Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

On Wed, Nov 04, 2020 at 12:39:13PM +0100, Hagen Paul Pfeifer wrote:

quoted

On 11/03/2020 5:30 PM Mike Rapoport [off-list ref] wrote:

quoted

quoted

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?

Yes, this will work. The processes that share the memfd_secret file
descriptor will have access to the same memory pages, pretty much like
with shared memory.

Hagen

-- 
Sincerely yours,
Mike.

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