linux-kernel - Re: [PATCH 09/24] kexec_file: Disable at runtime if securelevel has been set

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Date:   Fri, 7 Apr 2017 16:42:42 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:     David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org,
        Matthew Garrett <mjg59@...f.ucam.org>,
        linux-efi@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        Chun-Yi Lee <jlee@...e.com>, gregkh@...uxfoundation.org,
        kexec@...ts.infradead.org, linux-security-module@...r.kernel.org,
        keyrings@...r.kernel.org, matthew.garrett@...ula.com
Subject: Re: [PATCH 09/24] kexec_file: Disable at runtime if securelevel has
 been set

On 04/07/17 at 04:28am, Mimi Zohar wrote:
> On Fri, 2017-04-07 at 15:41 +0800, Dave Young wrote:
> > On 04/07/17 at 08:07am, David Howells wrote:
> > > Dave Young <dyoung@...hat.com> wrote:
> > > 
> > > > > > > +	/* Don't permit images to be loaded into trusted kernels if we're not
> > > > > > > +	 * going to verify the signature on them
> > > > > > > +	 */
> > > > > > > +	if (!IS_ENABLED(CONFIG_KEXEC_VERIFY_SIG) && kernel_is_locked_down())
> > > > > > > +		return -EPERM;
> > > > > > > +
> > > > > > >  
> > > > > 
> > > > > IMA can be used to verify file signatures too, based on the LSM hooks
> > > > > in  kernel_read_file_from_fd().  CONFIG_KEXEC_VERIFY_SIG should not be
> > > > > required.
> > > > 
> > > > Mimi, I remember we talked somthing before about the two signature 
> > > > verification. One can change IMA policy in initramfs userspace,
> > > > also there are kernel cmdline param to disable IMA, so it can break the
> > > > lockdown? Suppose kexec boot with ima disabled cmdline param and then
> > > > kexec reboot again..
> > > 
> > > I guess I should lock down the parameter to disable IMA too.
> > 
> > That is one thing, user can change IMA policy in initramfs userspace,
> > I'm not sure if IMA enforce the signed policy now, if no it will be also
> > a problem.
> 
> I'm not sure how this relates to the question of whether IMA verifies
> the kexec kernel image signature, as the test would not be based on a
> Kconfig option, but on a runtime variable.

I assumed one can change the policy to avoid kexec and initramfs check
And we use a global IMA status in the -EPERM check for the lockdown
checking.  But if there is some fine grained checking to ensure kernel
signature verification it should be fine.
> 
> To answer your question, the rule for requiring the policy to be
> signed is:  appraise func=POLICY_CHECK appraise_type=imasig
> 
> When the ability to append rules is Kconfig enabled, the builtin
> policy requires the new policy or additional rules to be signed.
>  Unfortunately, always requiring the policy to be signed, would have
> broken userspace.
> 
> Mimi
> 

Thanks
Dave