Re: [PATCH v2 0/2] unprivileged fanotify listener

On Mon, Mar 22, 2021 at 6:28 PM Christian Brauner
[off-list ref] wrote:

On Mon, Mar 22, 2021 at 02:44:20PM +0200, Amir Goldstein wrote:

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On Sat, Mar 20, 2021 at 2:57 PM Amir Goldstein [off-list ref] wrote:

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The code that sits in linux-next can give you pretty much a drop-in
replacement of inotify and nothing more. See example code:
https://github.com/amir73il/inotify-tools/commits/fanotify_name_fid

This is really great. Thank you for doing that work this will help quite
a lot of use-cases and make things way simpler. I created a TODO to port
our path-hotplug to this once this feature lands.

FWIW, I just tried to build this branch on Ubuntu 20.04.2 with LTS kernel
and there were some build issues, so rebased my branch on upstream
inotify-tools to fix those build issues.

I was not aware that the inotify-tools project is alive, I never intended
to upstream this demo code and never created a github pull request
but rebasing on upstream brought in some CI scripts, when I pushed the
branch to my github it triggered some tests that reported build failures on
Ubuntu 16.04 and 18.04.

Anyway, there is a pre-rebase branch 'fanotify_name' and the post rebase
branch 'fanotify_name_fid'. You can try whichever works for you.

FYI, fixed the CI build errors on fanotify_name_fid branch.

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You can look at the test script src/test_demo.sh for usage example.
Or just cd into a writable directory and run the script to see the demo.
The demo determines whether to use a recursive watch or "global"
watch by the uid of the user.

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If you think that is useful and you want to play with this feature I can
provide a WIP branch soon.

I would like to first play with the support for unprivileged fanotify
but sure, it does sound useful!

Just so you have an idea what I am talking about, this is a very early
POC branch:
https://github.com/amir73il/linux/commits/fanotify_userns

Thanks!  I'll try to pull this and take a look next week. I hope that's
ok.

Fine. I'm curious to know what it does.
Did not get to test it with userns yet :)

Now tested FAN_MARK_FILESYSTEM watch on tmpfs mounted
inside userns and works fine, with two wrinkles I needed to iron:

1. FAN_REPORT_FID not supported on tmpfs because tmpfs has
    zero f_fsid (easy to fix)
2. open_by_handle_at() is not userns aware (can relax for
    FS_USERNS_MOUNT fs)

Pushed these two fixes to branch fanotify_userns.

Pushed another fix to mnt refcount bug in WIP and another commit to
add the last piece that could make fanotify usable for systemd-homed
setup - a filesystem watch filtered by mnt_userns (not tested yet).

Sounds interesting.

So I'm looking and commenting on that branch a little.
One general question, when fanotify FANOTIFY_PERM_EVENTS is set fanotify
will return a file descriptor (for relevant events) referring to the
file/directory that e.g. got created. And there are no permissions
checks other than the capable(CAP_SYS_ADMIN) check when the fanotify
instance is created, right?

Correct.
fanotify_init() enforces that in a few maybe not so obvious steps:

1. Either CAP_SYS_ADMIN or fid_mode (no file descriptor in event):

        if (!capable(CAP_SYS_ADMIN)) {
                /*
                 * An unprivileged user can setup an fanotify group with
                 * limited functionality - an unprivileged group is limited to
                 * notification events with file handles and it cannot use
                 * unlimited queue/marks.
                 */
                if ((flags & FANOTIFY_ADMIN_INIT_FLAGS) || !fid_mode)
                        return -EPERM;
        }

2. fid_mode is not supported for high priority classes:

        if (fid_mode && class != FAN_CLASS_NOTIF)
                return -EINVAL;

3. Permission events are only allowed for high priority classes:
        /*
         * group->priority == FS_PRIO_0 == FAN_CLASS_NOTIF.  These are not
         * allowed to set permissions events.
         */
        ret = -EINVAL;
        if (mask & FANOTIFY_PERM_EVENTS &&
            group->priority == FS_PRIO_0)

You may want to look at the summary of all the limitations on
unprivileged listener here:
https://github.com/amir73il/man-pages/blob/fanotify_unpriv/man2/fanotify_init.2#L400

Thanks,
Amir.