Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v19 5/8] mm: introduce memfd_secret system call to create "secret" memory areas | Date | Fri, 14 May 2021 10:50:55 +0200 |
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On 13.05.21 20:47, Mike Rapoport wrote: > From: Mike Rapoport <rppt@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
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