Re: [PATCH 8/8] evm: introduce EVM hmac xattr list

[Mimi Zohar]

On Wed, 2014-03-05 at 11:26 +0200, Dmitry Kasatkin wrote: 
> On Tue, Mar 4, 2014 at 10:36 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, 2014-03-04 at 16:18 +0200, Dmitry Kasatkin wrote:
> >> On Tue, Mar 4, 2014 at 5:21 AM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
> >> > On Mon, 2014-03-03 at 19:00 -0800, Casey Schaufler wrote:
> >> >> On 3/3/2014 6:39 PM, Mimi Zohar wrote:
> >> >> > On Fri, 2014-02-28 at 16:59 +0200, Dmitry Kasatkin wrote:
> >> >> >> EVM currently uses source hard coded list of xattrs which needs to be
> >> >> >> included into the HMAC calculation. This is very unflexible.
> >> >> >> Adding new attributes requires modifcation of the source code and
> >> >> >> prevents building the kernel which works with previously labeled
> >> >> >> filesystems.
> >> >> >>
> >> >> >> Early versions of Smack used only one xattr security.SMACK64,
> >> >> >> which is protected by EVM. Now Smack has a few more attributes and
> >> >> >> they are not protected. Adding support to the source code makes it
> >> >> >> impossible to use new kernel with previousely labeled filesystems.
> >> >> > I think this patch will break any existing filesystems labeled with
> >> >> > 'security.smack64'.  Details inline.
> >> >> >
> >> >> >> This patch replaces hardcoded xattr array with dynamic list which is
> >> >> >> initialized from CONFIG_EVM_HMAC_XATTRS variable. It allows to build
> >> >> >> kernel with with support of older and newer EVM HMAC formats.
> >
> > So instead of having a single kernel, this allows you to build different
> > kernels with different xattr labels included in the HMAC.  Wouldn't you
> > want a migration mode, similar to 'fix' mode, that only updates the
> > HMAC, if the existing HMAC verified based on the prior set of xattrs?
> >
> 
> It would require to maintain 2 sets of xattrs: "old" and "new".
> What if "new" becomes "old" again, while there were still not updated
> previous "old" :)
> 
> I think it would bring unnecessary complexity.
> System designers define what xattrs they want to protect and stick with that.
> Filesystems stays anyway, but allows to upgrade to new kernels.
> 
> If someone really wants to upgrade the system, just use "evm=fix" and run
> recursive fix.., e.g. 'evmctl -r ima_fix /'

Right, but instead of fixing everything, we would want a "safe" fix.
Verify the existing HMAC, before updating the HMAC based on the new set
of xattrs.  With this design, only an "old" and "new" set of xattrs
would be needed.

Mimi

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