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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 39F51C43460 for ; Fri, 21 May 2021 13:19:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1B0BD613EB for ; Fri, 21 May 2021 13:19:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233878AbhEUNUu (ORCPT ); Fri, 21 May 2021 09:20:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:35354 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234368AbhEUNUl (ORCPT ); Fri, 21 May 2021 09:20:41 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 961E3AC11; Fri, 21 May 2021 13:19:17 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 5915E1F2C73; Fri, 21 May 2021 15:19:17 +0200 (CEST) Date: Fri, 21 May 2021 15:19:17 +0200 From: Jan Kara To: Amir Goldstein Cc: Jan Kara , Matthew Bobrowski , Christian Brauner , linux-fsdevel , Linux API Subject: Re: [PATCH 5/5] fanotify: Add pidfd info record support to the fanotify API Message-ID: <20210521131917.GM18952@quack2.suse.cz> References: <48d18055deb4617d97c695a08dca77eb573097e9.1621473846.git.repnop@google.com> <20210520081755.eqey4ryngngt4yqd@wittgenstein> <20210521102418.GF18952@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 Fri 21-05-21 14:10:32, Amir Goldstein wrote: > On Fri, May 21, 2021 at 1:24 PM Jan Kara wrote: > > > > On Fri 21-05-21 12:41:51, Amir Goldstein wrote: > > > On Fri, May 21, 2021 at 12:22 PM Matthew Bobrowski wrote: > > > > > > > > Hey Amir/Christian, > > > > > > > > On Thu, May 20, 2021 at 04:43:48PM +0300, Amir Goldstein wrote: > > > > > On Thu, May 20, 2021 at 11:17 AM Christian Brauner > > > > > wrote: > > > > > > > +#define FANOTIFY_PIDFD_INFO_HDR_LEN \ > > > > > > > + sizeof(struct fanotify_event_info_pidfd) > > > > > > > > > > > > > > static int fanotify_fid_info_len(int fh_len, int name_len) > > > > > > > { > > > > > > > @@ -141,6 +143,9 @@ static int fanotify_event_info_len(unsigned int info_mode, > > > > > > > if (fh_len) > > > > > > > info_len += fanotify_fid_info_len(fh_len, dot_len); > > > > > > > > > > > > > > + if (info_mode & FAN_REPORT_PIDFD) > > > > > > > + info_len += FANOTIFY_PIDFD_INFO_HDR_LEN; > > > > > > > + > > > > > > > return info_len; > > > > > > > } > > > > > > > > > > > > > > @@ -401,6 +406,29 @@ static int copy_fid_info_to_user(__kernel_fsid_t *fsid, > > > > > > > return info_len; > > > > > > > } > > > > > > > > > > > > > > +static int copy_pidfd_info_to_user(struct pid *pid, > > > > > > > + char __user *buf, > > > > > > > + size_t count) > > > > > > > +{ > > > > > > > + struct fanotify_event_info_pidfd info = { }; > > > > > > > + size_t info_len = FANOTIFY_PIDFD_INFO_HDR_LEN; > > > > > > > + > > > > > > > + if (WARN_ON_ONCE(info_len > count)) > > > > > > > + return -EFAULT; > > > > > > > + > > > > > > > + info.hdr.info_type = FAN_EVENT_INFO_TYPE_PIDFD; > > > > > > > + info.hdr.len = info_len; > > > > > > > + > > > > > > > + info.pidfd = pidfd_create(pid, 0); > > > > > > > + if (info.pidfd < 0) > > > > > > > + info.pidfd = FAN_NOPIDFD; > > > > > > > + > > > > > > > + if (copy_to_user(buf, &info, info_len)) > > > > > > > + return -EFAULT; > > > > > > > > > > > > Hm, well this kinda sucks. The caller can end up with a pidfd in their > > > > > > fd table and when the copy_to_user() failed they won't know what fd it > > > > > > > > > > Good catch! > > > > > > > > Super awesome catch Christian, thanks pulling this up! > > > > > > > > > But I prefer to solve it differently, because moving fd_install() to the > > > > > end of this function does not guarantee that copy_event_to_user() > > > > > won't return an error one day with dangling pidfd in fd table. > > > > > > > > I can see the angle you're approaching this from... > > > > > > > > > It might be simpler to do pidfd_create() next to create_fd() in > > > > > copy_event_to_user() and pass pidfd to copy_pidfd_info_to_user(). > > > > > pidfd can be closed on error along with fd on out_close_fd label. > > > > > > > > > > You also forgot to add CAP_SYS_ADMIN check before pidfd_create() > > > > > (even though fanotify_init() does check for that). > > > > > > > > I didn't really understand the need for this check here given that the > > > > administrative bits are already being checked for in fanotify_init() > > > > i.e. FAN_REPORT_PIDFD can never be set for an unprivileged listener; > > > > thus never walking any of the pidfd_mode paths. Is this just a defense > > > > in depth approach here, or is it something else that I'm missing? > > > > > > > > > > We want to be extra careful not to create privilege escalations, > > > so even if the fanotify fd is leaked or intentionally passed to a less > > > privileged user, it cannot get an open pidfd. > > > > > > IOW, it is *much* easier to be defensive in this case than to prove > > > that the change cannot introduce any privilege escalations. > > > > I have no problems with being more defensive (it's certainly better than > > being too lax) but does it really make sence here? I mean if CAP_SYS_ADMIN > > task opens O_RDWR /etc/passwd and then passes this fd to unpriviledged > > process, that process is also free to update all the passwords. > > Traditionally permission checks in Unix are performed on open and then who > > has fd can do whatever that fd allows... I've tried to follow similar > > philosophy with fanotify as well and e.g. open happening as a result of > > fanotify path events does not check permissions either. > > > > Agreed. > > However, because we had this issue with no explicit FAN_REPORT_PID > we added the CAP_SYS_ADMIN check for reporting event->pid as next > best thing. So now that becomes weird if priv process created fanotify fd > and passes it to unpriv process, then unpriv process gets events with > pidfd but without event->pid. > > We can change the code to: > > if (!capable(CAP_SYS_ADMIN) && !pidfd_mode && > task_tgid(current) != event->pid) > metadata.pid = 0; > > So the case I decscribed above ends up reporting both pidfd > and event->pid to unpriv user, but that is a bit inconsistent... Oh, now I see where you are coming from :) Thanks for explanation. And remind me please, cannot we just have internal FAN_REPORT_PID flag that gets set on notification group when priviledged process creates it and then test for that instead of CAP_SYS_ADMIN in copy_event_to_user()? It is mostly equivalent but I guess more in the spirit of how fanotify traditionally does things. Also FAN_REPORT_PIDFD could then behave in the same way... Honza -- Jan Kara SUSE Labs, CR