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Date:   Wed, 4 Nov 2020 12:39:13 +0100 (CET)
From:   Hagen Paul Pfeifer <hagen@jauu.net>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Idan Yaniv <idan.yaniv@ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
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References: <20200924132904.1391-1-rppt@kernel.org>
 <20201101110935.GA4105325@laniakea> <20201102154028.GD4879@kernel.org>
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Subject: Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create
 "secret" memory areas
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> On 11/03/2020 5:30 PM Mike Rapoport <rppt@kernel.org> wrote:
> 
> > > As long as the task share the file descriptor, they can share the
> > > secretmem pages, pretty much like normal memfd.
> > 
> > Including process_vm_readv() and process_vm_writev()? Let's take a hypothetical
> > "dbus-daemon-secure" service that receives data from process A and wants to
> > copy/distribute it to data areas of N other processes. Much like dbus but without
> > SOCK_DGRAM rather direct copy into secretmem/mmap pages (ring-buffer). Should be
> > possible, right?
> 
> I'm not sure I follow you here.
> For process_vm_readv() and process_vm_writev() secremem will be only
> accessible on the local part, but not on the remote.
> So copying data to secretmem pages using process_vm_writev wouldn't
> work.

A hypothetical "dbus-daemon-secure" service will not be *process related* with communication
peers. E.g. a password-input process (reading a password into secured-memory page) will
transfer the password to dbus-daemon-secure and this service will hand-over the password to
two additional applications: a IPsec process on CPU0 und CPU1 (which itself use a
secured-memory page).

So four applications IPC chain:
 password-input -> dbus-daemon-secure -> {IPsec0, IPsec1}

- password-input: uses a secured page to read/save the password locally after reading from TTY
- dbus-daemon-secure: uses a secured page for IPC (legitimate user can write and read into the secured page)
- IPSecN has secured page to save the password locally (and probably other data as well), IPC memory is memset'ed after copy

Goal: the whole password is never saved/touched on non secured pages during IPC transfer.

Question: maybe a *file-descriptor passing* mechanism can do the trick? I.e. dbus-daemon-secure
allocates via memfd_secret/mmap secure pages and permitted processes will get the descriptor/mmaped-page
passed so they can use the pages directly?

Hagen