Re: [PATCH v16 06/11] mm: introduce memfd_secret system call to create "secret" memory areas

Michal Hocko
2021-01-25T17:01:48Z

On Thu 21-01-21 14:27:18, 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 user will 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 owning mm.
>
> The secret memory remains accessible in the process context using uaccess
> primitives, but it is not accessible using direct/linear map addresses.
>
> Functions in the follow_page()/get_user_page() family will refuse to return
> a page that belongs to the secret memory area.
>
> A page that was a part of the secret memory area is cleared when it is
> freed.
>
> 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);

I do not see any access control or permission model for this feature.
Is this feature generally safe to anybody?
--
Michal Hocko
SUSE Labs