linux-kernel - Re: [PATCH] IMA: do not measure everything opened by root bydefault

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Date:	Tue, 12 May 2009 17:53:21 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	jmorris@...ei.org, linux-kernel@...r.kernel.org,
	Dave Safford <safford@...son.ibm.com>
Subject: Re: [PATCH] IMA: do not measure everything opened by root bydefault

On Tue, 2009-05-12 at 17:27 -0400, Eric Paris wrote:
> On Tue, 2009-05-12 at 17:18 -0400, Mimi Zohar wrote:
> > On Tue, 2009-05-12 at 15:14 -0400, Eric Paris wrote:
> > > The IMA default policy measures every single file opened by root.  This is
> > > terrible for most users.  Consider a system (like mine) with virtual machine
> > > images.  When those images are touched (which happens at boot for me) those
> > > images are measured.  This is just way too much for the default case.
> > > 
> > > Signed-off-by: Eric Paris <eparis@...hat.com>
> > 
> > The question of what to measure is a major issue. If you measure too
> > much, performance is affected, but if you measure too little, then the
> > measurement list will not contain everything that could affect the
> > Trusted Computing Base(TCB), such as configuration files and scripts.
> > 
> > The solution is not to remove the rule that measures everything read
> > by root, but to replace the default IMA configuration file with an LSM
> > specific one, which should be done early in the etc init scripts or
> > initrd.  LTP contains a sample script to replace the default IMA policy
> > (testcases/kernel/security/integrity/ima/tests/ima_policy.sh).
> > 
> > The following SELinux integrity rule, prevents /var/log/messages from
> > being measured. (Dependent on "integrity: lsm audit rule matching fix"
> > patch in the security-testing tree.)
> > 
> > dont_measure func=PATH_CHECK mask=MAY_READ obj_type=var_log_t
> > 
> > By defining an equivalent SELinux integrity rule for each virtual
> > machine image type, the virtual machine images will not be measured.
> > This is far better than not measuring everything in the TCB.
> > 
> > Mimi Zohar
> 
> While the TCB might be interesting to you I'm going to guess that 99% of
> users don't care at all.  I don't think the kernel should ship with such
> an overhead just to make the options available to the few.
> 
> Every distro that wants to ship with IMA compiled in the kernel is going
> to need to carry their own ima policy and they are going to have to
> change userspace so they can load that policy by default.  This is turn
> means that every distro is going to, by default, leave ima
> uncustomizable since we can only load a single policy.

I'm not sure I understand the problem here. Although the policy can only
be loaded once per boot, it could be based on a configuration file
like /etc/measure, which the distro could define. Any system specific
changes could be made to this file.

Mimi Zohar

> Maybe we'd like to allow multiple policy loads?  That doesn't seem great
> to me...
> 
> If the 'right default' for every distro's common user is to not read and
> measure every single file root touches it's the 'right default' in the
> kernel.  Any distro owner want to disagree?
> 
> -Eric
> 

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