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

Mike Rapoport
2021-01-26T08:33:50Z

On Tue, Jan 26, 2021 at 08:16:14AM +0100, Michal Hocko wrote:
> 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?

Why this should not be allowed for non-privileged processes? This behaves
similarly to mlocked memory, so I don't see a reason why secretmem should
have different permissions model.

> Please make sure to describe all those details in the changelog.

--
Sincerely yours,
Mike.