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 -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
