On 19.08.20 13:42, Mike Rapoport wrote:
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(a)linux.ibm.com>
>> This is an implementation of "secret" mappings backed by a file
>> 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
>> * 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
>> 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
>> 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.
Okay, thanks. AFAIU the difference to mlock() is that the pages here are
not movable, fragment memory, and limit compaction. Hm.
David / dhildenb