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Fri, 21 May 2021 09:53:44 -0400 Received: from mail-io1-xd2f.google.com (mail-io1-xd2f.google.com [IPv6:2607:f8b0:4864:20::d2f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0E11BC061763; Fri, 21 May 2021 06:52:20 -0700 (PDT) Received: by mail-io1-xd2f.google.com with SMTP id t11so20141098iol.9; Fri, 21 May 2021 06:52:20 -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=/z49A5xjYjstfAwWBD6lDPYDhb3OM0GCCVdyr2X27lk=; b=uwfoywKocI8W9RyMdxvN3Fyu2abc+f09wVFGFSqdsFho2MBA+jA3RiTz/L/E1KvtGR 8T4b7DtDgoH66320z2ekWTUzQWEbKdLSQaS/2sh5kGymWkkxvf3K/VKEjsyTExJISpOL PU8PG+FLfbxzHzwQgila4oGvxDH7DxKMmBodYAYwZSNzNF8wFyFsJUcldpYk2xXhFB/M sTmMsKCYN7wftxO04wsgyUPgPe8x2imFBAVf+h4Y9ntntF0X4Q+Cs7HR/AeN+WTOT5bC kQun7XEPHVR7Jc02L1erks7opluDvO9qmSZk/FcppClHNSVJtJJYR1njb7L6JOM/Bt3J Zm9g== 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=/z49A5xjYjstfAwWBD6lDPYDhb3OM0GCCVdyr2X27lk=; b=puLSHjZE0Ygb3DUBucb34toPrpIzxYOaTGTPi1KkTuwXAlbsEJVK59S/x/FofRKIZx gDR2ihXHk1aXdLw4rTz1kVe0iJ0yjBlwfWfZCFs/DXGD5v8JuxjJmO867q1sEbRxIy8Q baqesRf7J5f1EI3vdAWIBv9XmZeH+sGf9G11jonrGWdX4dNhmTIad5XJllOJdrd8uB8v XWIpnY+gTF1PSBRE1zyTLBRFLiT0/eSIu8zjhu8ZJY1133Fcu3MuL7kd6meRbQBjQD+4 vmRjd5TXuX5LSpErIVF5R+bwNXYAt+lG2w/IjsLVKpnlpr/aXQLPMkl7mh9VTRsSAN+X VsyA== X-Gm-Message-State: AOAM530IRBdTAZFf8k8Ab+O2ctRcW6xRiIk3pvIjMDUexefTB3lbOsuu AWObd0j2N98Y9X5GzTxSVPkuC/uvgYo5zPKp+gzWWpFcd5w= X-Google-Smtp-Source: ABdhPJx6/UfGhm7BGjDZbtF0JOc1IR8ovWq6nj8EYz3mPzb/CQBAZFEWz0XndpvCe9bLi3pnvXpvCQl9bJ43xz/FtwQ= X-Received: by 2002:a5d:814d:: with SMTP id f13mr11378158ioo.203.1621605139351; Fri, 21 May 2021 06:52:19 -0700 (PDT) MIME-Version: 1.0 References: <48d18055deb4617d97c695a08dca77eb573097e9.1621473846.git.repnop@google.com> <20210520081755.eqey4ryngngt4yqd@wittgenstein> <20210521102418.GF18952@quack2.suse.cz> <20210521131917.GM18952@quack2.suse.cz> In-Reply-To: <20210521131917.GM18952@quack2.suse.cz> From: Amir Goldstein Date: Fri, 21 May 2021 16:52:08 +0300 Message-ID: Subject: Re: [PATCH 5/5] fanotify: Add pidfd info record support to the fanotify API To: Jan Kara Cc: Matthew Bobrowski , Christian Brauner , linux-fsdevel , Linux API Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, May 21, 2021 at 4:19 PM Jan Kara wrote: > > 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... Yes, we can. In fact, we should call the internal flag FANOTIFY_UNPRIV as it described the situation better than FAN_REPORT_PID. This happens to be how I implemented it in the initial RFC [1]. It's not easy to follow our entire discussion on this thread, but I think we can resurrect the FANOTIFY_UNPRIV internal flag and use it in this case instead of CAP_SYS_ADMIN. Thanks, Amir. https://lore.kernel.org/linux-fsdevel/20210124184204.899729-3-amir73il@gmail.com/