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[209.132.180.67]) by mx.google.com with ESMTP id s13-v6si24573665pgp.382.2018.10.30.18.28.26; Tue, 30 Oct 2018 18:28:42 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@joelfernandes.org header.s=google header.b=PtFm6Va7; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728851AbeJaJxN (ORCPT + 99 others); Wed, 31 Oct 2018 05:53:13 -0400 Received: from mail-pf1-f194.google.com ([209.85.210.194]:32886 "EHLO mail-pf1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728591AbeJaJxN (ORCPT ); Wed, 31 Oct 2018 05:53:13 -0400 Received: by mail-pf1-f194.google.com with SMTP id a15-v6so6743200pfn.0 for ; Tue, 30 Oct 2018 17:57:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=3uDb/lmLWw3HWuGL3n/wi65BBdbtBwwSCJhrS1GpAXY=; b=PtFm6Va7Lwp5J7BdnMdJCNNVv8Jj3YTXEsN26NeK/TUa6Dl93rBpaRv6SMB83VO1BK AXX5Rd7NOY3f5OR6i8dzvnQoBdQAbo7TfFkP+xWmfh4jpTz+GJsew9NrTP6TWuG9VVem wjxVo2ENcO2aoPtk9xaxKv15AuCuL69iV39T4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=3uDb/lmLWw3HWuGL3n/wi65BBdbtBwwSCJhrS1GpAXY=; b=Jkh02z8QCl+AUEqNJLxEjlMTg4mRXaNb9o4wHZ+ud2OcCo7NlHhSfH0R7Yj6RqCK9Y 5cV7FFD4S9OqGs6yFoFGgSx5HjXZOO9aOB223WgLXa/xae8Si0pZ8UAYM2X5Dt3/D+Y/ yZgVigY8WdkhAjwLHMU4IwAlJCNy0R2bbTpy7yYimX1Gf+1O3YIhuKwXNKDWsODtidwm gc9eROQOExalWKkVc1XY+dsri1/1YxDfKvJFWc32+SCWJVG0UdXpkuhfhgOh2ZaeWXEK 5Y5i/xd0i9LCSodhCOFLqdH7ujxzrBhs1ELyrSs/Jt2avGpauSmnOq7CW95vWCC9u2JQ SDvQ== X-Gm-Message-State: AGRZ1gKOIZQiWzswk5vIPcEH3A+gaURwadGfYAHUWIx1WAYmcdPztN2I +S1alspPvMPaoUkCuwEeiFie0oebgcI= X-Received: by 2002:a63:42c1:: with SMTP id p184mr986241pga.202.1540947445713; Tue, 30 Oct 2018 17:57:25 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id r23-v6sm28492636pgm.48.2018.10.30.17.57.24 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 30 Oct 2018 17:57:24 -0700 (PDT) Date: Tue, 30 Oct 2018 17:57:23 -0700 From: Joel Fernandes To: Daniel Colascione Cc: Aleksa Sarai , linux-kernel , Tim Murray , Suren Baghdasaryan Subject: Re: [RFC PATCH] Implement /proc/pid/kill Message-ID: <20181031005723.GD224709@google.com> References: <20181029221037.87724-1-dancol@google.com> <20181030050012.u43lcvydy6nom3ul@yavin> <20181030204501.jnbe7dyqui47hd2x@yavin> <20181030214243.GB32621@google.com> <20181030222339.ud4wfp75tidowuo4@yavin> <20181030223343.GB105735@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 30, 2018 at 11:10:47PM +0000, Daniel Colascione wrote: > On Tue, Oct 30, 2018 at 10:33 PM, Joel Fernandes wrote: > > On Wed, Oct 31, 2018 at 09:23:39AM +1100, Aleksa Sarai wrote: > >> On 2018-10-30, Joel Fernandes wrote: > >> > On Wed, Oct 31, 2018 at 07:45:01AM +1100, Aleksa Sarai wrote: > >> > [...] > >> > > > > (Unfortunately > >> > > > > there are lots of things that make it a bit difficult to use /proc/$pid > >> > > > > exclusively for introspection of a process -- especially in the context > >> > > > > of containers.) > >> > > > > >> > > > Tons of things already break without a working /proc. What do you have in mind? > >> > > > >> > > Heh, if only that was the only blocker. :P > >> > > > >> > > The basic problem is that currently container runtimes either depend on > >> > > some non-transient on-disk state (which becomes invalid on machine > >> > > reboots or dead processes and so on), or on long-running processes that > >> > > keep file descriptors required for administration of a container alive > >> > > (think O_PATH to /dev/pts/ptmx to avoid malicious container filesystem > >> > > attacks). Usually both. > >> > > > >> > > What would be really useful would be having some way of "hiding away" a > >> > > mount namespace (of the pid1 of the container) that has all of the > >> > > information and bind-mounts-to-file-descriptors that are necessary for > >> > > administration. If the container's pid1 dies all of the transient state > >> > > has disappeared automatically -- because the stashed mount namespace has > >> > > died. In addition, if this was done the way I'm thinking with (and this > >> > > is the contentious bit) hierarchical mount namespaces you could make it > >> > > so that the pid1 could not manipulate its current mount namespace to > >> > > confuse the administrative process. You would also then create an > >> > > intermediate user namespace to help with several race conditions (that > >> > > have caused security bugs like CVE-2016-9962) we've seen when joining > >> > > containers. > >> > > > >> > > Unfortunately this all depends on hierarchical mount namespaces (and > >> > > note that this would just be that NS_GET_PARENT gives you the mount > >> > > namespace that it was created in -- I'm not suggesting we redesign peers > >> > > or anything like that). This makes it basically a non-starter. > >> > > > >> > > But if, on top of this ground-work, we then referenced containers > >> > > entirely via an fd to /proc/$pid then you could also avoid PID reuse > >> > > races (as well as being able to find out implicitly whether a container > >> > > has died thanks to the error semantics of /proc/$pid). And that's the > >> > > way I would suggest doing it (if we had these other things in place). > >> > > >> > I didn't fully follow exactly what you mean. If you can explain for the > >> > layman who doesn't know much experience with containers.. > >> > > >> > Are you saying that keeping open a /proc/$pid directory handle is not > >> > sufficient to prevent PID reuse while the proc entries under /proc/$pid are > >> > being looked into? If its not sufficient, then isn't that a bug? If it is > >> > sufficient, then can we not just keep the handle open while we do whatever we > >> > want under /proc/$pid ? > >> > >> Sorry, I went on a bit of a tangent about various internals of container > >> runtimes. My main point is that I would love to use /proc/$pid because > >> it makes reuse handling very trivial and is always correct, but that > >> there are things which stop us from being able to use it for everything > >> (which is what my incoherent rambling was on about). > > > > Ok thanks. So I am guessing if the following sequence works, then Dan's patch is not > > needed. > > > > 1. open /proc/ directory > > 2. inspect /proc/ or do whatever with > > 3. Issue the kill on > > 4. Close the /proc/ directory opened in step 1. > > > > So unless I missed something, the above sequence will not cause any PID reuse > > races. > > Keeping a /proc/$PID directory file descriptor open does not prevent > $PID being used to name some other process. If it could, you could > pretty quickly fill a whole system's process table. See the program > below, which demonstrates the PID collision. I know. We both were not sure about that earlier, that's why I requested you to write the program when we were privately chatting. Now I'm sure because Aleska answered that and the you program you wrote showed that too. I wonder if this cannot be plumbed by just making the /proc/$PID directory opens hold a reference to task_struct (and a reference to whatever else is supposed to prevent the PID from getting reused), instead of introducing a brand new API. > I think Aleksa's larger point is that it's useful to treat processes > as other file-descriptor-named, poll-able, wait-able resources. > Consistency is important. A process is just another system resource, > and like any other system resource, you should be open to hold a file > descriptor to it and do things to that process via that file > descriptor. The precise form of this process-handle FD is up for > debate. The existing /proc/$PID directory FD is a good candidate for a > process handle FD, since it does almost all of what's needed. But > regardless of what form a process handle FD takes, we need it. I don't > see a case for continuing to treat processes in a non-unixy, > non-file-descriptor-based manner. So wait, how is that supposed to address what you're now saying above "quickly fill a whole process table"? You either want this, or you don't :)