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

[Tianjia Zhang]

On 2020/4/28 15:29, Greg KH wrote:
> 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@xxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Tianjia Zhang <tianjia.zhang@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >
> > > > 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

Thanks for your explanation, I will reconsider whether this method is 
suitable.

Thanks and best,
Tianjia