Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

[Andy Lutomirski]

On Thu, Apr 17, 2014 at 11:57 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Thu, Apr 17, 2014 at 02:50:23PM -0400, Vivek Goyal wrote:
>> On Thu, Apr 17, 2014 at 02:23:33PM -0400, Simo Sorce wrote:
>> > On Thu, 2014-04-17 at 10:35 -0700, Andy Lutomirski wrote:
>> > > On Thu, Apr 17, 2014 at 10:33 AM, Simo Sorce <ssorce@xxxxxxxxxx> wrote:
>> > > > On Thu, 2014-04-17 at 10:26 -0700, Andy Lutomirski wrote:
>> > > >>
>> > > >> Not really.  write(2) can't send SCM_CGROUP.  Callers of sendmsg(2)
>> > > >> who supply SCM_CGROUP are explicitly indicating that they want their
>> > > >> cgroup associated with that message.  Callers of write(2) and send(2)
>> > > >> are simply indicating that they have some bytes that they want to
>> > > >> shove into whatever's at the other end of the fd.
>> > > >
>> > > > But there is no attack vector that passes by tricking setuid binaries to
>> > > > write to pre-opened file descriptors on sendmsg(), and for the other
>> > > > cases (connected socket) journald can always cross check with
>> > > > SO_PEERCGROUP, so why do we care again ?
>> > >
>> > > Because the proposed code does not do what I described, at least as
>> > > far I as I can tell.
>> >
>> > Ok let me backtrack, apparently if you explicitly use connect() on a
>> > datagram socket then you *can* write() (thanks to Vivek for checking
>> > this).
>> >
>> > So you can trick something to write() to it but you can't do
>> > SO_PEERCGROUP on the other side, because it is not really a connected
>> > socket, the connection is only faked on the sender side by constructing
>> > sendmsg() messages with the original address passed into connect().
>> >
>> > So given this unfortunate circumstance, requiring the client to
>> > explicitly pass cgroup data on unix datagram sockets may be an
>> > acceptable request IMO.
>> >
>> > Perhaps this could be done with a sendmsg() header flag or simplified
>> > ancillary data even, rather than forcing the sender process to retrieve
>> > and construct the whole information which is already available in
>> > kernel.
>>
>> So what would be the protocol here? When should somebody send an
>> SCM_CGROUP message using sendmsg()?
>
> I don't know how it will even be used for systemd logging case. systemd
> provides various ways to connect stdout of services. So say a service's
> stdout is connected to a connected datagram socket and all printf()
> messages to stdout are being logged by receiver in journal. Now how
> would sender know that it is supposed to send SCM_CGROUP? One needs
> to modify printf() now?

Does connecting stdout to a datagram socket really work well?  The
systemd function connect_logger_as looks like it's using stream
sockets, one per service, connected to /run/systemd/journal/stdout.
There's some rather strange logic in journald to authenticate the
thing that connects (using SO_PEERCRED!), but I don't see why this
code would even want to use SCM_CGROUP.

IOW, write(2) issues notwithstanding, I'm still wondering what the use
case for this whole thing is.

>
> Thanks
> Vivek



-- 
Andy Lutomirski
AMA Capital Management, LLC
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