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[46.142.197.178]) by smtp.gmail.com with ESMTPSA id o47sm7838488edc.37.2019.05.23.02.55.08 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Thu, 23 May 2019 02:55:08 -0700 (PDT) Date: Thu, 23 May 2019 11:55:07 +0200 From: Christian Brauner To: Amir Goldstein Cc: Jan Kara , linux-fsdevel , linux-kernel Subject: Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s Message-ID: <20190523095506.nyei5nogvv63lm4a@brauner.io> References: <20190522163150.16849-1-christian@brauner.io> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, May 22, 2019 at 11:00:22PM +0300, Amir Goldstein wrote: > On Wed, May 22, 2019 at 9:57 PM Christian Brauner wrote: > > > > On May 22, 2019 8:29:37 PM GMT+02:00, Amir Goldstein wrote: > > >On Wed, May 22, 2019 at 7:32 PM Christian Brauner > > > wrote: > > >> > > >> This removes two redundant capable(CAP_SYS_ADMIN) checks from > > >> fanotify_init(). > > >> fanotify_init() guards the whole syscall with capable(CAP_SYS_ADMIN) > > >at the > > >> beginning. So the other two capable(CAP_SYS_ADMIN) checks are not > > >needed. > > > > > >It's intentional: > > > > > >commit e7099d8a5a34d2876908a9fab4952dabdcfc5909 > > >Author: Eric Paris > > >Date: Thu Oct 28 17:21:57 2010 -0400 > > > > > > fanotify: limit the number of marks in a single fanotify group > > > > > >There is currently no limit on the number of marks a given fanotify > > >group > > >can have. Since fanotify is gated on CAP_SYS_ADMIN this was not seen > > >as > > >a serious DoS threat. This patch implements a default of 8192, the > > >same as > > >inotify to work towards removing the CAP_SYS_ADMIN gating and > > >eliminating > > > the default DoS'able status. > > > > > > Signed-off-by: Eric Paris > > > > > >There idea is to eventually remove the gated CAP_SYS_ADMIN. > > >There is no reason that fanotify could not be used by unprivileged > > >users > > >to setup inotify style watch on an inode or directories children, see: > > >https://patchwork.kernel.org/patch/10668299/ > > > > > >> > > >> Fixes: 5dd03f55fd2 ("fanotify: allow userspace to override max queue > > >depth") > > >> Fixes: ac7e22dcfaf ("fanotify: allow userspace to override max > > >marks") > > > > > >Fixes is used to tag bug fixes for stable. > > >There is no bug. > > > > > >Thanks, > > >Amir. > > > > Interesting. When do you think the gate can be removed? > > Nobody is working on this AFAIK. > What I posted was a simple POC, but I have no use case for this. > In the patchwork link above, Jan has listed the prerequisites for > removing the gate. > > One of the prerequisites is FAN_REPORT_FID, which is now merged. > When events gets reported with fid instead of fd, unprivileged user > (hopefully) cannot use fid for privilege escalation. > > > I was looking into switching from inotify to fanotify but since it's not usable from > > non-initial userns it's a no-no > > since we support nested workloads. > > One of Jan's questions was what is the benefit of using inotify-compatible > fanotify vs. using inotify. > So what was the reason you were looking into switching from inotify to fanotify? > Is it because of mount/filesystem watch? Because making those available for Yeah. Well, I would need to look but you could probably do it safely for filesystems mountable in user namespaces (which are few). Can you do a bind-mount and then place a watch on the bind-mount or is this superblock based? Thanks! Christian