Re: [PATCH 1/1] NAX LSM: Add initial support support

[THOBY Simon <Simon.THOBY@viveris.fr>]

Hi Igor,

On 8/12/21 6:43 PM, Igor Zhbanov wrote:
> Hi Simon,
> 
> Thanks for thorough review. Will post the corrected version soon.
> 
>>> @@ -278,11 +279,11 @@ endchoice
>>>
>>>  config LSM
>>>       string "Ordered list of enabled LSMs"
>>> -     default "landlock,lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
>>> -     default "landlock,lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
>>> -     default "landlock,lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
>>> -     default "landlock,lockdown,yama,loadpin,safesetid,integrity,bpf" if DEFAULT_SECURITY_DAC
>>> -     default "landlock,lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor,bpf"
>>> +     default "nax,landlock,lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
>>> +     default "nax,landlock,lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
>>> +     default "nax,landlock,lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
>>> +     default "nax,landlock,lockdown,yama,loadpin,safesetid,integrity,bpf" if DEFAULT_SECURITY_DAC
>>> +     default "nax,landlock,lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor,bpf"
>>
>> I don't know the policy with regard to new LSMs, but enabling this one by default is maybe a bit violent?
>> That said, considering the default mode is SECURITY_NAX_MODE_LOG, this would just log kernel messages and
>> not break existing systems, so this shouldn't be a problem.
> 
> I've just oriended on landlock and lockdown. They are handled in the similar
> way. But, yes, by default NAX module doesn't block anything.
> If you suggest not to do that, I can remove it.

If other LSMs are also enabled by default when added to the kernel, and keeping the idea that the default behavior
is warning-only (warning in a rate-limited fashion, as you rightfully did, so people running JIT engines as root
don't get their kernel log flooded with warnings), then I have no objection to that change.

> 
>>> +__setup("nax_mode=", setup_mode);
>>> +
>>> +static int __init setup_quiet(char *str)
>>> +{
>>> +     unsigned long val;
>>> +
>>> +     if (!locked && !kstrtoul(str, 0, &val))
>>> +             quiet = val ? 1 : 0;
>>
>> The order of the kernel parameters will have an impact on NAX behavior.
>>
>> "nax_mode=1 nax_locked=1" and "nax_locked=1 nax_mode=1" will end up with the same behavior.
>> in the first case the mode is enforced, in the second case it isn't (well unless you changed
>>  the kernel configuration from the default) and it can't be enabled later either.
>>
>> Is that desired?
> 
> Yes. The idea is that on boot you can set-up the proper options then block
> further changing (especially turning the module off).
> Initially it was implemented for sysctl parameters, so, you can change
> something in init-scripts, then lock. Then, I've extended it to command-line
> parameters.
> I can ignore "locked" flag in setup_* functions to defer locking to sysctl
> parsing. (Unless our command-line is glued by the bootloader from several
> parts, so we want to lock it as early as possible...).
> 

I may have badly made my point (especially considering I made a lot of typos when writing
that mail, for which I would like to apologize).
My sentence
	"nax_mode=1 nax_locked=1" and "nax_locked=1 nax_mode=1" will end up with the same behavior.
lacked the "not" word, and both options will NOT have the same behavior.
What I wanted to say was that I think both parameter should have the same behavior, and that
the ordering of the options shouldn't impact the end result, because order-dependent options
may confuse users.

For the matter of have a kernel commandline being the result of concatenations from multiple
sources, I think that if any attacker is able to alter part of the command line, they can
already write 'lsm=' to it and completely disable NAX, thus I'm not sure 'nax_locked' should
impact other setup_* functions.

I believe keeping the nax_locked parameter, but not checking for the 'locked' status in the other setup_*
functions should be enought, as sysctls writes will still be protected by the 'locked' variable.


> Thanks.
> 

Thanks,
Simon