Re: [PATCH 3/6] mm: introduce secretmemfd system call to create "secret" memory areas

Arnd Bergmann
2020-07-20T14:34:36Z

On Mon, Jul 20, 2020 at 4:21 PM Mike Rapoport <rppt@kernel.org> wrote:
> On Mon, Jul 20, 2020 at 01:30:13PM +0200, Arnd Bergmann wrote:
> > On Mon, Jul 20, 2020 at 11:25 AM Mike Rapoport <rppt@kernel.org> wrote:
> > >
> > > From: Mike Rapoport <rppt@linux.ibm.com>
> > >
> > > Introduce "secretmemfd" 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 secretmemfd 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.
> > > * uncached - the memory area is present only in the page tables of the
> > > owning mm and it is mapped there as uncached.
> > >
> > > For instance, the following example will create an uncached mapping (error
> > > handling is omitted):
> > >
> > > fd = secretmemfd(SECRETMEM_UNCACHED);
> > > ftruncate(fd, MAP_SIZE);
> > > ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED,
> > > fd, 0);
> > >
> > > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> >
> > I wonder if this should be more closely related to dmabuf file
> > descriptors, which
> > are already used for a similar purpose: sharing access to secret memory areas
> > that are not visible to the OS but can be shared with hardware through device
> > drivers that can import a dmabuf file descriptor.
>
> TBH, I didn't think about dmabuf, but my undestanding is that is this
> case memory areas are not visible to the OS because they are on device
> memory rather than normal RAM and when dmabuf is backed by the normal
> RAM, the memory is visible to the OS.

No, dmabuf is normally about normal RAM that is shared between multiple
devices, the idea is that you can have one driver allocate a buffer in RAM
and export it to user space through a file descriptor. The application can then
go and mmap() it or pass it into one or more other drivers.

This can be used e.g. for sharing a buffer between a video codec and the
gpu, or between a crypto engine and another device that accesses
unencrypted data while software can only observe the encrypted version.

Arnd