On Thu, May 23, 2019 at 02:40:39PM +0300, Amir Goldstein wrote: > On Thu, May 23, 2019 at 1:42 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > On Thu, May 23, 2019 at 01:25:08PM +0300, Amir Goldstein wrote: > > > On Thu, May 23, 2019 at 12:55 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > > > On Wed, May 22, 2019 at 11:00:22PM +0300, Amir Goldstein wrote: > > > > > On Wed, May 22, 2019 at 9:57 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > > > > > > > On May 22, 2019 8:29:37 PM GMT+02:00, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > > > >On Wed, May 22, 2019 at 7:32 PM Christian Brauner > > > > > > ><christian@xxxxxxxxxx> 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 <eparis@xxxxxxxxxx> > > > > > > >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 <eparis@xxxxxxxxxx> > > > > > > > > > > > > > >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? > > > > > > > > > > Either. > > > FAN_MARK_MOUNT was there from day 1 of fanotify. > > > FAN_MARK_FILESYSTEM was merged to Linux Linux 4.20. > > > > > > But directory modification events that are supported since v5.1 are > > > not available > > > with FAN_MARK_MOUNT, see: > > > > Because you're worried about unprivileged users spying on events? Or > > something else? > > Something else. The current fsnotify_move/create/delete() VFS hooks > have no path/mount information, so it is not possible to filter them by > mount only by inode/sb. > Fixing that would not be trivial, but first a strong use case would need > to be presented. > > > Because if you can do a bind-mount there's nothing preventing an > > unprivileged user to do a hand-rolled recursive inotify that would > > amount to the same thing anyway. > > There is. unprivileged user cannot traverse into directories it is not > allowed to read/search. Right, I should've mentioned: when you're userns root and you have access to all files. The part that is interesting to me is getting rid of capable(CAP_SYS_ADMIN). > > > (And btw, v5.1 really is a major step forward and I would really like to > > use this api tbh.) > > > > You haven't answered my question. What is the reason you are interested > in the new API? What does it provide that the old API does not? > I know the 2 APIs differ. I just want to know which difference interests *you*, > because without a strong use case, it will be hard for me to make progress > upstream. > > Is what you want really a "bind-mount" watch or a "subtree watch"? > The distinction is important. I am thinking about solutions for the latter, > although there is no immediate solution in the horizon - only ideas. Both cases would be interesting. But subtree watch is what would probably help a lot already. So let me explain. For LXD - not sure if you know what that is - we allow user to "hotplug" mounts or certain whitelisted devices into a user namespace container. One of the nifty features is that we let users specify a "required" property. When "required" is "false" the user can give us a path, e.g. /bla/bla/bla/target and then we place a watch on the closest existing ancestor of my-device. When the target shows up we hotplug it for the user. Now, as you imagine maintaining that cache until "target" shows up is a royal pain. I think that we can get rid of at least some of the complexity if subtree watch and bind-mount watches would work. Thanks! Christian
