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

Michal Hocko
2021-01-26T09:20:52Z

On Tue 26-01-21 10:00:14, Michal Hocko wrote:
> On Tue 26-01-21 10:33:11, Mike Rapoport wrote:
> > 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.
>
> Because appart from the reclaim aspect it fragments the direct mapping
> IIUC. That might have an impact on all others, right?

Also forgot to mention that you rely on a contiguous allocations and
that can become a very scarce resource so what does prevent one abuser
from using it all and deny the access to others. And unless I am missing
something allocation failure would lead to OOM which cannot really help
because the oom killer cannot compensate for the CMA reservation.
--
Michal Hocko
SUSE Labs