Re: [LSF/MM/BPF TOPIC] Restricted kernel address spaces

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Feb 06, 2020 at 06:59:00PM +0200, Mike Rapoport wrote:
> 
> Restricted mappings in the kernel mode may improve mitigation of hardware
> speculation vulnerabilities and minimize the damage exploitable kernel bugs
> can cause.
> 
> There are several ongoing efforts to use restricted address spaces in
> Linux kernel for various use cases:
> * speculation vulnerabilities mitigation in KVM [1]
> * support for memory areas visible only in a single owning context, or more
>   generically, a memory areas with more restrictive protection that the
>   defaults ("secret" memory) [2], [3], [4]
> * hardening of the Linux containers [ no reference yet :) ]
> 
> Last year we had vague ideas and possible directions, this year we have
> several real challenges and design decisions we'd like to discuss:
> 
> * "Secret" memory userspace APIs
> 
>   Should such API follow "native" MM interfaces like mmap(), mprotect(),
>   madvise() or it would be better to use a file descriptor , e.g. like
>   memfd-create does?

I don't really see a point in such file-descriptor. It suppose to be very
private secret data. What functionality that provide a file descriptor do
you see valuable in this scenario?

File descriptor makes it easier to spill the secrets to other process: over
fork(), UNIX socket or via /proc/PID/fd/.

>   MM "native" APIs would require VM_something flag and probably a page flag
>   or page_ext. With file-descriptor VM_SPECIAL and custom implementation of
>   .mmap() and .fault() would suffice. On the other hand, mmap() and
>   mprotect() seem better fit semantically and they could be more easily
>   adopted by the userspace.

You mix up implementation and interface. You can provide an interface which
doesn't require a file descriptor, but still use a magic file internally to
the VMA distinct.

> * Direct/linear map fragmentation
> 
>   Whenever we want to drop some mappings from the direct map or even change
>   the protection bits for some memory area, the gigantic and huge pages
>   that comprise the direct map need to be broken and there's no THP for the
>   kernel page tables to collapse them back. Moreover, the existing API
>   defined in <asm/set_memory.h> by several architectures do not really
>   presume it would be widely used.
> 
>   For the "secret" memory use-case the fragmentation can be minimized by
>   caching large pages, use them to satisfy smaller "secret" allocations and
>   than collapse them back once the "secret" memory is freed. Another
>   possibility is to pre-allocate physical memory at boot time.

I would rather go with pre-allocation path. At least at first. We always
can come up with more dynamic and complicated solution later if the
interface would be wildly adopted.

>   Yet another idea is to make page allocator aware of the direct map layout.
> 
> * Kernel page table management
> 
>   Currently we presume that only one kernel page table exists (well,
>   mostly) and the page table abstraction is required only for the user page
>   tables. As such, we presume that 'page table == struct mm_struct' and the
>   mm_struct is used all over by the operations that manage the page tables.
> 
>   The management of the restricted address space in the kernel requires
>   ability to create, update and remove kernel contexts the same way we do
>   for the userspace.
> 
>   One way is to overload the mm_struct, like EFI and text poking did. But
>   it is quite an overkill, because most of the mm_struct contains
>   information required to manage user mappings.

In what way is it overkill? Just memory overhead? How many of such
contexts do you expect to see in the system?

>   My suggestion is to introduce a first class abstraction for the page
>   table and then it could be used in the same way for user and kernel
>   context management. For now I have a very basic POC that slitted several
>   fields from the mm_struct into a new 'struct pg_table' [5]. This new
>   abstraction can be used e.g. by PTI implementation of the page table
>   cloning and the KVM ASI work.
> 
> 
> [1] https://lore.kernel.org/lkml/1557758315-12667-1-git-send-email-alexandre.chartre@oracle.com/
> [2] https://lore.kernel.org/lkml/20190612170834.14855-1-mhillenb@amazon.de/
> [3] https://lore.kernel.org/lkml/1572171452-7958-1-git-send-email-rppt@kernel.org/
> [4] https://lore.kernel.org/lkml/20200130162340.GA14232@rapoport-lnx/
> [5] https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=pg_table/v0.0
> 
> -- 
> Sincerely yours,
> Mike.
> 
> 

-- 
 Kirill A. Shutemov