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

[Dmitry Kasatkin]

On Wed, Mar 5, 2014 at 6:04 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
> 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.
>

This can be the next step further.
Now we need to support new xattrs in the kernel, without breaking
existing labeled filesystem.

Dmitry


> Mimi
>



-- 
Thanks,
Dmitry
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