On 24.09.20 15:28, Mike Rapoport wrote:
From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Hi,
This is an implementation of "secret" mappings backed by a file descriptor.
I've dropped the boot time reservation patch for now as it is not strictly
required for the basic usage and can be easily added later either with or
without CMA.
Hi Mike,
I'd like to stress again that I'd prefer *any* secretmem allocations
going via CMA as long as these pages are unmovable. The user can
allocate a non-significant amount of unmovable allocations only fenced
by the mlock limit, which behave very different to mlocked pages - they
are not movable for page compaction/migration.
Assume you have a system with quite some ZONE_MOVABLE memory (esp. in
virtualized environments), eating up a significant amount of
!ZONE_MOVABLE memory dynamically at runtime can lead to non-obvious
issues. It looks like you have plenty of free memory, but the kernel
might still OOM when trying to do kernel allocations e.g., for
pagetables. With CMA we at least know what we're dealing with - it
behaves like ZONE_MOVABLE except for the owner that can place unmovable
pages there. We can use it to compute statically the amount of
ZONE_MOVABLE memory we can have in the system without doing harm to the
system.
Ideally, we would want to support page migration/compaction and allow
for allocation from ZONE_MOVABLE as well. Would involve temporarily
mapping, copying, unmapping. Sounds feasible, but not sure which
roadblocks we would find on the way.
[...]
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.
For demonstration of secret memory usage we've created a userspace library
[1] that does two things: the first is act as a preloader for openssl to
redirect all the OPENSSL_malloc calls to secret memory meaning any secret
keys get automatically protected this way and the other thing it does is
expose the API to the user who needs it. We anticipate that a lot of the
use cases would be like the openssl one: many toolkits that deal with
secret keys already have special handling for the memory to try to give
them greater protection, so this would simply be pluggable into the
toolkits without any need for user application modification.
I've hesitated whether to continue to use new flags to memfd_create() or to
add a new system call and I've decided to use a new system call after I've
started to look into man pages update. There would have been two completely
independent descriptions and I think it would have been very confusing.
This was also raised on lwn.net by "dullfire" [1]. I do wonder if it
would be the right place as well.
[1] https://lwn.net/Articles/835342/#Comments
Hiding secret memory mappings behind an anonymous file allows (ab)use of
the page cache for tracking pages allocated for the "secret" mappings as
well as using address_space_operations for e.g. page migration callbacks.
The anonymous file may be also used implicitly, like hugetlb files, to
implement mmap(MAP_SECRET) and use the secret memory areas with "native" mm
ABIs in the future.
As the fragmentation of the direct map was one of the major concerns raised
during the previous postings, I've added an amortizing cache of PMD-size
pages to each file descriptor that is used as an allocation pool for the
secret memory areas.
--
Thanks,
David / dhildenb
