From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>,
Mike Rapoport <rppt@linux.ibm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
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Dan Williams <dan.j.williams@intel.com>,
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Elena Reshetova <elena.reshetova@intel.com>,
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James Bottomley <jejb@linux.ibm.com>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
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x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Thu, 11 Feb 2021 13:07:10 +0100 [thread overview]
Message-ID: <05082284-bd85-579f-2b3e-9b1af663eb6f@redhat.com> (raw)
In-Reply-To: <20210211112702.GI242749@kernel.org>
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
next prev parent reply other threads:[~2021-02-11 12:07 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-08 8:49 [PATCH v17 00/10] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 01/10] mm: add definition of PMD_PAGE_ORDER Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 02/10] mmap: make mlock_future_check() global Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 03/10] riscv/Kconfig: make direct map manipulation options depend on MMU Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 04/10] set_memory: allow set_direct_map_*_noflush() for multiple pages Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 05/10] set_memory: allow querying whether set_direct_map_*() is actually enabled Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 06/10] arm64: kfence: fix header inclusion Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2021-02-08 10:49 ` Michal Hocko
2021-02-08 21:26 ` Mike Rapoport
2021-02-09 8:47 ` Michal Hocko
2021-02-09 9:09 ` Mike Rapoport
2021-02-09 13:17 ` Michal Hocko
2021-02-11 7:13 ` Mike Rapoport
2021-02-11 8:39 ` Michal Hocko
2021-02-11 9:01 ` David Hildenbrand
2021-02-11 9:38 ` Michal Hocko
2021-02-11 9:48 ` David Hildenbrand
2021-02-11 10:02 ` David Hildenbrand
2021-02-11 11:29 ` Mike Rapoport
2021-02-11 11:27 ` Mike Rapoport
2021-02-11 12:07 ` David Hildenbrand [this message]
2021-02-11 23:09 ` Mike Rapoport
2021-02-12 9:18 ` David Hildenbrand
2021-02-14 9:19 ` Mike Rapoport
2021-02-14 9:58 ` David Hildenbrand
2021-02-14 19:21 ` James Bottomley
2021-02-15 9:13 ` Michal Hocko
2021-02-15 18:14 ` James Bottomley
2021-02-15 19:20 ` Michal Hocko
2021-02-16 16:25 ` James Bottomley
2021-02-16 16:34 ` David Hildenbrand
2021-02-16 16:44 ` James Bottomley
2021-02-16 17:16 ` David Hildenbrand
2021-02-17 16:19 ` James Bottomley
2021-02-22 9:38 ` David Hildenbrand
2021-02-22 10:50 ` David Hildenbrand
2021-02-16 16:51 ` Michal Hocko
2021-02-11 11:20 ` Mike Rapoport
2021-02-11 12:30 ` Michal Hocko
2021-02-11 22:59 ` Mike Rapoport
2021-02-12 9:02 ` Michal Hocko
2021-02-08 8:49 ` [PATCH v17 08/10] PM: hibernate: disable when there are active secretmem users Mike Rapoport
2021-02-08 10:18 ` Michal Hocko
2021-02-08 10:32 ` David Hildenbrand
2021-02-08 10:51 ` Michal Hocko
2021-02-08 10:53 ` David Hildenbrand
2021-02-08 10:57 ` Michal Hocko
2021-02-08 11:13 ` David Hildenbrand
2021-02-08 11:14 ` David Hildenbrand
2021-02-08 11:26 ` David Hildenbrand
2021-02-08 12:17 ` Michal Hocko
2021-02-08 13:34 ` Michal Hocko
2021-02-08 13:40 ` David Hildenbrand
2021-02-08 21:28 ` Mike Rapoport
2021-02-22 7:34 ` Matthew Garrett
2021-02-22 10:23 ` Mike Rapoport
2021-02-22 18:27 ` Matthew Garrett
2021-02-22 19:17 ` Dan Williams
2021-02-22 19:21 ` James Bottomley
2021-02-08 8:49 ` [PATCH v17 09/10] arch, mm: wire up memfd_secret system call where relevant Mike Rapoport
2021-02-08 8:49 ` [PATCH v17 10/10] secretmem: test: add basic selftest for memfd_secret(2) Mike Rapoport
2021-02-08 9:27 ` [PATCH v17 00/10] mm: introduce memfd_secret system call to create "secret" memory areas David Hildenbrand
2021-02-08 21:13 ` Mike Rapoport
2021-02-08 21:38 ` David Hildenbrand
2021-02-09 8:59 ` Michal Hocko
2021-02-09 9:15 ` David Hildenbrand
2021-02-09 9:53 ` Michal Hocko
2021-02-09 10:23 ` David Hildenbrand
2021-02-09 10:30 ` David Hildenbrand
2021-02-09 13:25 ` Michal Hocko
2021-02-09 16:17 ` David Hildenbrand
2021-02-09 20:08 ` Michal Hocko
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