[PATCH v19 5/8] mm: introduce memfd_secret system call to create "secret" memory areas

[David Hildenbrand]

On 13.05.21 20:47, Mike Rapoport wrote:
> From: Mike Rapoport <rppt at 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 secretmem feature is off by default and the user must explicitly
> enable it at the boot time.
> 
> Once secretmem is enabled, the user will be able to 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 processes that have access to the file descriptor.
> 
> The file descriptor based memory has several advantages over the 
> "traditional" mm interfaces, such as mlock(), mprotect(), madvise().
> File descriptor approach allows explict and controlled sharing of the
> memory

s/explict/explicit/

> areas, it allows to seal the operations. Besides, file descriptor
> based memory paves the way for VMMs to remove the secret memory range
> from the userpace hipervisor process, for instance QEMU. Andy
> Lutomirski says:

s/userpace hipervisor/userspace hypervisor/

> 
> "Getting fd-backed memory into a guest will take some possibly major
> work in the kernel, but getting vma-backed memory into a guest
> without mapping it in the host user address space seems much, much
> worse."
> 
> memfd_secret() is made a dedicated system call rather than an
> extention to

s/extention/extension/

> memfd_create() because it's purpose is to allow the user to create
> more secure memory mappings rather than to simply allow file based
> access to the memory. Nowadays a new system call cost is negligible
> while it is way simpler for userspace to deal with a clear-cut system
> calls than with a multiplexer or an overloaded syscall. Moreover, the
> initial implementation of memfd_secret() is completely distinct from
> memfd_create() so there is no much sense in overloading
> memfd_create() to begin with. If there will be a need for code
> sharing between these implementation it can be easily achieved
> without a need to adjust user visible APIs.
> 
> The secret memory remains accessible in the process context using
> uaccess primitives, but it is not exposed to the kernel otherwise;
> secret memory areas are removed from the direct map and functions in
> the follow_page()/get_user_page() family will refuse to return a page
> that belongs to the secret memory area.
> 
> Once there will be a use case that will require exposing secretmem to
> the kernel it will be an opt-in request in the system call flags so
> that user would have to decide what data can be exposed to the
> kernel.

Maybe spell out an example: like page migration.

> 
> Removing of the pages from the direct map may cause its fragmentation
> on architectures that use large pages to map the physical memory
> which affects the system performance. However, the original Kconfig
> text for CONFIG_DIRECT_GBPAGES said that gigabyte pages in the direct
> map "... can improve the kernel's performance a tiny bit ..." (commit
> 00d1c5e05736 ("x86: add gbpages switches")) and the recent report [1]
> showed that "... although 1G mappings are a good default choice,
> there is no compelling evidence that it must be the only choice".
> Hence, it is sufficient to have secretmem disabled by default with
> the ability of a system administrator to enable it at boot time.

Maybe add a link to the Intel performance evaluation.

> 
> Pages in the secretmem regions are unevictable and unmovable to
> avoid accidental exposure of the sensitive data via swap or during
> page migration.
> 
> Since the secretmem mappings are locked in memory they cannot exceed 
> RLIMIT_MEMLOCK. Since these mappings are already locked independently
> from mlock(), an attempt to mlock()/munlock() secretmem range would
> fail and mlockall()/munlockall() will ignore secretmem mappings.

Maybe add something like "similar to pages pinned by VFIO".

> 
> However, unlike mlock()ed memory, secretmem currently behaves more
> like long-term GUP: secretmem mappings are unmovable mappings
> directly consumed by user space. With default limits, there is no
> excessive use of secretmem and it poses no real problem in
> combination with ZONE_MOVABLE/CMA, but in the future this should be
> addressed to allow balanced use of large amounts of secretmem along
> with ZONE_MOVABLE/CMA.
> 
> A page that was a part of the secret memory area is cleared when it
> is freed to ensure the data is not exposed to the next user of that
> page.

You could skip that with init_on_free (and eventually also with 
init_on_alloc) set to avoid double clearing.

> 
> 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);
> 
> [1]
> https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/

[my mail client messed up the remainder of the mail for whatever reason, 
will comment in a separate mail if there is anything to comment :) ]

-- 
Thanks,

David / dhildenb