linux-kernel - Re: [PATCH] Make kernel taint on invalid module signatures configurable

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Message-ID: <20180220212357.wts5c5c7uqcfguzu@redbean>
Date:   Tue, 20 Feb 2018 22:23:59 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Matthew Garrett <mjg59@...gle.com>
Cc:     Ben Hutchings <ben@...adent.org.uk>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Make kernel taint on invalid module signatures
 configurable

+++ Matthew Garrett [20/02/18 20:37 +0000]:
>On Tue, Feb 20, 2018 at 11:21 AM Jessica Yu <jeyu@...nel.org> wrote:
>
>> Ah, OK. So if I'm understanding correctly, you want to use the same kernel
>> image/configuration but for two different use cases, one where the module
>> signatures do not matter, and one where they do matter. But the config you
>> want to use in both use cases would have CONFIG_MODULE_SIG=y and
>> CONFIG_MODULE_SIG_TAINT=n (to avoid tainting of unsigned/invalidly signed
>> modules).
>
>Right. Distributions generally have to try to satisfy multiple use cases
>with as few kernel images as possible.
>
>> In any case, I think I'd be willing to merge it as a module_param made
>> available under CONFIG_MODULE_SIG=y (rather than as a new separate config
>> option), while preserving the default behavior of tainting on
>> unsigned/invalidly signed module loads (so let's keep the param parts of
>> your patch). I think it makes sense to consider the turning-off-the-taint
>> param as a behavioral tweak under CONFIG_MODULE_SIG. Then you could turn
>> off the tainting behavior on the kernel command line, would this
>sufficient
>> enough for your use cases?
>
>I think that's probably not practical - distributions often aren't in
>control of the kernel command line after initial installation, so they'd
>end up with different behaviour depending on whether a machine was a clean
>install or not (which is why several things that are module_params have
>defaults controlled by additional kernel config options)

Ah I see.. Fair enough!

>> >Not entirely. There's two cases where the current situation causes
>problems:
>> >
>> >1) Distributions that build out of tree kernel modules and don't have
>> >infrastructure to sign them will end up with kernel taint. That's
>something
>> >that can be resolved by implementing that infrastructure.
>> >2) End-users who build out of tree kernel modules will end up with kernel
>> >taint and will file bugs. This cannot be fixed but will increase
>> >distribution load anyway.
>
>> I thought these two cases (particularly #2) were the very situations
>> where distros might find the unsigned module taint useful (especially
>> in the use case where you do benefit from module signatures). From my
>> understanding, the unsigned module taint is intended to be useful when
>> looking at crashes/OOPses, to provide a clear indication of whether or
>> not a developer could reliably debug the crash, or choose to tread
>> carefully, because the end-user has loaded an unsigned/out-of-tree
>> module that wasn't signed/shipped by the distribution. Is the taint
>> just not useful to distros in this manner anymore?
>
>The module list is usually sufficient for that - users tend not to replace
>individual distribution modules without rebuilding their entire kernel.