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

[Tianjia Zhang]

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?

Thanks and best,
Tianjia