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

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Date:   Tue, 28 Apr 2020 16:24:21 +0800
From:   Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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 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@...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
> 

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

Thanks and best,
Tianjia