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

Mike Rapoport
2021-01-26T09:20:57Z

On Tue, Jan 26, 2021 at 10:00:13AM +0100, 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?

It does fragment the direct map, but first it only splits 1G pages to 2M
pages and as was discussed several times already it's not that clear which
page size in the direct map is the best and this is very much workload
dependent.

These are the results of the benchmarks I've run with the default direct
mapping covered with 1G pages, with disabled 1G pages using "nogbpages" in
the kernel command line and with the entire direct map forced to use 4K
pages using a simple patch to arch/x86/mm/init.c.

https://docs.google.com/spreadsheets/d/1tdD-cu8e93vnfGsTFxZ5YdaEfs2E1GELlvWNOGkJV2U/edit?usp=sharing

--
Sincerely yours,
Mike.