linux-kernel - Re: [PATCH v3] module: Allow to disable modsign in kernel cmdline

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Date:   Tue, 28 Apr 2020 09:29:44 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
Cc:     jeyu@...nel.org, corbet@....net, rdunlap@...radead.org,
        mchehab+samsung@...nel.org, tglx@...utronix.de,
        akpm@...ux-foundation.org, pawan.kumar.gupta@...ux.intel.com,
        jgross@...e.com, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3] module: Allow to disable modsign in kernel cmdline

On Tue, Apr 28, 2020 at 03:07:10PM +0800, Tianjia Zhang wrote:
> 
> 
> On 2020/4/28 14:35, Greg KH wrote:
> > On Tue, Apr 28, 2020 at 02:00:08PM +0800, Tianjia Zhang wrote:
> > > This option allows to disable modsign completely at the beginning,
> > > and turn off by set the kernel cmdline `no_modsig_enforce` when
> > > `CONFIG_MODULE_SIG_FORCE` is enabled.
> > > 
> > > Yet another change allows to always show the current status of
> > > modsign through `/sys/module/module/parameters/sig_enforce`.
> > > 
> > > Signed-off-by: Jia Zhang <zhang.jia@...ux.alibaba.com>
> > > Signed-off-by: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
> > > ---
> > > 
> > > v3 change:
> > >    Beautify the document description according to the recommendation.
> > > 
> > > v2 change:
> > >    document this new option.
> > > 
> > >   Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
> > >   kernel/module.c                                 | 8 ++++++++
> > >   2 files changed, 14 insertions(+)
> > > 
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 7bc83f3d9bdf..b30f013fb8c5 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -3190,6 +3190,12 @@
> > >   	noirqdebug	[X86-32] Disables the code which attempts to detect and
> > >   			disable unhandled interrupt sources.
> > > +	no_modsig_enforce
> > > +			[KNL] When CONFIG_MODULE_SIG_FORCE is set, this option
> > > +			allows to disable modsign completely at the beginning.
> > > +			This means that modules without (valid) signatures will
> > > +			be loaded successfully.
> > > +
> > 
> > So now we have module.sig_enforce and this one?  That feels really
> > confusing, why can't you just use the existing option?
> > 
> > And why would you want to allow the bootloader to override a kernel
> > build option like this?  That feels risky.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> 
> If CONFIG_MODULE_SIG_FORCE is set, `module.sig_enforce` is always true and
> read-only. There is indeed a risk in doing this, but it will allow the
> system to boot normally in some emergency situations, such as certificate
> expiration.
> 
> On the other hand, would it be a good solution to make `module.sig_enforce`
> readable and writable?

Readable is fine :)

And you really can't modify the existing option to change how it works,
but my question is, why would you want to override
CONFIG_MODULE_SIG_FORCE at all?  I wouldn't want my bootloader to have
the ability to change the kernel's protection model, that's a huge
security hole you are adding to the kernel that it can not protect
itself from at all.

thanks,

greg k-h