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From: Petr Mladek <pmladek@suse.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Miroslav Benes <mbenes@suse.cz>,
	jikos@kernel.org, joe.lawrence@redhat.com,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal
Date: Fri, 16 Aug 2019 11:46:08 +0200
Message-ID: <20190816094608.3p2z73oxcoqavnm4@pathway.suse.cz> (raw)
In-Reply-To: <20190814151244.5xoaxib5iya2qjco@treble>

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

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-19 12:28 [RFC PATCH 0/2] " Miroslav Benes
2019-07-19 12:28 ` [PATCH 1/2] livepatch: Nullify obj->mod in klp_module_coming()'s error path Miroslav Benes
2019-07-28 19:45   ` Josh Poimboeuf
2019-08-19 11:26     ` Petr Mladek
2019-07-19 12:28 ` [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal Miroslav Benes
2019-07-22  9:33   ` Petr Mladek
2019-08-14 12:33     ` Miroslav Benes
2019-07-28 20:04   ` Josh Poimboeuf
2019-08-14 11:06     ` Miroslav Benes
2019-08-14 15:12       ` Josh Poimboeuf
2019-08-16  9:46         ` Petr Mladek [this message]
2019-08-22 22:36           ` Josh Poimboeuf
2019-08-23  8:13             ` Petr Mladek
2019-08-26 14:54               ` Josh Poimboeuf
2019-08-27 15:05                 ` Joe Lawrence
2019-08-27 15:37                   ` Josh Poimboeuf
2019-09-02 16:13                 ` Miroslav Benes
2019-09-02 17:05                   ` Joe Lawrence
2019-09-03 13:02                     ` Miroslav Benes
2019-09-04  8:49                       ` Petr Mladek
2019-09-04 16:26                         ` Joe Lawrence
2019-09-05  2:50                         ` Josh Poimboeuf
2019-09-05 11:09                           ` Petr Mladek
2019-09-05 11:19                             ` Jiri Kosina
2019-09-05 13:23                               ` Josh Poimboeuf
2019-09-05 13:31                                 ` Jiri Kosina
2019-09-05 13:42                                   ` Josh Poimboeuf
2019-09-05 11:39                             ` Joe Lawrence
2019-09-05 13:08                             ` Josh Poimboeuf
2019-09-05 13:15                               ` Josh Poimboeuf
2019-09-05 13:52                                 ` Petr Mladek
2019-09-05 14:28                                   ` Josh Poimboeuf
2019-09-05 12:03                           ` Miroslav Benes
2019-09-05 12:35                             ` Josh Poimboeuf
2019-09-05 12:49                               ` Miroslav Benes
2019-09-05 11:52                         ` Miroslav Benes
2019-09-05  2:32                       ` Josh Poimboeuf
2019-09-05 12:16                         ` Miroslav Benes
2019-09-05 12:54                           ` Josh Poimboeuf
2019-09-06 12:51                             ` Miroslav Benes
2019-09-06 15:38                               ` Joe Lawrence
2019-09-06 16:45                               ` Josh Poimboeuf
2019-08-26 13:44         ` Nicolai Stange
2019-08-26 15:02           ` Josh Poimboeuf

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