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

Michal Hocko
2021-01-26T09:49:24Z

On Tue 26-01-21 11:20:11, Mike Rapoport wrote:
> 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.

I do appreciate this has been discussed but this changelog is not
specific on any of that reasoning and I am pretty sure nobody will
remember details in few years in the future. Also some numbers would be
appropriate.

> 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

A good start for the data I am asking above.
--
Michal Hocko
SUSE Labs