Re: [RFC][PATCH] fanotify: allow to set errno in FAN_DENY permission response

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Feb 19, 2024 at 1:01 PM Jan Kara <jack@suse.cz> wrote:
>
> On Thu 15-02-24 17:40:07, Amir Goldstein wrote:
> > > > Last time we discussed this the conclusion was an API of a group-less
> > > > default mask, for example:
> > > >
> > > > 1. fanotify_mark(FAN_GROUP_DEFAULT,
> > > >                            FAN_MARK_ADD | FAN_MARK_MOUNT,
> > > >                            FAN_PRE_ACCESS, AT_FDCWD, path);
> > > > 2. this returns -EPERM for access until some group handles FAN_PRE_ACCESS
> > > > 3. then HSM is started and subscribes to FAN_PRE_ACCESS
> > > > 4. and then the mount is moved or bind mounted into a path exported to users
> > >
> > > Yes, this was the process I was talking about.
> > >
> > > > It is a simple solution that should be easy to implement.
> > > > But it does not involve "register the HSM app with the filesystem",
> > > > unless you mean that a process that opens an HSM group
> > > > (FAN_REPORT_FID|FAN_CLASS_PRE_CONTENT) should automatically
> > > > be given FMODE_NONOTIFY files?
> > >
> > > Two ideas: What you describe above seems like what the new mount API was
> > > intended for? What if we introduced something like an "hsm" mount option
> > > which would basically enable calling into pre-content event handlers
> >
> > I like that.
> > I forgot that with my suggestion we'd need a path to setup
> > the default mask.
> >
> > > (for sb without this flag handlers wouldn't be called and you cannot place
> > > pre-content marks on such sb).
> >
> > IMO, that limitation (i.e. inside brackets) is too restrictive.
> > In many cases, the user running HSM may not have control over the
> > mount of the filesystem (inside containers?).
> > It is true that HSM without anti-crash protection is less reliable,
> > but I think that it is still useful enough that users will want the
> > option to run it (?).
> >
> > Think of my HttpDirFS demo - it's just a simple lazy mirroring
> > of a website. Even with low reliability I think it is useful (?).
>
> Yeah, ok, makes sense. But for such "unpriviledged" usecases we don't have
> a deadlock-free way to fill in the file contents because that requires a
> special mountpoint?
>

True, unless we also keep the FMODE_NONOTIFY event->fd
for the simple cases. I'll need to think about this some more.

> > > These handlers would return EACCESS unless
> > > there's somebody handling events and returning something else.
> > >
> > > You could then do:
> > >
> > > fan_fd = fanotify_init()
> > > ffd = fsopen()
> > > fsconfig(ffd, FSCONFIG_SET_STRING, "source", device, 0)
> > > fsconfig(ffd, FSCONFIG_SET_FLAG, "hsm", NULL, 0)
> > > rootfd = fsconfig(ffd, FSCONFIG_CMD_CREATE, NULL, NULL, 0)
> > > fanotify_mark(fan_fd, FAN_MARK_ADD, ... , rootfd, NULL)
> > > <now you can move the superblock into the mount hierarchy>
> >
> > Not too bad.
> > I think that "hsm_deny_mask=" mount options would give more flexibility,
> > but I could be convinced otherwise.
> >
> > It's probably not a great idea to be running two different HSMs on the same
> > fs anyway, but if user has an old HSM version installed that handles only
> > pre-content events, I don't think that we want this old version if it happens
> > to be run by mistake, to allow for unsupervised create,rename,delete if the
> > admin wanted to atomically mount a fs that SHOULD be supervised by a
> > v2 HSM that knows how to handle pre-path events.
> >
> > IOW, and "HSM bit" on sb is too broad IMO.
>
> OK. So "hsm_deny_mask=" would esentially express events that we require HSM
> to handle, the rest would just be accepted by default. That makes sense.

Yes.

> The only thing I kind of dislike is that this ties fanotify API with mount
> API. So perhaps hsm_deny_mask should be specified as a string? Like
> preaccess,premodify,prelookup,... and transformed into a bitmask only
> inside the kernel? It gives us more maneuvering space for the future.
>

Urgh. I see what you are saying, but this seems so ugly to me.
I have a strong feeling that we are trying to reinvent something
and that we are going to reinvent it badly.
I need to look for precedents, maybe in other OS.
I believe that in Windows, there is an API to register as a
Cloud Engine Provider, so there is probably a way to have multiple HSMs
working on different sections of the filesystem in some reliable
crash safe manner.

> > > This would elegantly solve the "what if HSM handler dies" problem as well
> > > as cleanly handle the setup. And we don't have to come up with a concept of
> > > "default mask".
> >
> > We can still have a mask, it's just about the API to set it up.
> >
> > > Now we still have the problem how to fill in the filesystem
> > > on pre-content event without deadlocking. As I was thinking about it the
> > > most elegant solution would IMHO be if the HSM handler could have a private
> > > mount flagged so that HSM is excluded from there (or it could place ignore
> > > mark on this mount for HSM events).
> >
> > My HttpDirFS demo does it the other way around - HSM uses a mount
> > without a mark mount - but ignore mark works too.
>
> Yes, the HSM handler is free to setup whatever works for it. I was just
> thinking to make sure there is at least one sane way how to do it :)
>

Yeh, we need to write "best practice" guidelines.

> > > I think we've discarded similar ideas
> > > in the past because this is problematic with directory pre-content events
> > > because security hooks don't get the mountpoint. But what if we used
> > > security_path_* hooks for directory pre-content events?
> >
> > No need for security_path_ * hooks.
> > In my POC, the pre-path hooks have the path argument.
> > For people who are not familiar with the term, here is man page draft
> > for "pre-path" events:
> > https://github.com/amir73il/man-pages/commits/fan_pre_path/
> >
> > This is an out of date branch from the time that I called them
> > FAN_PRE_{CREATE,DELETE,MOVE_*} events:
> > https://github.com/amir73il/linux/commit/29c60e4db3068ff2cd7b2c5a73108afb2c19b868
> >
> > They are implemented by replacing the mnt_want_write() calls
> > with mnt_want_write_{path,parent,parents}() calls.
> >
> > This was done to make sure that they take the sb write srcu and call
> > the pre-path hook before taking sb writers freeze protection.
>
> Ok, so AFAIU you agree we don't need to rely on FMODE_NONOTIFY for HSM and
> can just use access through dedicated mount for filling in the filesystem?
>

It seems like a decent and simple way to avoid difficult questions,
so I will try to start with that...
whenever I manage to context switch back ;)

Thanks,
Amir.