Re: [RFC] Raceless module interface

[Daniel Phillips <phillips@arcor.de>]

On Friday 13 September 2002 17:13, Roman Zippel wrote:
> Hi,
> 
> On Fri, 13 Sep 2002, Daniel Phillips wrote:
> 
> > > The exit function should always be called after the init function (even if
> > > it failed, I don't do it in the patch, that's a bug). The fs init/exit
> > > would like this then:
> >
> > Perhaps, but if so, the module itself should call the exit function in
> > its failure path itself.  Doing the full exit whether it needs to be
> > done or not is wasteful and opens up new DoS opportunities.
> 
> The exit itself can fail as well, so it has to be done by the module code
> anyway (until it suceeds).

That's debatable.  Arguably, a failed ->module_cleanup() should be
retried on every rmmod -a, but expecting module.c to just keep
retrying mindlessly on its own sounds too much like a busy wait.

> What DoS opportunities are there?

Suppose the module exit relies on synchronize_kernel.  The attacker
can force repeated synchronize_kernels, knowing that module.c will
mindlessly do a synchronize_kernel every time a module init fails,
whether needed or not.  Each synchronize_kernel takes an unbounded
amount of time to complete, across which module.c holds a lock.

> Module init failure is the exception
> case and usally needs further attention, so we could actually disable
> further attempts to load this module, unless the user tells us
> specifically so.

Sure, you can fix it by lathering on more complexity.  What you have
to do is explain why we should do that, when there is a simpler and
faster approach that doesn't introduce the problem in the first
place.

> > In the example you give below you must rely on register_filesystem
> > tolerating unregistering a nonexistent filesystem.  That's sloppy at
> > best, and you will have to ensure *every* helper used by ->exit is
> > similarly sloppy.
> 
> Why is that sloppy? E.g. kfree() happily accepts NULL pointers as well.

That is sloppy.  Different discussion.

I take it that the points you didn't reply to are points that you
agree with?  (The main point being, that we both advocate a simple,
two-method interface for module load/unload.)

-- 
Daniel