Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932713AbaDVOHz (ORCPT ); Tue, 22 Apr 2014 10:07:55 -0400 Received: from cantor2.suse.de ([195.135.220.15]:40477 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751534AbaDVOHu (ORCPT ); Tue, 22 Apr 2014 10:07:50 -0400 Date: Tue, 22 Apr 2014 16:07:47 +0200 From: Jan Kara To: "Michael Kerrisk (man-pages)" Cc: Jan Kara , Heinrich Schuchardt , Eric Paris , lkml Subject: Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor Message-ID: <20140422140747.GF366@quack.suse.cz> References: <1397940833-6386-1-git-send-email-xypron.glpk@gmx.de> <20140422134044.GE366@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Honza > >> fs/notify/fanotify/fanotify_user.c | 20 +++++++++++++++----- > >> 1 file changed, 15 insertions(+), 5 deletions(-) > >> > >> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > >> index 4e565c8..5d22a20 100644 > >> --- a/fs/notify/fanotify/fanotify_user.c > >> +++ b/fs/notify/fanotify/fanotify_user.c > >> @@ -62,6 +62,8 @@ static int create_fd(struct fsnotify_group *group, > >> { > >> int client_fd; > >> struct file *new_file; > >> + int mask; > >> + int ret; > >> > >> pr_debug("%s: group=%p event=%p\n", __func__, group, event); > >> > >> @@ -75,11 +77,19 @@ static int create_fd(struct fsnotify_group *group, > >> */ > >> /* it's possible this event was an overflow event. in that case dentry and mnt > >> * are NULL; That's fine, just don't call dentry open */ > >> - if (event->path.dentry && event->path.mnt) > >> - new_file = dentry_open(&event->path, > >> - group->fanotify_data.f_flags | FMODE_NONOTIFY, > >> - current_cred()); > >> - else > >> + if (event->path.dentry && event->path.mnt) { > >> + /* check permissions before granting access to file */ > >> + mask = MAY_READ; > >> + if (group->fanotify_data.f_flags & (O_RDWR | O_WRONLY)) > >> + mask |= MAY_WRITE; > >> + ret = inode_permission(event->path.dentry->d_inode, mask); > >> + if (ret) > >> + new_file = ERR_PTR(ret); > >> + else > >> + new_file = dentry_open(&event->path, > >> + group->fanotify_data.f_flags | FMODE_NONOTIFY, > >> + current_cred()); > >> + } else > >> new_file = ERR_PTR(-EOVERFLOW); > >> if (IS_ERR(new_file)) { > >> /* > >> -- > >> 1.9.1 > >> > > -- > > Jan Kara > > SUSE Labs, CR > > > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ -- Jan Kara SUSE Labs, CR -- 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/