Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

From: David Hildenbrand
Date: Thu Feb 11 2021 - 07:12:48 EST

On 11.02.21 12:27, Mike Rapoport wrote:
On Thu, Feb 11, 2021 at 10:01:32AM +0100, David Hildenbrand wrote:
On 11.02.21 09:39, Michal Hocko wrote:
On Thu 11-02-21 09:13:19, Mike Rapoport wrote:
On Tue, Feb 09, 2021 at 02:17:11PM +0100, Michal Hocko wrote:
On Tue 09-02-21 11:09:38, Mike Rapoport wrote:
[...]
Citing my older email:

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.

Could you elaborate? Unmapping from the kernel address space can work
both for sealed or hugetlb memfds, no? Those features are completely
orthogonal AFAICS. With a dedicated syscall you will need to introduce
this functionality on top if that is required. Have you considered that?
I mean hugetlb pages are used to back guest memory very often. Is this
something that will be a secret memory usecase?

Please be really specific when giving arguments to back a new syscall
decision.

Isn't "syscalls have completely independent description" specific enough?

No, it's not as you can see from questions I've had above. More on that
below.

We are talking about API here, not the implementation details whether
secretmem supports large pages or not.

The purpose of memfd_create() is to create a file-like access to memory.
The purpose of memfd_secret() is to create a way to access memory hidden
from the kernel.

I don't think overloading memfd_create() with the secretmem flags because
they happen to return a file descriptor will be better for users, but
rather will be more confusing.

This is quite a subjective conclusion. I could very well argue that it
would be much better to have a single syscall to get a fd backed memory
with spedific requirements (sealing, unmapping from the kernel address
space). Neither of us would be clearly right or wrong. A more important
point is a future extensibility and usability, though. So let's just
think of few usecases I have outlined above. Is it unrealistic to expect
that secret memory should be sealable? What about hugetlb? Because if
the answer is no then a new API is a clear win as the combination of
flags would never work and then we would just suffer from the syscall
multiplexing without much gain. On the other hand if combination of the
functionality is to be expected then you will have to jam it into
memfd_create and copy the interface likely causing more confusion. See
what I mean?

I by no means do not insist one way or the other but from what I have
seen so far I have a feeling that the interface hasn't been thought
through enough. Sure you have landed with fd based approach and that
seems fair. But how to get that fd seems to still have some gaps IMHO.


I agree with Michal. This has been raised by different
people already, including on LWN (https://lwn.net/Articles/835342/).

I can follow Mike's reasoning (man page), and I am also fine if there is
a valid reason. However, IMHO the basic description seems to match quite good:

memfd_create() creates an anonymous file and returns a file descriptor that refers to it. The
file behaves like a regular file, and so can be modified, truncated, memory-mapped, and so on.
However, unlike a regular file, it lives in RAM and has a volatile backing storage. Once all
references to the file are dropped, it is automatically released. Anonymous memory is used
for all backing pages of the file. Therefore, files created by memfd_create() have the same
semantics as other anonymous memory allocations such as those allocated using mmap(2) with the
MAP_ANONYMOUS flag.

Even despite my laziness and huge amount of copy-paste you can spot the
differences (this is a very old version, update is due):

memfd_secret() creates an anonymous file and returns a file descriptor
that refers to it. The file can only be memory-mapped; the memory in
such mapping will have stronger protection than usual memory mapped
files, and so it can be used to store application secrets. Unlike a
regular file, a file created with memfd_secret() lives in RAM and has a
volatile backing storage. Once all references to the file are dropped,
it is automatically released. The initial size of the file is set to
0. Following the call, the file size should be set using ftruncate(2).

The memory areas obtained with mmap(2) from the file descriptor are ex‐
clusive to the owning context. These areas are removed from the kernel
page tables and only the page table of the process holding the file de‐
scriptor maps the corresponding physical memory.

So let's talk about the main user-visible differences to other memfd files (especially, other purely virtual files like hugetlbfs). With secretmem:

- File content can only be read/written via memory mappings.
- File content cannot be swapped out.

I think there are still valid ways to modify file content using syscalls: e.g., fallocate(PUNCH_HOLE). Things like truncate also seems to work just fine.

What else?


AFAIKS, we would need MFD_SECRET and disallow
MFD_ALLOW_SEALING and MFD_HUGETLB.

So here we start to multiplex.

Yes. And as Michal said, maybe we can support combinations in the future.


In addition, we could add MFD_SECRET_NEVER_MAP, which could disallow any kind of
temporary mappings (eor migration). TBC.

Never map is the default. When we'll need to map we'll add an explicit flag
for it.

No strong opinion. (I'd try to hurt the kernel less as default)

--
Thanks,

David / dhildenb