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

[Tianjia Zhang]

On 2020/4/28 18:02, Jessica Yu wrote:
> +++ Greg KH [28/04/20 09:29 +0200]:
> > 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:
> > > > >ÂÂÂ Beautify the document description according to the 
> > > recommendation.
> > > > >
> > > > > v2 change:
> > > > >ÂÂÂ document this new option.
> > > > >
> > > > >ÂÂ Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
> > > > >ÂÂ kernel/module.cÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | 8 ++++++++
> > > > >ÂÂ 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 @@
> > > > >ÂÂÂÂÂÂ noirqdebugÂÂÂ [X86-32] Disables the code which attempts to 
> > > detect and
> > > > >ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ disable unhandled interrupt sources.
> > > > > +ÂÂÂ no_modsig_enforce
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂ [KNL] When CONFIG_MODULE_SIG_FORCE is set, this option
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂ allows to disable modsign completely at the beginning.
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂ This means that modules without (valid) signatures will
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂ 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.
>
> I agree with Greg's reasoning here. We had an almost identical thread
> about this two years ago:
>
>  Âhttp://lore.kernel.org/r/20180312132823.dixp7gkjypjlgymt@xxxxxxxxxxxxxxxxxxx
>
> I generally view module signature enforcement as a one way street. You
> can go from unenforced to enforced, but not the other way around. If
> you are anticipating the need to load unsigned modules or undo this
> protection in general, then why are you building the kernel with
> CONFIG_MODULE_SIG_FORCE? It seems to defeat the purpose of enabling
> this option. You could achieve the same behavior by building without
> it and toggling module.sig_enforce on boot, no?
>
> Thanks,
>
> Jessica

I'm sorry I didn't pay attention to the previous email, your information 
helped me a lot, I think this scenario can be solved by toggling 
module.sig_enforce, thank you very much.

Thanks and best,
Tianjia