[PATCH] LSM: Allow syzbot to ignore security= parameter.

[Casey Schaufler]

On 2/6/2019 6:30 PM, Tetsuo Handa wrote:
> Casey Schaufler wrote:
>> On 2/6/2019 2:23 AM, Tetsuo Handa wrote:
>>> But as I update the documentation ( https://tomoyo.osdn.jp/2.6/chapter-3.html.en#3.6 ),
>>> I came to think that we should ignore security= parameter when lsm= parameter is specified.
>>>
>>> Currently, it is possible to enable TOMOYO and only one of SELinux/Smack/AppArmor. Therefore,
>>> it is possible to disable only TOMOYO by specifying security=selinux when we want to enable
>>> only SELinux, by specifying security=smack when we want to enable only Smack, by specifying
>>> security=apparmor when we want to enable only AppArmor. That is, we can use security= parameter
>>> in order to specify the other LSM module which should not be disabled.
>>>
>>> But when it becomes possible to enable TOMOYO and more than one of SELinux/Smack/AppArmor,
>>> we will no longer be able to selectively disable one LSM module using security= parameter, for
>>> security= parameter is intended for specifying only one LSM module which should be enabled.
>>> That is, we will need to use lsm= parameter in order to selectively disable LSM modules.
>> Yes. That is correct. The existing behavior of security= is maintained.
> But the existing behavior of CONFIG_DEFAULT_SECURITY is not maintained.

That's a developer interface, not a user interface. I realize
that may be splitting hairs, but it had to change.

> This might cause a problem like
>
>   commit e5a3b95f581da62e2054ef79d3be2d383e9ed664
>   Author: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
>   Date:   Sat Feb 14 11:46:56 2009 +0900
>
>       TOMOYO: Don't create securityfs entries unless registered.
>
>       TOMOYO should not create /sys/kernel/security/tomoyo/ interface unless
>       TOMOYO is registered.
>
> for Ubuntu users because Ubuntu kernels are built with
>
>   CONFIG_SECURITY_SELINUX=y
>   CONFIG_SECURITY_SMACK=y
>   CONFIG_SECURITY_TOMOYO=y
>   CONFIG_SECURITY_APPARMOR=y
>   CONFIG_SECURITY_YAMA=y
>   CONFIG_DEFAULT_SECURITY="apparmor"
>
> . Due to CONFIG_DEFAULT_SECURITY="apparmor", majority of Ubuntu users are enabling
> only AppArmor without explicitly specifying "security=apparmor".
>
> Currently default CONFIG_LSM setting is
>
>   "yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor"
>
> but Ubuntu kernels would have to be built with non-default CONFIG_LSM setting like
>
>   "yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo"
>
> in order to make sure that AppArmor is by default chosen for the LSM_FLAG_EXCLUSIVE module.

Yes, and Yocto Project is likely to want Smack specified first.

> Now that TOMOYO becomes a !LSM_FLAG_EXCLUSIVE module, not specifying "security=apparmor" will
> automatically enable TOMOYO. And majority of Ubuntu users will unexpectedly encounter TOMOYO
> messages. But removing "tomoyo" from CONFIG_LSM setting in order to save majority of Ubuntu
> users from unexpectedly encountering TOMOYO messages also has a problem; Ubuntu users who want
> to enable only TOMOYO from LSM_FLAG_LEGACY_MAJOR modules can specify "security=tomoyo", but
> Ubuntu users who want to enable TOMOYO and one of SELinux,Smack,AppArmor (including syzbot)
> will have to explicitly specify "lsm=" because "security=" can't allow enabling multiple
> LSM_FLAG_LEGACY_MAJOR modules.

I believe we got general buy in from Ubuntu, and I understand
that the LSM list is awkward, but I don't see a rational alternate.
I know that I played with a half dozen, and nothing was closer to
maintaining the status quo.

>> The new behavior of lsm= is provided to allow general handling of a list
>> of security modules. It uses the same form of data as CONFIG_LSM.
>>
>>> Then, I think that it is straightforward (and easier to manage) to ignore security= parameter
>>> when lsm= parameter is specified.
>> That reduces flexibility somewhat. If I am debugging security modules
>> I may want to use lsm= to specify the order while using security= to
>> identify a specific exclusive module. I could do that using lsm= by
>> itself, but habits die hard.
> "lsm=" can be used for identifying a specific exclusive module, and Ubuntu kernels would
> have to use CONFIG_LSM (or "lsm=") for identifying the default exclusive module (in order
> to allow enabling both TOMOYO and one of SELinux,Smack,AppArmor at the same time).
>
> Since "security=" can't be used for selectively enable/disable more than one of
> SELinux,Smack,TOMOYO,AppArmor, I think that recommending users to migrate to "lsm=" is the
> better direction. And ignoring "security=" when "lsm=" is specified is easier to understand.

I added Kees to the CC list. Kees, what to you think about
ignoring security= if lsm= is specified? I'm ambivalent.