even when it has
to be enabled by an admin it is still all or nothing approach. Mlock
limit is not really useful because it is per mm rather than per user.

Is there any reason why this is allowed for non-privileged processes?
Maybe this has been discussed in the past but is there any reason why
this cannot be done by a special device which will allow to provide at
least some permission policy?

Please make sure to describe all those details in the changelog.
--
Michal Hocko
SUSE Labs