Re: [PATCH v4 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

[Mike Rapoport]

On Wed, Aug 19, 2020 at 12:47:54PM +0200, David Hildenbrand wrote:
> On 18.08.20 16:15, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > Hi,
> > 
> > This is an implementation of "secret" mappings backed by a file descriptor. 
> > 
> > v4 changes:
> > * rebase on v5.9-rc1
> > * Do not redefine PMD_PAGE_ORDER in fs/dax.c, thanks Kirill
> > * Make secret mappings exclusive by default and only require flags to
> >   memfd_secret() system call for uncached mappings, thanks again Kirill :)
> > 
> > v3 changes:
> > * Squash kernel-parameters.txt update into the commit that added the
> >   command line option.
> > * Make uncached mode explicitly selectable by architectures. For now enable
> >   it only on x86.
> > 
> > v2 changes:
> > * Follow Michael's suggestion and name the new system call 'memfd_secret'
> > * Add kernel-parameters documentation about the boot option
> > * Fix i386-tinyconfig regression reported by the kbuild bot.
> >   CONFIG_SECRETMEM now depends on !EMBEDDED to disable it on small systems
> >   from one side and still make it available unconditionally on
> >   architectures that support SET_DIRECT_MAP.
> > 
> > 
> > The file descriptor backing secret memory mappings is created using a
> > dedicated memfd_secret system call The desired protection mode for the
> > memory is configured using flags parameter of the system call. The mmap()
> > of the file descriptor created with memfd_secret() will create a "secret"
> > memory mapping. The pages in that mapping will be marked as not present in
> > the direct map and will have desired protection bits set in the user page
> > table. For instance, current implementation allows uncached mappings.
> > 
> > Although normally Linux userspace mappings are protected from other users, 
> > such secret mappings are useful for environments where a hostile tenant is
> > trying to trick the kernel into giving them access to other tenants
> > mappings.
> > 
> > Additionally, the secret mappings may be used as a mean to protect guest
> > memory in a virtual machine host.
> > 
> 
> Just a general question. I assume such pages (where the direct mapping
> was changed) cannot get migrated - I can spot a simple alloc_page(). So
> essentially a process can just allocate a whole bunch of memory that is
> unmovable, correct? Is there any limit? Is it properly accounted towards
> the process (memctl) ?

The memory as accounted in the same way like with mlock(), so normal
user won't be able to allocate more than RLIMIT_MEMLOCK.

> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.