From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0B63CC432BE for ; Thu, 5 Aug 2021 08:55:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC875610CD for ; Thu, 5 Aug 2021 08:55:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239812AbhHEI4F (ORCPT ); Thu, 5 Aug 2021 04:56:05 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:51800 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232637AbhHEI4E (ORCPT ); Thu, 5 Aug 2021 04:56:04 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 5A18F22351; Thu, 5 Aug 2021 08:55:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1628153749; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lysYHXX5nF6qwVi9SFXu+jOT/ettQMPO3mcm668+sXY=; b=1pL6X4TK5xuD7TkM+JlADQaNHPCwKjyhvtyW+wY75CQmVeABBFlF9D00upAllYUa9dLtXt O/zCEz25hpt05InWL0pYqkQAV/dgzvSa+M+S1dK/+NPenkaXKDMn0+bdlZqpcnuAjfgUav ZLHSV40dM9ZCf69+0t7pZKqSoimCl2s= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1628153749; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=lysYHXX5nF6qwVi9SFXu+jOT/ettQMPO3mcm668+sXY=; b=BuFWpYQxUwlfM1VarFKlfqdMrZsX8FgOXbSDWimTQB/2AQ0X8hxHsvI/9aRjOGAtLBjkjR PAVQOZ4NM7WcOWCw== Received: from quack2.suse.cz (unknown [10.163.43.118]) by relay2.suse.de (Postfix) with ESMTP id 0765FA3B95; Thu, 5 Aug 2021 08:55:49 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 9B5D31E1514; Thu, 5 Aug 2021 10:55:48 +0200 (CEST) Date: Thu, 5 Aug 2021 10:55:48 +0200 From: Jan Kara To: Matthew Bobrowski Cc: Jan Kara , Amir Goldstein , Christian Brauner , Jann Horn , linux-fsdevel , Linux API , Andy Lutomirski Subject: Re: [PATCH v3 5/5] fanotify: add pidfd support to the fanotify API Message-ID: <20210805085548.GA14483@quack2.suse.cz> References: <20210802123428.GB28745@quack2.suse.cz> <20210803093753.mxcn6nzgj55erpuw@wittgenstein> <20210803140421.GE10621@quack2.suse.cz> <20210804123940.GD4578@quack2.suse.cz> 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) Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu 05-08-21 15:51:16, Matthew Bobrowski wrote: > On Wed, Aug 04, 2021 at 02:39:40PM +0200, Jan Kara wrote: > > On Wed 04-08-21 13:46:05, Matthew Bobrowski wrote: > > > On Tue, Aug 03, 2021 at 04:04:21PM +0200, Jan Kara wrote: > > > > On Tue 03-08-21 13:07:57, Amir Goldstein wrote: > > > > > On Tue, Aug 3, 2021 at 12:37 PM Christian Brauner > > > > > wrote: > > > > > > > > > > > > On Mon, Aug 02, 2021 at 05:38:20PM +0300, Amir Goldstein wrote: > > > > > > > On Mon, Aug 2, 2021 at 3:34 PM Jan Kara wrote: > > > > > > > > > > > > > > > > On Fri 30-07-21 08:03:01, Amir Goldstein wrote: > > > > > > > > > On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein wrote: > > > > > > > > > > On Thu, Jul 29, 2021 at 4:39 PM Jan Kara wrote: > > > > > > > > > > > Well, but pidfd also makes sure that /proc// keeps belonging to the > > > > > > > > > > > same process while you read various data from it. And you cannot achieve > > > > > > > > > > > that with pid+generation thing you've suggested. Plus the additional > > > > > > > > > > > concept and its complexity is non-trivial So I tend to agree with > > > > > > > > > > > Christian that we really want to return pidfd. > > > > > > > > > > > > > > > > > > > > > > Given returning pidfd is CAP_SYS_ADMIN priviledged operation I'm undecided > > > > > > > > > > > whether it is worth the trouble to come up with some other mechanism how to > > > > > > > > > > > return pidfd with the event. We could return some cookie which could be > > > > > > > > > > > then (by some ioctl or so) either transformed into real pidfd or released > > > > > > > > > > > (so that we can release pid handle in the kernel) but it looks ugly and > > > > > > > > > > > complicates things for everybody without bringing significant security > > > > > > > > > > > improvement (we already can pass fd with the event). So I'm pondering > > > > > > > > > > > whether there's some other way how we could make the interface safer - e.g. > > > > > > > > > > > so that the process receiving the event (not the one creating the group) > > > > > > > > > > > would also need to opt in for getting fds created in its file table. > > > > > > > > > > > > > > > > > > > > > > But so far nothing bright has come to my mind. :-| > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > There is a way, it is not bright, but it is pretty simple - > > > > > > > > > > store an optional pid in group->fanotify_data.fd_reader. > > > > > > > > > > > > > > > > > > > > With flag FAN_REPORT_PIDFD, both pidfd and event->fd reporting > > > > > > > > > > will be disabled to any process other than fd_reader. > > > > > > > > > > Without FAN_REPORT_PIDFD, event->fd reporting will be disabled > > > > > > > > > > if fd_reaader is set to a process other than the reader. > > > > > > > > > > > > > > > > > > > > A process can call ioctl START_FD_READER to set fd_reader to itself. > > > > > > > > > > With FAN_REPORT_PIDFD, if reaader_fd is NULL and the reader > > > > > > > > > > process has CAP_SYS_ADMIN, read() sets fd_reader to itself. > > > > > > > > > > > > > > > > > > > > Permission wise, START_FD_READER is allowed with > > > > > > > > > > CAP_SYS_ADMIN or if fd_reader is not owned by another process. > > > > > > > > > > We may consider YIELD_FD_READER ioctl if needed. > > > > > > > > > > > > > > > > > > > > I think that this is a pretty cheap price for implementation > > > > > > > > > > and maybe acceptable overhead for complicating the API? > > > > > > > > > > Note that without passing fd, there is no need for any ioctl. > > > > > > > > > > > > > > > > > > > > An added security benefit is that the ioctl adds is a way for the > > > > > > > > > > caller of fanotify_init() to make sure that even if the fanotify_fd is > > > > > > > > > > leaked, that event->fd will not be leaked, regardless of flag > > > > > > > > > > FAN_REPORT_PIDFD. > > > > > > > > > > > > > > > > > > > > So the START_FD_READER ioctl feature could be implemented > > > > > > > > > > and documented first. > > > > > > > > > > And then FAN_REPORT_PIDFD could use the feature with a > > > > > > > > > > very minor API difference: > > > > > > > > > > - Without the flag, other processes can read fds by default and > > > > > > > > > > group initiator can opt-out > > > > > > > > > > - With the flag, other processes cannot read fds by default and > > > > > > > > > > need to opt-in > > > > > > > > > > > > > > > > > > Or maybe something even simpler... fanotify_init() flag > > > > > > > > > FAN_PRIVATE (or FAN_PROTECTED) that limits event reading > > > > > > > > > to the initiator process (not only fd reading). > > > > > > > > > > > > > > > > > > FAN_REPORT_PIDFD requires FAN_PRIVATE. > > > > > > > > > If we do not know there is a use case for passing fanotify_fd > > > > > > > > > that reports pidfds to another process why implement the ioctl. > > > > > > > > > We can always implement it later if the need arises. > > > > > > > > > If we contemplate this future change, though, maybe the name > > > > > > > > > FAN_PROTECTED is better to start with. > > > > > > > > > > > > > > > > Good ideas. I think we are fine with returning pidfd only to the process > > > > > > > > creating the fanotify group. Later we can add an ioctl which would indicate > > > > > > > > that the process is also prepared to have fds created in its file table. > > > > > > > > But I have still some open questions: > > > > > > > > Do we want threads of the same process to still be able to receive fds? > > > > > > > > > > > > > > I don't see why not. > > > > > > > They will be bloating the same fd table as the thread that called > > > > > > > fanotify_init(). > > > > > > > > > > > > > > > Also pids can be recycled so they are probably not completely reliable > > > > > > > > identifiers? > > > > > > > > > > > > > > Not sure I follow. The group hold a refcount on struct pid of the process that > > > > > > > called fanotify_init() - I think that can used to check if reader process is > > > > > > > the same process, but not sure. Maybe there is another way (Christian?). > > > > > > > > > > > > If the fanotify group hold's a reference to struct pid it won't get > > > > > > recycled. And it can be used to check if the reader thread is the same > > > > > > thread with some care. You also have to be specific what exactly you > > > > > > want to know. If you're asking if the reading process is the same as > > > > > > the fanotify_init() process you can be asking one of two things. > > > > > > > > > > > > You can be asking if the reader is a thread in the same thread-group as > > > > > > the thread that called fanotify_init(). In that case you might need to > > > > > > do something like > > > > > > > > > > > > rcu_read_lock(); > > > > > > struct task_struct *fanotify_init_task_struct = pid_task(stashed_struct_pid, PIDTYPE_PID); > > > > > > if (!fanotify_init_task_struct) { > > > > > > /* The thread which called fanotify_init() has died already. */ > > > > > > return -ESRCH; > > > > > > } > > > > > > if (same_thread_group(fanotify_init_task_struct, current)) > > > > > > rcu_read_unlock(); > > > > > > > > > > > > though thinking about it makes me realise that there's a corner case. If > > > > > > the thread that called fanotify_init() is a thread in a non-empty > > > > > > thread-group it can already have died and been reaped. This would mean, > > > > > > pid_task(..., PIDTYPE_PID) will return NULL but there are still other > > > > > > threads alive in the thread-group. Handling that case might be a bit > > > > > > complicated. > > > > > > > > > > > > If you're asking whether the reading thread is really the same as the > > > > > > thread that created the fanotify instance then you might need to do sm > > > > > > like > > > > > > > > > > > > rcu_read_lock(); > > > > > > if (pid_task(stashed_struct_pid, PIDTYPE_PID) == current) > > > > > > rcu_read_unlock(); > > > > > > > > > > > > Just for completeness if I remember all of this right: there's a corner > > > > > > case because of how de_thread() works. > > > > > > During exec the thread that is execing will assume the struct pid of the > > > > > > old thread-group leader. (All other threads in the same thread-group > > > > > > will get killed.) > > > > > > Assume the thread that created the fanotify instance is not the > > > > > > thread-group leader in its non-empty thread-group. And further assume it > > > > > > exec's. Then it will assume the struct pid of the old thread-group > > > > > > leader during de_thread(). > > > > > > Assume the thread inherits the fanotify fd across the exec. Now, when it > > > > > > tries to read a new event after the exec then pid_task() will return > > > > > > NULL. > > > > > > However, if the thread was already the thread-group leader before the > > > > > > exec then pid_task() will return the same task struct as before after > > > > > > the exec (because no struct pid swapping needed to take place). > > > > > > > > > > > > I hope this causes more clarity ?then confusion. :) > > > > > > > > > > I'm afraid it's the latter :D > > > > > > > > > > Sigh! We must simplify. > > > > > > > > > > Thinking out loud, instead of sealing the possibility of another > > > > > process reading pidfd, maybe just avoid the most obvious unintentional > > > > > leak of fanotify_fd to another process by mandating FAN_CLOEXEC? > > > > > > > > Well, I don't think we need any protection from leaking fanotify_fd. It is > > > > special fd with special priviledges as any other. If you leak it, well, bad > > > > luck but that's how Unix priviledge model works. > > > > > > > > The threat IMO is that you have a process X, that process expects to > > > > receive fd to work with from process Y. Now process Y is malicious (or > > > > taken over by an attacker) and passes to X fanotify_fd. X reads from > > > > fanotify_fd to get data to process, it performs all kinds of validity > > > > checks on untrusted input but it does not expect that the read has side > > > > effects on X's file_table and in the worst case can lead to some compromise > > > > of X or easily to DoS on X by exhausting its file_table space. > > > > > > > > Currently this attack vector is moot because you have to have CAP_SYS_ADMIN > > > > to get to fanotify_fd and then you can certainly do worse things. But OTOH > > > > I can see why Jann was uneasy about this. > > > > > > As I have breifly expressed in my previous emails, the cause for concern > > > here is flakey IMO. If there's sensible something that I'm clearly missing, > > > then please explain. > > > > No, I think your understanding is correct. > > > > > From my perspective, the only sensible attack vector that's maybe worth > > > worrying about here is the possibility of exhausting the fdtable of a given > > > process, which yes, can be considered as a form of DoS. However, in any > > > case, there are other defensive protections/measures that a programmer > > > could employ in their application code which could prevent such from ever > > > happening. > > > > > > The whole passing of file descriptors between process Y and process X and > > > the leaking of a file descriptor thing simply goes back to what you've > > > mentioned above Jan. I consider it a very weak argument. When enabling > > > FAN_REPORT_PIDFD, the process requires CAP_SYS_ADMIN. If that process ever > > > has its execution flow hijacked by an attacker, then I'm sorry, I think > > > there's other larger causes for concern at that point rather then worrying > > > about the state of some other child processes fdtable. > > > > > > In general cases, I get that passing a file descriptor between process Y > > > and process X and then having process X's fdtable modified as result of > > > calling functions like read() is considered undesired. But, for > > > applications that makes use of fanotify is there ever a case where we pass > > > the fanotify file descriptor to a random/unexpected process and have it > > > process events? I don't think so. So, I suppose what I'm trying to say is > > > that, if an application chooses to opt-in and use a flag like > > > FAN_REPORT_PIDFD or any other future file descriptor generating variant, > > > the expectation is that which ever process is created and event processing > > > is passed to that process, then it should always expect to have its fdtable > > > modified when reading events. > > > > Yes, I was thinking about this some more and at this point, given the lack > > of convenient options for the hardening, I think the best option is to keep > > the interface as originally planned. Because I'm afraid the hardening options > > we were able to come up with would only cause confusion (and from confusion > > bugs easily arise) for little security gain. > > OK, in that case are you happy for me to post hopefully the last iteration > of this series with the minor nits addressed? Yes. Honza -- Jan Kara SUSE Labs, CR