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

[David Hildenbrand]

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) ?

-- 
Thanks,

David / dhildenb