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=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 D6A6FC4320A for ; Tue, 3 Aug 2021 10:08:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BD82360E09 for ; Tue, 3 Aug 2021 10:08:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235063AbhHCKIc (ORCPT ); Tue, 3 Aug 2021 06:08:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49986 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235371AbhHCKIW (ORCPT ); Tue, 3 Aug 2021 06:08:22 -0400 Received: from mail-io1-xd32.google.com (mail-io1-xd32.google.com [IPv6:2607:f8b0:4864:20::d32]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B75DDC061796; Tue, 3 Aug 2021 03:08:09 -0700 (PDT) Received: by mail-io1-xd32.google.com with SMTP id y9so23667448iox.2; Tue, 03 Aug 2021 03:08:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=35tRKDbNn9ff+ylWBphE4a+eYYDOmGypG4uKiqPgB1c=; b=hVoAGzA+MOEMwCbGBLceJS8DXCkK4eV0YUVg9BjIncBGdC6TUcCorzUoaecCbSVWbA gxvi+C7KciqOPCFTJOXTHdhi7/Kws8Z2lrSXTceGzQfN1R6DGRjRHi9fI1SXhjvPQ8Tl tvKhurAVpurUBb4lbZ1pxnSCjn+6Lhq8lH3lswlYNINVkD62XjwfdR0E90qh9bPt+lhT f/dywQRnr6rSSH6sWJi/NP9+w/77Fh3ppLCOl2d6+GDj2k1ZwUdgaQe5jvgOoIDks855 f7LFhQ7zud4CLNmxRjjyZUuWUL+ZL/coYxi6QUO5jRWXEvV53TX25uQq8Zl6HYEw7ThJ 15hQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=35tRKDbNn9ff+ylWBphE4a+eYYDOmGypG4uKiqPgB1c=; b=D2nxqKHm95R24qB81rTbslaG2wPJk4ylL3BKw/vJLReTEfhPjxYTR65omTqY3Ovbyi vfY8kXemuGsmEY7SV1ZM/TyME20L639zbQQGp4Y/vfjYO0WSaoQUHQ9pGa8xQ6qeqSzp 0AMI76XGCX4c7iuMsMKC3AMCAJ7eNyiAX6Wz+9g9VehIYm6Ue6BeZLNaPkH8UyFDZaOB 8+xix2GRVMregZHKSlERuYobKMAd9gymHNqZ9V7qml0b/NNXhcQVG9OEP621CYA07y57 25NNirZ/0e2Uk6elCUdZlwN9yvkiLXs/cplJOpM05VvXLXrDXw07zRs6CRLKNOWmVGvc RfRQ== X-Gm-Message-State: AOAM532QA4inrkrfYzhCcHYPhVe67u0864TUTv+26jbqkk14r7x4N1Vf S3YqdPsm7Rj0ari+JuFVP+fyzIavPPANDfh0zULsklak5eY= X-Google-Smtp-Source: ABdhPJzuarxFNtBuHaixXfKZAWGpbaNAA9disL8/5QnLBbqFW0S5vYi08W9p1jgeY0a3TWJCjp8nputGeOpNsVyke08= X-Received: by 2002:a05:6602:3304:: with SMTP id b4mr1717368ioz.186.1627985289140; Tue, 03 Aug 2021 03:08:09 -0700 (PDT) MIME-Version: 1.0 References: <02ba3581fee21c34bd986e093d9eb0b9897fa741.1626845288.git.repnop@google.com> <20210729133953.GL29619@quack2.suse.cz> <20210802123428.GB28745@quack2.suse.cz> <20210803093753.mxcn6nzgj55erpuw@wittgenstein> In-Reply-To: <20210803093753.mxcn6nzgj55erpuw@wittgenstein> From: Amir Goldstein Date: Tue, 3 Aug 2021 13:07:57 +0300 Message-ID: Subject: Re: [PATCH v3 5/5] fanotify: add pidfd support to the fanotify API To: Christian Brauner Cc: Jan Kara , Jann Horn , Matthew Bobrowski , linux-fsdevel , Linux API , Andy Lutomirski Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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? If users want to fork and use fanotify fd with pidfd reporting they can opt-in back in with fcntl(). Thanks, Amir.