From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boaz Harrosh Subject: Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced Date: Thu, 23 May 2013 15:25:20 +0300 Message-ID: <519E0AB0.7040704@panasas.com> References: <20130522072840.27720.85023.stgit@localhost.localdomain> <878v36ex6n.fsf@xmission.com> <519DCE5D.6070204@parallels.com> <87k3mq9fsu.fsf@xmission.com> <519DF109.9010309@parallels.com> <20130523073108.13afafa6@tlielax.poochiereds.net> <519DFFA9.3010606@parallels.com> <20130523075620.21abf79a@tlielax.poochiereds.net> <519E0474.5000606@parallels.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Jeff Layton , "Eric W. Biederman" , , , , , , , , , , To: Stanislav Kinsbursky Return-path: In-Reply-To: <519E0474.5000606@parallels.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 23/05/13 14:58, Stanislav Kinsbursky wrote: > 23.05.2013 15:56, Jeff Layton =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >> On Thu, 23 May 2013 15:38:17 +0400 >> Stanislav Kinsbursky wrote: >> >>> 23.05.2013 15:31, Jeff Layton =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>>> On Thu, 23 May 2013 14:35:53 +0400 >>>> Stanislav Kinsbursky wrote: >>>> >>>>> 23.05.2013 14:00, Eric W. Biederman =D0=BF=D0=B8=D1=88=D0=B5=D1=82= : >>>>>> Stanislav Kinsbursky writes: >>>>>> >>>>>>> 22.05.2013 21:33, Eric W. Biederman =D0=BF=D0=B8=D1=88=D0=B5=D1= =82: >>>>>>>> Stanislav Kinsbursky writes: >>>>>>>> >>>>>>>>> Usermode helper executes all binaries in global "init" root c= ontext. This >>>>>>>>> doesn't allow to call a binary from other root context (for e= xample in a >>>>>>>>> container). >>>>>>>>> Currently, both containerized NFS client and NFS server requi= res an ability to >>>>>>>>> execute a binary in a container's root context. Root swap can= be done in >>>>>>>>> "init" callback, passed by UMH caller. >>>>>>>>> But since we have 2 callers already (and more of them are exp= ected to appear >>>>>>>>> in future) and because set_fs_root() in not exported, it look= s reasonable to >>>>>>>>> add one more generic UMH helper to generic fs code. >>>>>>>>> Root path reference must be hold by the caller, since it will= be put on UMH >>>>>>>>> thread exit. >>>>>>>> >>>>>>>> Awesome. With this patch as an uprivilieged user I get to pic= k which >>>>>>>> binary the kernel will execute. At least if nfs and nfsd ever= runs in a >>>>>>>> user namespace (something that looks like only matter of time)= =2E >>>>>>>> >>>>>>> >>>>>>> Not really. Only by using a kernel module to call the UMH. >>>>>>> And an unprivileged can't load a module as far a I know. >>>>>>> I.e. NFSd, for example, will use unprivileged user's root to pe= rform this call. >>>>>> >>>>>> To help me understand the context which instances of call user m= ode >>>>>> helper are you expecting to use this facility? >>>>>> >>>>> >>>>> Ok. Here is how the NFSd uses UMH: >>>>> UMH is used on NFSd service to start user-space client tracker da= emon >>>>> ("/sbin/nfsdcltarck"), which is used to store some per-client loc= ks data on >>>>> persistent storage. >>>>> >>>>>>>> I think this is a seriously bad idea. >>>>>>>> >>>>>>>> Why can't we do this in userspace with setns as we do with the= core dump >>>>>>>> helper? >>>>>>>> >>>>>>> >>>>>>> Could you, please, clarify, how setns can help here? >>>>>> >>>>>> setns can change the mount namespace, and chroot can change to r= oot >>>>>> directory in the specified mount namespace. Essentially you can= enter >>>>>> into a containers complete context (pid, mnt, root, etc) comming= from >>>>>> the outside. >>>>>> >>>>> >>>>> So, you are actually suggesting to move the binary start from the= kernel to user-space. >>>>> IOW, you are suggesting to do not using UMH at all. >>>>> Am I right? >>>>> I don't know the reasons, why it was done by using UMH and not in= userspace. >>>>> Could you clarify this, Jeff? >>>>> >>>> >>>> nfsdcltrack is a "one-shot" program for managing and querying the = nfsd >>>> client tracking database. When knfsd needs to query or modify the >>>> db, it uses the UMH infrastructure to call this program that does >>>> what's requested and then exits. >>>> >>>> So, I'm not sure I really understand your question. It wasn't done= in >>>> userspace since the whole purpose of this program is to handle upc= alls >>>> from the kernel. >>>> >>> >>> The question is what was the reason to start this binary from kerne= l by UMH? >> >> Manipulating and querying the client tracking database is an infrequ= ent >> event, so having a continuously running daemon is wasteful and means >> that the admin has to ensure that it's running. A UMH upcall is much >> simpler and generally "just works" if the program is present. >> >>> I.e. why it can't be started by some user-space process before or a= fter NFSd start? >>> I don't familiar with this client tracking facility and that's the = only reason why I'm asking. >>> >> >> This program is not a daemon that runs continuously. It's only calle= d >> when the kernel needs to manipulate the database. Are you asking >> whether we could turn this into a continuously running daemon? If so >> then the answer is "yes", but that's not really a good idea either. >> >> In fact, we had that with the nfsdcld program, but no one liked it >> (including me) for the reasons I detailed above. >> >=20 > No, I'm just asking to understand. > Eric was, actually, asking the same. I.e. how does NFSd uses UMH and = why this can't be done in userspace? > Thanks you for your answer. >=20 I'm not familiar with nfsdcltrack but I would imagine it receives it's = information from Kernel as a command line parameters. Would it not be the simplest approach to add a --chroot=3D/path/to/root= optional parameter to nfsdcltrack so it should access an alternate DB relative t= o=20 --chroot. This would address Eric's concern of not executing user-privileged exec= utable from Kernel. I think Just my $0.017 Boaz