On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote:

On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote:

quoted

On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote:

quoted

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
where flags supplied as a parameter to this system call will
define
the
desired protection mode for the memory associated with that file
descriptor.

  Currently there are two protection modes:

* exclusive - the memory area is unmapped from the kernel direct
map
and it
               is present only in the page tables of the owning
mm.

Seems like there were some concerns raised around direct map
efficiency, but in case you are going to rework this...how does
this
memory work for the existing kernel functionality that does things
like
this?

get_user_pages(, &page);
ptr = kmap(page);
foo = *ptr;

Not sure if I'm missing something, but I think apps could cause the
kernel to access a not-present page and oops.

The idea is that this memory should not be accessible by the kernel,
so
the sequence you describe should indeed fail.

Probably oops would be to noisy and in this case the report needs to
be
less verbose.

I was more concerned that it could cause kernel instabilities.

I see, so it should not be accessed even at the userspace address? I
wonder if it should be prevented somehow then. At least
get_user_pages() should be prevented I think. Blocking copy_*_user()
access might not be simple.

I'm also not so sure that a user would never have any possible reason
to copy data from this memory into the kernel, even if it's just
convenience. In which case a user setup could break if a specific
kernel implementation switched to get_user_pages()/kmap() from using
copy_*_user(). So seems maybe a bit thorny without fully blocking
access from the kernel, or deprecating that pattern.

You should probably call out these "no passing data to/from the kernel"
expectations, unless I missed them somewhere.