Re: fanotify HSM open issues

[Christian Brauner <brauner@kernel.org>]

On Mon, Nov 20, 2023 at 06:59:47PM +0200, Amir Goldstein wrote:
> On Mon, Nov 20, 2023 at 4:06 PM Jan Kara <jack@suse.cz> wrote:
> >
> > Hi Amir,
> >
> > sorry for a bit delayed reply, I did not get to "swapping in" HSM
> > discussion during the Plumbers conference :)
> >
> > On Mon 13-11-23 13:50:03, Amir Goldstein wrote:
> > > On Wed, Aug 23, 2023 at 7:31 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > On Wed, Aug 23, 2023 at 5:37 PM Jan Kara <jack@suse.cz> wrote:
> > > > > > Recap for new people joining this thread.
> > > > > >
> > > > > > The following deadlock is possible in upstream kernel
> > > > > > if fanotify permission event handler tries to make
> > > > > > modifications to the filesystem it is watching in the context
> > > > > > of FAN_ACCESS_PERM handling in some cases:
> > > > > >
> > > > > > P1                             P2                      P3
> > > > > > -----------                    ------------            ------------
> > > > > > do_sendfile(fs1.out_fd, fs1.in_fd)
> > > > > > -> sb_start_write(fs1.sb)
> > > > > >   -> do_splice_direct()                         freeze_super(fs1.sb)
> > > > > >     -> rw_verify_area()                         -> sb_wait_write(fs1.sb) ......
> > > > > >       -> security_file_permission()
> > > > > >         -> fsnotify_perm() --> FAN_ACCESS_PERM
> > > > > >                                  -> do_unlinkat(fs1.dfd, ...)
> > > > > >                                    -> sb_start_write(fs1.sb) ......
> > > > > >
> > > > > > start-write-safe patches [1] (not posted) are trying to solve this
> > > > > > deadlock and prepare the ground for a new set of permission events
> > > > > > with cleaner/safer semantics.
> > > > > >
> > > > > > The cases described above of sendfile from a file in loop mounted
> > > > > > image over fs1 or overlayfs over fs1 into a file in fs1 can still
> > > > > > deadlock despite the start-write-safe patches [1].
> > > > >
> > > > > Yep, nice summary.
> > ...
> > > > > > > As I wrote above I don't like the abuse of FMODE_NONOTIFY much.
> > > > > > > FMODE_NONOTIFY means we shouldn't generate new fanotify events when using
> > > > > > > this fd. It says nothing about freeze handling or so. Furthermore as you
> > > > > > > observe FMODE_NONOTIFY cannot be set by userspace but practically all
> > > > > > > current fanotify users need to also do IO on other files in order to handle
> > > > > > > fanotify event. So ideally we'd have a way to do IO to other files in a
> > > > > > > manner safe wrt freezing. We could just update handling of RWF_NOWAIT flag
> > > > > > > to only trylock freeze protection - that actually makes a lot of sense to
> > > > > > > me. The question is whether this is enough or not.
> > > > > > >
> > > > > >
> > > > > > Maybe, but RWF_NOWAIT doesn't take us far enough, because writing
> > > > > > to a file is not the only thing that HSM needs to do.
> > > > > > Eventually, event handler for lookup permission events should be
> > > > > > able to also create files without blocking on vfs level freeze protection.
> > > > >
> > > > > So this is what I wanted to clarify. The lookup permission event never gets
> > > > > called under a freeze protection so the deadlock doesn't exist there. In
> > > > > principle the problem exists only for access and modify events where we'd
> > > > > be filling in file data and thus RWF_NOWAIT could be enough.
> > > >
> > > > Yes, you are right.
> > > > It is possible that RWF_NOWAIT could be enough.
> > > >
> > > > But the discovery of the loop/ovl corner cases has shaken my
> > > > confidence is the ability to guarantee that freeze protection is not
> > > > held somehow indirectly.
> > > >
> > > > If I am not mistaken, FAN_OPEN_PERM suffers from the exact
> > > > same ovl corner case, because with splice from ovl1 to fs1,
> > > > fs1 freeze protection is held and:
> > > >   ovl_splice_read(ovl1.file)
> > > >     ovl_real_fdget()
> > > >       ovl_open_realfile(fs1.file)
> > > >          ... security_file_open(fs1.file)
> > > >
> > > > > That being
> > > > > said I understand this may be assuming too much about the implementations
> > > > > of HSM daemons and as you write, we might want to provide a way to do IO
> > > > > not blocking on freeze protection from any hook. But I wanted to point this
> > > > > out explicitly so that it's a conscious decision.
> > > > >
> > >
> > > I agree and I'd like to explain using an example, why RWF_NOWAIT is
> > > not enough for HSM needs.
> > >
> > > The reason is that often, when HSM needs to handle filling content
> > > in FAN_PRE_ACCESS, it is not just about writing to the accessed file.
> > > HSM needs to be able to avoid blocking on freeze protection
> > > for any operations on the filesystem, not just pwrite().
> > >
> > > For example, the POC HSM code [1], stores the DATA_DIR_fd
> > > from the lookup event and uses it in the handling of access events to
> > > update the metadata files that store which parts of the file were already
> > > filled (relying of fiemap is not always a valid option).
> > >
> > > That is the reason that in the POC patches [2], FMODE_NONOTIFY
> > > is propagated from dirfd to an fd opened with openat(dirfd, ...), so
> > > HSM has an indirect way to get a FMODE_NONOTIFY fd on any file.
> > >
> > > Another use case is that HSM may want to download content to a
> > > temp file on the same filesystem, verify the downloaded content and
> > > then clone the data into the accessed file range.
> > >
> > > I think that a PF_ flag (see below) would work best for all those cases.
> >
> > Ok, I agree that just using RWF_NOWAIT from the HSM daemon need not be
> > enough for all sensible usecases to avoid deadlocks with freezing. However
> > note that if we want to really properly handle all possible operations, we
> > need to start handling error from all sb_start_write() and
> > file_start_write() calls and there are quite a few of those.
> >
> 
> Darn, forgot about those.
> I am starting to reconsider adding a freeze level.
> I cannot shake the feeling that there is a simpler solution that escapes us...
> Maybe fs anti-freeze (see blow).
> 
> > > > > > In theory, I am not saying we should do it, but as a thought experiment:
> > > > > > if the requirement from permission event handler is that is must use a
> > > > > > O_PATH | FMODE_NONOTIFY event->fd provided in the event to make
> > > > > > any filesystem modifications, then instead of aiming for NOWAIT
> > > > > > semantics using sb_start_write_trylock(), we could use a freeze level
> > > > > > SB_FREEZE_FSNOTIFY between
> > > > > > SB_FREEZE_WRITE and SB_FREEZE_PAGEFAULT.
> > > > > >
> > > > > > As a matter of fact, HSM is kind of a "VFS FAULT", so as long as we
> > > > > > make it clear how userspace should avoid nesting "VFS faults" there is
> > > > > > a model that can solve the deadlock correctly.
> > > > >
> > > > > OK, yes, in principle another freeze level which could be used by handlers
> > > > > of fanotify permission events would solve the deadlock as well. Just you
> > > > > seem to like to tie this functionality to the particular fd returned from
> > > > > fanotify and I'm not convinced that is a good idea. What if the application
> > > > > needs to do write to some other location besides the one fd it got passed
> > > > > from fanotify event? E.g. imagine it wants to fetch a whole subtree on
> > > > > first access to any file in a subtree. Or maybe it wants to write to some
> > > > > DB file containing current state or something like that.
> > > > >
> > > > > One solution I can imagine is to create an open flag that can be specified
> > > > > on open which would result in the special behavior wrt fs freezing. If the
> > > > > special behavior would be just trylocking the freeze protection then it
> > > > > would be really easy. If the behaviour would be another freeze protection
> > > > > level, then we'd need to make sure we don't generate another fanotify
> > > > > permission event with such fd - autorejecting any such access is an obvious
> > > > > solution but I'm not sure if practical for applications.
> > > > >
> > > >
> > > > I had also considered marking the listener process with the FSNOTIFY
> > > > context and enforcing this context on fanotify_read().
> > > > In a way, this is similar to the NOIO and NOFS process context.
> > > > It could be used to both act as a stronger form of FMODE_NONOTIFY
> > > > and to activate the desired freeze protection behavior
> > > > (whether trylock or SB_FREEZE_FSNOTIFY level).
> > > >
> > >
> > > My feeling is that the best approach would be a PF_NOWAIT task flag:
> > >
> > > - PF_NOWAIT will prevent blocking on freeze protection
> > > - PF_NOWAIT + FMODE_NOWAIT would imply RWF_NOWAIT
> > > - PF_NOWAIT could be auto-set on the reader of a permission event
> > > - PF_NOWAIT could be set on init of group FAN_CLASS_PRE_PATH
> > > - We could add user API to set this personality explicitly to any task
> > > - PF_NOWAIT without FMODE_NONOTIFY denies permission events
> > >
> > > Please let me know if you agree with this design and if so,
> > > which of the methods to set PF_NOWAIT are a must for the first version
> > > in your opinion?
> >
> > Yeah, the PF flag could work. It can be set for the process(es) responsible
> > for processing the fanotify events and filling in filesystem contents. I
> > don't think automatic setting of this flag is desirable though as it has
> > quite wide impact and some of the consequences could be surprising.  I
> > rather think it should be a conscious decision when setting up the process
> > processing the events. So I think API to explicitly set / clear the flag
> > would be the best. Also I think it would be better to capture in the name
> > that this is really about fs freezing. So maybe PF_NOWAIT_FREEZE or
> > something like that?
> >
> 
> Sure.
> 
> > Also we were thinking about having an open(2) flag for this (instead of PF
> > flag) in the past. That would allow finer granularity control of the
> > behavior but I guess you are worried that it would not cover all the needed
> > operations?
> >
> 
> Yeh, it seems like an API that is going to be harder to write safe HSM
> programs with.
> 
> > > Do you think we should use this method to fix the existing deadlocks
> > > with FAN_OPEN_PERM and FAN_ACCESS_PERM? without opt-in?
> >
> > No, I think if someone cares about these, they should explicitly set the
> > PF flag in their task processing the events.
> >
> 
> OK.
> 
> I see an exit hatch in this statement -
> If we are going leave the responsibility to avoid deadlock in corner
> cases completely in the hands of the application, then I do not feel
> morally obligated to create the PF_NOWAIT_FREEZE API *before*
> providing the first HSM API.
> 
> If the HSM application is running in a controlled system, on a filesystem
> where fsfreeze is not expected or not needed, then a fully functional and
> safe HSM does not require PF_NOWAIT_FREEZE API.
> 
> Perhaps an API to make an fs unfreezable is just as practical and a much
> easier option for the first version of HSM API?
> 
> Imagine that HSM opens an fd and sends an EXCLUSIVE_FSFREEZER
> ioctl. Then no other task can freeze the fs, for as long as the fd is open
> apart from the HSM itself using this fd.

This would mean you also prevent FREEZE_HOLDER_KERNEL requests which xfs
uses for filesystem scrubbing iirc. I would reckon that you also run
into problems with device mapper workloads where freeze/thaw requests
from the block layer and into the filesystem layer are quite common.

Have you given any thought to the idea - similar to a FUSE daemon - that
you could register with a given filesystem as an HSM? Maybe integration
like this is really undesirable for some reason but that may be an
alternative.