On Fri, Aug 16, 2019 at 11:46:08AM +0200, Petr Mladek wrote:

On Wed 2019-08-14 10:12:44, Josh Poimboeuf wrote:

quoted

On Wed, Aug 14, 2019 at 01:06:09PM +0200, Miroslav Benes wrote:

quoted

quoted

Really, we should be going in the opposite direction, by creating module
dependencies, like all other kernel modules do, ensuring that a module
is loaded *before* we patch it.  That would also eliminate this bug.

Yes, but it is not ideal either with cumulative one-fixes-all patch 
modules. It would load also modules which are not necessary for a 
customer and I know that at least some customers care about this. They 
want to deploy only things which are crucial for their systems.

If you frame the question as "do you want to destabilize the live
patching infrastucture" then the answer might be different.

We should look at whether it makes sense to destabilize live patching
for everybody, for a small minority of people who care about a small
minority of edge cases.

I do not see it that simple. Forcing livepatched modules to be
loaded would mean loading "random" new modules when updating
livepatches:

I don't want to start a long debate on this, because this idea isn't
even my first choice.  But we shouldn't dismiss it outright.

<devils-advocate>

  + It means more actions and higher risk to destabilize
    the system. Different modules have different quality.

Maybe the distro shouldn't ship modules which would destabilize the
system.

  + It might open more security holes that are not fixed by
    the livepatch.

Following the same line of thinking, the livepatch infrastructure might
open security holes because of the inherent complexity of late module
patching.

  + It might require some extra configuration actions to handle
    the newly opened interfaces (devices). For example, updating
    SELinux policies.

I assume you mean user-created policies, not distro ones?  Is this even
a realistic concern?

  + Are there conflicting modules that might need to get
    livepatched?

Again is this realistic?

This approach has a strong no-go from my side.

</devils-advocate>

I agree it's not ideal, but nothing is ideal at this point.  Let's not
to rule it out prematurely.  I do feel that our current approach is not
the best.  It will continue to create problems for us until we fix it.

quoted

Or maybe there's some other solution we haven't thought about, which
fits more in the framework of how kernel modules already work.

quoted

We could split patch modules as you proposed in the past, but that have 
issues as well.

quoted

Right, I'm not really crazy about that solution either.

Yes, this would just move the problem somewhere else.

quoted

Here's another idea: per-object patch modules.  Patches to vmlinux are
in a vmlinux patch module.  Patches to kvm.ko are in a kvm patch module.
That would require:

- Careful management of dependencies between object-specific patches.
  Maybe that just means that exported function ABIs shouldn't change.

- Some kind of hooking into modprobe to ensure the patch module gets
  loaded with the real one.

I see this just as a particular approach how to split livepatches
per-object. The above points suggest how to handle dependencies
on the kernel side.

Yes, they would need to be done on the distro / patch creation /
operational side.  They probably wouldn't affect livepatch code.

quoted

- Changing 'atomic replace' to allow patch modules to be per-object.

The problem might be how to transition all loaded objects atomically
when the needed code is loaded from different modules.

I'm not sure what you mean.

My idea was that each patch module would be specific to an object, with
no inter-module change dependencies.  So when using atomic replace, if
the patch module is only targeted to vmlinux, then only vmlinux-targeted
patch modules would be replaced.

In other words, 'atomic replace' would be object-specific.

Alternative would be to support only per-object consitency. But it
might reduce the number of supported scenarios too much. Also it
would make livepatching more error-prone.

Again, I don't follow.

-- 
Josh