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

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Message-ID: <CAK8P3a07jAec4hKyNMcha032TT6OXjYHaZZ4Za9ncDsvapeg8Q@mail.gmail.com>
Date:   Mon, 20 Jul 2020 16:34:12 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Idan Yaniv <idan.yaniv@....com>,
        Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, linux-nvdimm@...ts.01.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linaro-mm-sig@...ts.linaro.org,
        Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH 3/6] mm: introduce secretmemfd system call to create
 "secret" memory areas

On Mon, Jul 20, 2020 at 4:21 PM Mike Rapoport <rppt@...nel.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@...nel.org> wrote:
> > >
> > > From: Mike Rapoport <rppt@...ux.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@...ux.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