Messages in this thread | | | Date | Fri, 16 Aug 2019 11:46:08 +0200 | From | Petr Mladek <> | Subject | Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal |
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On Wed 2019-08-14 10:12:44, Josh Poimboeuf wrote: > On Wed, Aug 14, 2019 at 01:06:09PM +0200, Miroslav Benes wrote: > > > 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:
+ It means more actions and higher risk to destabilize the system. Different modules have different quality.
+ It might open more security holes that are not fixed by the livepatch.
+ It might require some extra configuration actions to handle the newly opened interfaces (devices). For example, updating SELinux policies.
+ Are there conflicting modules that might need to get livepatched?
This approach has a strong no-go from my side.
> Or maybe there's some other solution we haven't thought about, which > fits more in the framework of how kernel modules already work. > > > We could split patch modules as you proposed in the past, but that have > > issues as well.
> Right, I'm not really crazy about that solution either.
Yes, this would just move the problem somewhere else.
> 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.
> - 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.
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.
I would like to see updated variant of this patch to see how much arch-specific code is really necessary.
IMHO, if reverting relocations is too complicated then the least painful compromise is to "deny the patched modules to be removed".
> > Anyway, that is why I proposed "Rethinking late module patching" talk at > > LPC and we should try to come up with a solution there. > > Thanks, I saw that. It's definitely worthy of more discussion. The > talk may be more productive if there were code to look at. For example, > a patch which removes all the "late module patching" gunk, so we can at > least quantify the cost of the current approach.
+1
Best Regards, Petr
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