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

[Tianjia Zhang <tianjia.zhang@linux.alibaba.com>]

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@linux.alibaba.com>
>> Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.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?

Thanks and best,
Tianjia