Re: [PATCH] x86/ima: require signed kernel modules

[Luis Chamberlain <mcgrof@kernel.org>]

On Mon, Feb 04, 2019 at 05:05:10PM -0500, Mimi Zohar wrote:
> On Mon, 2019-02-04 at 12:38 -0800, Luis Chamberlain wrote:
> > On Thu, Jan 31, 2019 at 02:18:59PM -0500, Mimi Zohar wrote:
> > > diff --git a/kernel/module.c b/kernel/module.c
> > > index 2ad1b5239910..70a9709d19eb 100644
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -275,16 +275,23 @@ static void module_assert_mutex_or_preempt(void)
> > >  
> > >  static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
> > >  module_param(sig_enforce, bool_enable_only, 0644);
> > > +static bool sig_required;
> > >  
> > >  /*
> > >   * Export sig_enforce kernel cmdline parameter to allow other subsystems rely
> > >   * on that instead of directly to CONFIG_MODULE_SIG_FORCE config.
> > 
> > But the docs were't updated.
> 
> Neither "CONFIG_MODULE_SIG_FORCE" nor "module.sig_enforce" has
> changed.  Which docs are you referring to? 

You renamed is_module_sig_enforced() to is_module_sig_enforced_or_required()
and left the above doc which only justifies the enforced path.

> > >   */
> > > -bool is_module_sig_enforced(void)
> > > +bool is_module_sig_enforced_or_required(void)
> > >  {
> > > -	return sig_enforce;
> > > +	return sig_enforce || sig_required;
> > >  }
> > > -EXPORT_SYMBOL(is_module_sig_enforced);
> > > +EXPORT_SYMBOL(is_module_sig_enforced_or_required);
> > 
> > Meh, this is getting sloppy, the module signing infrastructure should
> > just be LSM'ified now that we have stacked LSMs. That would
> > compartamentaliz that code and make this much easier to read / understand
> > and mantain.
> > 
> > Can you take a look at doing it that way instead?
> 
> This patch is about coordinating the existing methods of verifying
> kernel module signatures.

I understand.

> > 
> > >  /* Block module loading/unloading? */
> > >  int modules_disabled = 0;
> > > @@ -2789,7 +2796,7 @@ static int module_sig_check(struct load_info *info, int flags)
> > >  	}
> > >  
> > >  	/* Not having a signature is only an error if we're strict. */
> > > -	if (err == -ENOKEY && !is_module_sig_enforced())
> > > +	if (err == -ENOKEY && !is_module_sig_enforced_or_required())
> > 
> > This is where I think a proper LSM hook would make sense. I think
> > that these "questions" model for signing don't work well on the LSM
> > hook model, perhaps just:
> > 
> > kernel_module_signed()
> > 
> > Suffices, therefore if not enforced or required its signed. If its
> > enforced or required and really signed, then it signed.
> > 
> > >  		err = 0;
> > >  
> > >  	return err;
> > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > index 357edd140c09..bbaf87f688be 100644
> > > --- a/security/integrity/ima/ima_main.c
> > > +++ b/security/integrity/ima/ima_main.c
> > > @@ -563,7 +563,7 @@ int ima_load_data(enum kernel_load_data_id id)
> > >  		}
> > >  		break;
> > >  	case LOADING_MODULE:
> > > -		sig_enforce = is_module_sig_enforced();
> > > +		sig_enforce = is_module_sig_enforced_or_required();
> > 
> > Yet another user.
> > 
> > >  		if (ima_enforce && (!sig_enforce
> > >  				    && (ima_appraise & IMA_APPRAISE_MODULES))) {
> > > -- 
> > > 2.7.5
> > 
> > Plus I think LSM'ifying module signing may help cleaning up some of the
> > #ifdery and config options around module signing. I'm suggestin this now 
> > as this has been on my mental TODO list for a while, and just not sure
> > when we'd get to it, if not you, not sure when it'd get done.
> > 
> > Then, do we have proper unit tests for the mixture of options to ensure
> > we can easily ensure we don't regress?
> > 
> 
> There are already two methods  - appended signatures and IMA xattrs -
> for validating kernel modules.
> 
> Kernel modules shouldn't be treated any differently than any other
> file.

The good 'ol kernel module signing code *does* treat it as such.

> Based on the IMA policy, the kernel module signature can be
> verified.  Also based on the IMA policy, the file hash added to the
> measurement list, and the file hash used to extend the TPM PCR.
>  Lastly, based on policy the file hash can be added to the audit log.

Sure...

> I don't see a need for an additional LSM just for verifying kernel
> module signatures.

But it is one, module signing was just spawned pre the boom of LSMs.

I do believe that treating the code as such would help with its reading
and long term maintenance.

Anyway, I had to try to convince you.

 Luis