From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stanislav Kinsbursky Subject: Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced Date: Thu, 23 May 2013 15:38:17 +0400 Message-ID: <519DFFA9.3010606@parallels.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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Eric W. Biederman" , , , , , , , , , , , To: Jeff Layton Return-path: In-Reply-To: <20130523073108.13afafa6@tlielax.poochiereds.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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 cont= ext. This >>>>>> doesn't allow to call a binary from other root context (for exam= ple in a >>>>>> container). >>>>>> Currently, both containerized NFS client and NFS server requires= 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 expect= ed to appear >>>>>> in future) and because set_fs_root() in not exported, it looks r= easonable 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 pick w= hich >>>>> binary the kernel will execute. At least if nfs and nfsd ever ru= ns in a >>>>> user namespace (something that looks like only matter of time). >>>>> >>>> >>>> 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 perfo= rm this call. >>> >>> To help me understand the context which instances of call user mode >>> 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 daemo= n >> ("/sbin/nfsdcltarck"), which is used to store some per-client locks = 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 co= re dump >>>>> helper? >>>>> >>>> >>>> Could you, please, clarify, how setns can help here? >>> >>> setns can change the mount namespace, and chroot can change to root >>> directory in the specified mount namespace. Essentially you can en= ter >>> into a containers complete context (pid, mnt, root, etc) comming fr= om >>> the outside. >>> >> >> So, you are actually suggesting to move the binary start from the ke= rnel 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 us= erspace. >> Could you clarify this, Jeff? >> > > nfsdcltrack is a "one-shot" program for managing and querying the nfs= d > 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 upcall= s > from the kernel. > The question is what was the reason to start this binary from kernel by= UMH? I.e. why it can't be started by some user-space process before or after= NFSd start? I don't familiar with this client tracking facility and that's the only= reason why I'm asking. --=20 Best regards, Stanislav Kinsbursky