[pulseaudio-discuss] [PATCH v2 00/12] Introduce memfd support

Fri Feb 12 15:58:43 UTC 2016

On 2016-02-12 16:04, Ahmed S. Darwish wrote:
> On Fri, Feb 12, 2016 at 11:57:00AM +0100, David Henningsson wrote:
>>
>> On 2016-02-12 00:59, Ahmed S. Darwish wrote:
>>> Hello!
>>
>> Hi and thanks for working on it!
>>
>> Having skimmed through the patches, I'm still not convinced w r t auto-send
>> mechanism of the memfds.
>>
>> E g, consider we have clients 1 and 2, and client 1 plays back audio to a
>> sink. Client 2 monitors the sink-input of client 1 (like parec does if you
>> specify --monitor-stream). Could this cause the memfd used to communicate
>> with client 1 to also be shared with client 2? If so, that's a security
>> breach.
>>
>> The mechanism seems fragile to such breaches but I'm not sure, maybe I'm
>> wrong and you can explain what would happen instead?
>>
>
> Thanks for the quick response and patch reviews ;-)
>
> We've just discussed this over IRC, so I'll detail things here
> for archival purposes. This might also be useful to add somewhere
> in the code or in future changelogs..

Thanks for the explanations! This is a good summary.

>
> - We now have 3 mempools in the system: a global mempool, and 2
>    per-client mempools. One created by the client for passing
>    playback audio. One created by the server for srbchannels.
>
> - For any per-client memfd mempool, the file descriptors are
>    instaneously closed after sending it to other side. The
>    receiving end also instantaneously close all received file
>    descriptors after doing an mmap().
>
>    Thus we have no risks of data leaks in that case. The 'secret'
>    shared by two PA endpoints is directly discarded after use.
>
> - A special case is the global server-wide mempool. Its fd is
>    kept open by the server, so whenever a new client connects, it
>    passes it the fd for memfd-fd<->SHM-ID negotiation.
>
>    Even in this case, communication is then done using IDs and
>    no further FDs are passed. The receiving end also does not
>    distinguish between per-client and global mempools and directly
>    close the fd after doing an mmap().
>
> - A question then arises: as was done with srbchannels, why not
>    transform the global mempool to a per-client one?
>
>    This is planned, but is to be done in a follow-up project. The
>    global mempool is touched *everywhere* in the system -- in
>    quite different manners and usage scenarioes. It's also used
>    by a huge set of modules, including quite esoteric ones.
>
>    Touching this will require extensive testing for each affected
>    part. So this will be quite a HUGE patch series of it own,
>    possibly done in 10 patches by 10 patches chunks.

Hmm. I'm thinking, to get the security without 100 patches first, we can 
start by not sharing the global mempool with the clients. That way, it 
will fallback to going over the socket. Which might mean an extra 
memcpy, even if that socket is an srbchannel. But still, it would be 
secure. Right?

Then, we can work on cleaning the global mempool up, by fixing modules 
one by one, the commonly used ones (such as the ALSA source modules) 
first. Indeed now we'll have to copy memory to each 
source_output->client mempool instead of to just one global mempool, so 
that will be a change.

>
>    But when it's done, we'll have all the necessary infrastructure
>    to directly secure it.
>
>    For now, we can completely disable posix SHM and things should
>    function as expected. This is a win.. yes, it's incomplete
>    until we personalize the global mempool too, but it's still a
>    step in the right direction. The memfd code path will also be
>    heavily tested in the process.
>
> Thanks,
> Darwish
>
>>>
>>> The all-improved memfd patch series ;-)
>>>
>>> ==> v1 cover letter:
>>>
>>> Memfd is a simple memory sharing mechanism, added by the systemd/kdbus
>>> developers, to share pages between processes in an anonymous, no
>>> global registry needed, no mount-point required, relatively secure,
>>> manner.
>>>
>>> This patch series introduces memfd support to PulseAudio, laying out
>>> the necessary (but not yet sufficient) groundwork for sandboxing,
>>> protecting PulseAudio from its clients, and protecting clients from
>>> each other.
>>>
>>> Memfd support is added in quite a transparent manner, respecting
>>> current PA mechanisms and abstractions. The lower-level layers are
>>> properly refactored and extended: the srbchannel communication path
>>> is transformed to memfds by only changing a single line of code.
>>>
>>> ==> v2 changes:
>>>
>>> - All pools (srbchannel + client audio data mempool + global mempool)
>>>    now use memfds by default. An empty /dev/shm FTW! ;-)
>>>
>>> - Design issues pointed by David now fixed, leading to a much smaller
>>>    code.. New memfd implementation now shares almost all of the
>>>    existing posix SHM code paths.
>>>
>>> - Heavy focus on not leaking any file descriptors throughout the
>>>    code. Every fd, sent and received, has a clear owner responsible
>>>    for closing it at appropriate times.
>>>
>>> - If a mempool is memfd-backed, we now require registering it with
>>>    the pstream before sending any blocks it covers. This is done to
>>>    to minimize fd passing overhead and the possibility of fd leaks.
>>>
>>> - A new, special, pstream packet type is introduced. That special
>>>    packet can be sent anytime during the pstrem's lifetime and is
>>>    used for creating on demand SHM ID to memfd mappings.
>>>
>>> - All issues pointed by Tanu's very detailed review (vacuuming,
>>>    memfd enablement over pstream, protocol changes, etc.) now fixed
>>>
>>> - Testing! I've been heavily using these patches now for three
>>>    weeks without visible issues.
>>>
>>> ==> references:
>>>
>>> - v1 submission
>>>    http://thread.gmane.org/gmane.comp.audio.pulseaudio.general/24110
>>>
>>> - memfd_create(2), David Herrmann [memfd author]
>>>    https://dvdhrm.wordpress.com/2014/06/10/memfd_create2/
>>>
>>> - xdg-app
>>>    https://wiki.gnome.org/Projects/SandboxedApps
>>>
>>> ==> diffstat:
>>>
>>> Ahmed S. Darwish (12):
>>>    pulsecore: Cache daemon shm size inside pa_core
>>>    pulsecore: srbchannel: Introduce per-client SHM files
>>>    pulsecore: Transform pa_mempool_new() into a factory method
>>>    pulsecore: Split pa_shm mempool backend into pa_shm and pa_privatemem
>>>    pulsecore: SHM: Introduce memfd support
>>>    pulsecore: memexport/memimport: Support memfd blocks
>>>    pulsecore, tests: Use new memexport/memimport API
>>>    pulsecore: Specially mark global mempools
>>>    pulsecore: pstream: Support memfd blocks transport
>>>    srbchannel: Use memfd transport by default; pump protocol version
>>>    pulse: client audio: Use memfd transport by default
>>>    core: Use memfd transport by default
>>>
>>>   PROTOCOL                         |  13 ++
>>>   configure.ac                     |  21 +++-
>>>   man/pulse-client.conf.5.xml.in   |   5 +
>>>   man/pulse-daemon.conf.5.xml.in   |   5 +
>>>   man/pulseaudio.1.xml.in          |  11 ++
>>>   shell-completion/bash/pulseaudio |   4 +-
>>>   shell-completion/zsh/_pulseaudio |   1 +
>>>   src/Makefile.am                  |   7 ++
>>>   src/daemon/cmdline.c             |  13 +-
>>>   src/daemon/daemon-conf.c         |   1 +
>>>   src/daemon/daemon-conf.h         |   1 +
>>>   src/daemon/main.c                |   4 +-
>>>   src/pulse/client-conf.c          |   1 +
>>>   src/pulse/client-conf.h          |   2 +-
>>>   src/pulse/context.c              |  48 +++++++-
>>>   src/pulse/internal.h             |   2 +
>>>   src/pulsecore/core.c             |  31 +++--
>>>   src/pulsecore/core.h             |  13 +-
>>>   src/pulsecore/mem.h              |  67 ++++++++++
>>>   src/pulsecore/memblock.c         | 245 ++++++++++++++++++++++++++++++++-----
>>>   src/pulsecore/memblock.h         |  18 ++-
>>>   src/pulsecore/memfd-wrappers.h   |  68 +++++++++++
>>>   src/pulsecore/privatemem.c       |  78 ++++++++++++
>>>   src/pulsecore/privatemem.h       |  35 ++++++
>>>   src/pulsecore/protocol-native.c  |  92 ++++++++++++--
>>>   src/pulsecore/pstream.c          | 257 +++++++++++++++++++++++++++++++++++----
>>>   src/pulsecore/pstream.h          |   2 +
>>>   src/pulsecore/shm.c              | 247 +++++++++++++++++++------------------
>>>   src/pulsecore/shm.h              |  33 ++++-
>>>   src/tests/cpu-mix-test.c         |   2 +-
>>>   src/tests/lfe-filter-test.c      |   2 +-
>>>   src/tests/mcalign-test.c         |   2 +-
>>>   src/tests/memblock-test.c        |  15 +--
>>>   src/tests/memblockq-test.c       |   2 +-
>>>   src/tests/mix-test.c             |   2 +-
>>>   src/tests/remix-test.c           |   2 +-
>>>   src/tests/resampler-test.c       |   2 +-
>>>   src/tests/srbchannel-test.c      |   2 +-
>>>   38 files changed, 1119 insertions(+), 237 deletions(-)
>>>   create mode 100644 src/pulsecore/mem.h
>>>   create mode 100644 src/pulsecore/memfd-wrappers.h
>>>   create mode 100644 src/pulsecore/privatemem.c
>>>   create mode 100644 src/pulsecore/privatemem.h
>>>
>>> Regards,
>>>
>>> --
>>> Darwish
>>> http://darwish.chasingpointers.com
>>>
>>
>> --
>> David Henningsson, Canonical Ltd.
>> https://launchpad.net/~diwic
>

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic