Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932100AbaDXPGK (ORCPT ); Thu, 24 Apr 2014 11:06:10 -0400 Received: from mail-qc0-f170.google.com ([209.85.216.170]:33445 "EHLO mail-qc0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754178AbaDXPGH (ORCPT ); Thu, 24 Apr 2014 11:06:07 -0400 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <5359177E.1070907@gmx.de> References: <1397940833-6386-1-git-send-email-xypron.glpk@gmx.de> <20140422134044.GE366@quack.suse.cz> <20140422140747.GF366@quack.suse.cz> <20140424090441.GA6723@quack.suse.cz> <5359177E.1070907@gmx.de> From: "Michael Kerrisk (man-pages)" Date: Thu, 24 Apr 2014 17:05:45 +0200 Message-ID: Subject: Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor To: Heinrich Schuchardt Cc: Jan Kara , Eric Paris , lkml Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 24, 2014 at 3:54 PM, Heinrich Schuchardt wrote: > On 24.04.2014 11:04, Jan Kara wrote: >> >> On Tue 22-04-14 16:07:47, Jan Kara wrote: >>> >>> On Tue 22-04-14 15:50:26, Michael Kerrisk (man-pages) wrote: >>>> >>>> On Tue, Apr 22, 2014 at 3:40 PM, Jan Kara wrote: >>>>> >>>>> On Sat 19-04-14 22:53:53, Heinrich Schuchardt wrote: >>>>>> >>>>>> When monitoring a directory or a mount with the fanotify API >>>>>> the call to fanotify_init checks, >>>>>> * the process has cap_sys_admin capability >>>>>> >>>>>> The call to fanotify_mark checks, >>>>>> * the process has read authorization for directory or mount >>>>>> >>>>>> A directory or mount may contain files for which the process >>>>>> has no read or write authorization. >>>>>> Yet when reading from the fanotify file descriptor, structures >>>>>> fanotify_event_metadata are returned, which contain a file >>>>>> descriptor for these files, and will allow to read or write. >>>>>> >>>>>> The patch adds an authorization check for read and write >>>>>> permission. In case of missing permission, reading from the >>>>>> fanotify file descriptor returns EACCES. >>>>> >>>>> OK, am I right you are concerned about a situation where fanotify >>>>> group >>>>> descriptor is passed to an unpriviledged process which handles all the >>>>> incoming events? I'm asking because the permission checking can be >>>>> relatively expensive (think of acls) so we better do it for a reason. >>>>> I'd prefer to hear from Eric what the original intention regarding >>>>> permissions was... >>>> >>>> >>>> If I understand correctly, passing to an unprivileged process is the >>>> point. The point is I think that supposedly one only needs to >>>> CAP_SYS_ADMIN to use fanotify. However, once you have that capability, >>>> then you implicitly get the effect of CAP_DAC_READ_SEARCH and >>>> CAP_DAC_OVERRIDE as well. >>> >>> Ah, OK. Thanks for explanation. Then I'm OK with the patch. So feel >>> free >>> to add: >>> >>> Reviewed-by: Jan Kara >> >> Hum, when digging more around this code, I've found out that >> fanotify_mark() checks whether it has a read permission to a watched file >> when creating the mark (in fanotify_find_path()). So I don't think it's >> really worth it to recheck the permissions when creating a file >> .gnupg/secring.gpgdescriptor >> >> for the event. Sure it may be somewhat surprising that read fd is created >> after a process doesn't have access to the file anymore but OTOH it is >> similar to a situation where the process has opened the file long time >> ago. >> > > fanotify_mark checks for the read authorization for the marked object, > not for the object for which the event occurs. > > This means a listener may have read authorization for /home and mark this > mount. > Afterwards, while you sign a git tag, it will receive a FAN_OPEN event > and use the file descriptor supplied in the event to overwrite your > /home/jankara/.gnupg/secring.pgp Sweet! -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/